Merge pull request #28 from GiVeMeRoOt/sec-module-review

Security module review

courses/security/fundamentals.md

@@ -10,7 +10,7 @@
 ![The Wide Area of security](images/image1.png)
 
 - SREs should be involved in both significant design discussions and actual system changes.
-  - They have quite a big role in System design & hence are quite sometimes the first line of defense.
+  - They have quite a big role in System design & hence are quite sometimes the first line of defence.
   - SRE’s help in preventing bad design & implementations which can affect the overall security of the infrastructure.  
 - Successfully designing, implementing, and maintaining systems requires a commitment to **the full system lifecycle**. This commitment is possible only when security and reliability are central elements in the architecture of systems.
 - Core Pillars of Information Security :
@@ -26,17 +26,17 @@
 
 - Security Principles By OWASP (Open Web Application Security Project)
   - Minimize attack surface area :
-    - Every feature that is added to an application adds a certain amount of risk to the overall application. The aim for secure development is to reduce the overall risk by reducing the attack surface area.
-    - For example, a web application implements online help with a search function. The search function may be vulnerable to SQL injection attacks. If the help feature was limited to authorized users, the attack likelihood is reduced. If the help feature’s search function was gated through centralized data validation routines, the ability to perform SQL injection is dramatically reduced. However, if the help feature was re-written to eliminate the search function (through better user interface, for example), this almost eliminates the attack surface area, even if the help feature was available to the Internet at large.
+    - Every feature that is added to an application adds a certain amount of risk to the overall application. The aim of secure development is to reduce the overall risk by reducing the attack surface area.
+    - For example, a web application implements online help with a search function. The search function may be vulnerable to SQL injection attacks. If the help feature was limited to authorized users, the attack likelihood is reduced. If the help feature’s search function was gated through centralized data validation routines, the ability to perform SQL injection is dramatically reduced. However, if the help feature was re-written to eliminate the search function (through a better user interface, for example), this almost eliminates the attack surface area, even if the help feature was available to the Internet at large.
   - Establish secure defaults:
     - There are many ways to deliver an “out of the box” experience for users. However, by default, the experience should be secure, and it should be up to the user to reduce their security – if they are allowed.
-    - For example, by default, password aging and complexity should be enabled. Users might be allowed to turn these two features off to simplify their use of the application and increase their risk.
-    - Default Passwords of routers, IOT devices should be changed
+    - For example, by default, password ageing and complexity should be enabled. Users might be allowed to turn these two features off to simplify their use of the application and increase their risk.
+    - Default Passwords of routers, IoT devices should be changed
   - Principle of Least privilege
     - The principle of least privilege recommends that accounts have the least amount of privilege required to perform their business processes. This encompasses user rights, resource permissions such as CPU limits, memory, network, and file system permissions.
     - For example, if a middleware server only requires access to the network, read access to a database table, and the ability to write to a log, this describes all the permissions that should be granted. Under no circumstances should the middleware be granted administrative privileges.
   - Principle of Defense in depth
-    - The principle of defense in depth suggests that where one control would be reasonable, more controls that approach risks in different fashions are better. Controls, when used in-depth, can make severe vulnerabilities extraordinarily difficult to exploit and thus unlikely to occur.
+    - The principle of defence in depth suggests that where one control would be reasonable, more controls that approach risks in different fashions are better. Controls, when used in depth, can make severe vulnerabilities extraordinarily difficult to exploit and thus unlikely to occur.
     - With secure coding, this may take the form of tier-based validation, centralized auditing controls, and requiring users to be logged on all pages.
     - For example, a flawed administrative interface is unlikely to be vulnerable to an anonymous attack if it correctly gates access to production management networks, checks for administrative user authorization, and logs all access.
   - Fail securely
@@ -58,26 +58,26 @@
 
   - Don’t trust services
     - Many organizations utilize the processing capabilities of third-party partners, who more than likely have different security policies and posture than you. It is unlikely that you can influence or control any external third party, whether they are home users or major suppliers or partners.
-    - Therefore, the implicit trust of externally run systems is not warranted. All external systems should be treated in a similar fashion.
-    - For example, a loyalty program provider provides data that is used by Internet Banking, providing the number of reward points and a small list of potential redemption items. However, the data should be checked to ensure that it is safe to display to end-users, and that the reward points are a positive number, and not improbably large.
+    - Therefore, the implicit trust of externally run systems is not warranted. All external systems should be treated similarly.
+    - For example, a loyalty program provider provides data that is used by Internet Banking, providing the number of reward points and a small list of potential redemption items. However, the data should be checked to ensure that it is safe to display to end-users and that the reward points are a positive number, and not improbably large.
   - Separation of duties
     - The key to fraud control is the separation of duties. For example, someone who requests a computer cannot also sign for it, nor should they directly receive the computer. This prevents the user from requesting many computers and claiming they never arrived.
     - Certain roles have different levels of trust than normal users. In particular, administrators are different from normal users. In general, administrators should not be users of the application.
     - For example, an administrator should be able to turn the system on or off, set password policy but shouldn’t be able to log on to the storefront as a super privileged user, such as being able to “buy” goods on behalf of other users.
   - Avoid security by obscurity
     - Security through obscurity is a weak security control, and nearly always fails when it is the only control. This is not to say that keeping secrets is a bad idea, it simply means that the security of systems should not be reliant upon keeping details hidden.
-    - For example, the security of an application should not rely upon knowledge of the source code being kept secret. The security should rely upon many other factors, including reasonable password policies, defense in depth, business transaction limits, solid network architecture, and fraud, and audit controls.
+    - For example, the security of an application should not rely upon knowledge of the source code being kept secret. The security should rely upon many other factors, including reasonable password policies, defence in depth, business transaction limits, solid network architecture, and fraud, and audit controls.
     - A practical example is Linux. Linux’s source code is widely available, and yet when properly secured, Linux is a secure and robust operating system.
   - Keep security simple
     - Attack surface area and simplicity go hand in hand. Certain software engineering practices prefer overly complex approaches to what would otherwise be a relatively straightforward and simple design.
     - Developers should avoid the use of double negatives and complex architectures when a simpler approach would be faster and simpler.
     - For example, although it might be fashionable to have a slew of singleton entity beans running on a separate middleware server, it is more secure and faster to simply use global variables with an appropriate mutex mechanism to protect against race conditions.
   - Fix security issues correctly
-    - Once a security issue has been identified, it is important to develop a test for it and to understand the root cause of the issue. When design patterns are used, it is likely that the security issue is widespread amongst all codebases, so developing the right fix without introducing regressions is essential.
+    - Once a security issue has been identified, it is important to develop a test for it and to understand the root cause of the issue. When design patterns are used, the security issue is likely widespread amongst all codebases, so developing the right fix without introducing regressions is essential.
     - For example, a user has found that they can see another user’s balance by adjusting their cookie. The fix seems to be relatively straightforward, but as the cookie handling code is shared among all applications, a change to just one application will trickle through to all other applications. The fix must, therefore, be tested on all affected applications.
   - Reliability & Security
-    - Reliability and security are both crucial components of a truly trustworthy system,but building systems that are both reliable and secure is difficult. While the requirements for reliability and security share many common properties, they also require different design considerations. It is easy to miss the subtle interplay between reliability and security that can cause unexpected outcomes
-    - Ex: A password management application failure was triggered by a reliability problem i.e poor load-balancing and load-shedding strategies and its recovery was later complicated by multiple measures (HSM mechanism which needs to be plugged into server racks , which works as an authentication & the HSM token supposedly locked inside a case.. & the problem can be further elongated ) designed to increase the security of the system.
+    - Reliability and security are both crucial components of a truly trustworthy system, but building systems that are both reliable and secure is difficult. While the requirements for reliability and security share many common properties, they also require different design considerations. It is easy to miss the subtle interplay between reliability and security that can cause unexpected outcomes
+    - Ex: A password management application failure was triggered by a reliability problem i.e poor load-balancing and load-shedding strategies and its recovery were later complicated by multiple measures (HSM mechanism which needs to be plugged into server racks, which works as an authentication & the HSM token supposedly locked inside a case.. & the problem can be further elongated ) designed to increase the security of the system.
 
 ---
 
@@ -111,7 +111,7 @@
 
 ### Ciphers
 
-- Ciphers are the cornerstone of cryptography. A cipher is a set of algorithms that performs encryption or decryption on a message. An encryption algorithm (E) takes a secret key (k) and a message (m), and produces a ciphertext (c). Similarly, a Decryption algorithm (D) takes a secret key (K) and the previous resulting Ciphertext (C). They are represented as follows:
+- Ciphers are the cornerstone of cryptography. A cipher is a set of algorithms that performs encryption or decryption on a message. An encryption algorithm (E) takes a secret key (k) and a message (m) and produces a ciphertext (c). Similarly, a Decryption algorithm (D) takes a secret key (K) and the previous resulting Ciphertext (C). They are represented as follows:
 
 ```
 
@@ -120,7 +120,7 @@ D(k,c) = m
 
 ```
 
-- This also means that in order for it to be a cipher, it must satisfy the consistency equation as follows, making it possible to decrypt.
+- This also means that for it to be a cipher, it must satisfy the consistency equation as follows, making it possible to decrypt.
 
 ```
 
@@ -130,7 +130,7 @@ D(k,E(k,m)) = m
 Stream Ciphers:
 
 - The message is broken into characters or bits and enciphered with a key or keystream(should be random and generated independently of the message stream) that is as long as the plaintext bitstream.
-- sIf the keystream is random, this scheme would be unbreakable unless the keystream was acquired, making it unconditionally secure. The keystream must be provided to both parties in a secure way to prevent its release.
+- If the keystream is random, this scheme would be unbreakable unless the keystream was acquired, making it unconditionally secure. The keystream must be provided to both parties in a secure way to prevent its release.
 
 Block Ciphers:
 
@@ -182,15 +182,15 @@ Asymmetric Key Algorithm
 Diffie-Hellman
 
 - The protocol has two system parameters, p and g. They are both public and may be used by everybody. Parameter p is a prime number, and parameter g (usually called a generator) is an integer that is smaller than p, but with the following property: For every number n between 1 and p – 1 inclusive, there is a power k of g such that n = gk mod p.
-- Diffie Hellman algorithm is an asymmetric algorithm used to establish a shared secret for a symmetric key algorithm. Nowadays most of the people use hybrid cryptosystem i.e, combination of symmetric and asymmetric encryption. Asymmetric Encryption is used as a technique in key exchange mechanism to share secret key and after the key is shared between sender and receiver, the communication will take place using symmetric encryption. The shared secret key will be used to encrypt the communication.
+- Diffie Hellman algorithm is an asymmetric algorithm used to establish a shared secret for a symmetric key algorithm. Nowadays most of the people use hybrid cryptosystem i.e, a combination of symmetric and asymmetric encryption. Asymmetric Encryption is used as a technique in key exchange mechanism to share a secret key and after the key is shared between sender and receiver, the communication will take place using symmetric encryption. The shared secret key will be used to encrypt the communication.
 - Refer: <https://medium.com/@akhigbemmanuel/what-is-the-diffie-hellman-key-exchange-algorithm-84d60025a30d>
 
 RSA
 
 - The RSA algorithm is very flexible and has a variable key length where, if necessary, speed can be traded for the level of security of the algorithm. The RSA keys are usually 512 to 2048 bits long. RSA has withstood years of extensive cryptanalysis. Although those years neither proved nor disproved RSA's security, they attest to a confidence level in the algorithm. RSA security is based on the difficulty of factoring very large numbers. If an easy method of factoring these large numbers were discovered, the effectiveness of RSA would be destroyed.
-- Refer : <https://medium.com/curiositypapers/a-complete-explanation-of-rsa-asymmetric-encryption-742c5971e0f>
+- Refer: <https://medium.com/curiositypapers/a-complete-explanation-of-rsa-asymmetric-encryption-742c5971e0f>
 
-    **NOTE** : RSA Keys can be used for key exchange just like Deffie Hellman
+    **NOTE**: RSA Keys can be used for key exchange just like Diffie Hellman
 
 Hashing Algorithms
 
@@ -220,10 +220,10 @@ Digital Certificates
 
 - Key management is often considered the most difficult task in designing and implementing cryptographic systems. Businesses can simplify some of the deployment and management issues that are encountered with secured data communications by employing a Public Key Infrastructure (PKI). Because corporations often move security-sensitive communications across the Internet, an effective mechanism must be implemented to protect sensitive information from the threats presented on the Internet.
 
-- PKI provides a hierarchical framework for managing digital security attributes. Each PKI participant holds a digital certificate that has been issued by a CA (either public or private). The certificate contains a number of attributes that are used when parties negotiate a secure connection. These attributes must include the certificate validity period, end-host identity information, encryption keys that will be used for secure communications, and the signature of the issuing CA. Optional attributes may be included, depending on the requirements and capability of the PKI.
+- PKI provides a hierarchical framework for managing digital security attributes. Each PKI participant holds a digital certificate that has been issued by a CA (either public or private). The certificate contains several attributes that are used when parties negotiate a secure connection. These attributes must include the certificate validity period, end-host identity information, encryption keys that will be used for secure communications, and the signature of the issuing CA. Optional attributes may be included, depending on the requirements and capability of the PKI.
 - A CA can be a trusted third party, such as VeriSign or Entrust, or a private (in-house) CA that you establish within your organization.
 - The fact that the message could be decrypted using the sender's public key means that the holder of the private key created the message. This process relies on the receiver having a copy of the sender's public key and knowing with a high degree of certainty that it really does belong to the sender and not to someone pretending to be the sender.
-- To validate the CA's signature, the receiver must know the CA's public key. Normally, this is handled out-of-band or through an operation performed during installation of the certificate. For instance, most web browsers are configured with the root certificates of several CAs by default.
+- To validate the CA's signature, the receiver must know the CA's public key. Normally, this is handled out-of-band or through an operation performed during the installation of the certificate. For instance, most web browsers are configured with the root certificates of several CAs by default.
 
 CA Enrollment process
 
@@ -257,21 +257,21 @@ The major features and guarantees of the SSH protocol are:
 - Integrity of communications, guaranteeing they haven’t been altered
 - Authentication, i.e., proof of identity of senders and receivers
 - Authorization, i.e., access control to accounts
-- Forwarding or tunneling to encrypt other TCP/IP-based sessions
+- Forwarding or tunnelling to encrypt other TCP/IP-based sessions
 
 ### Kerberos
 
 - According to Greek mythology Kerberos (Cerberus) was the gigantic, three-headed dog that guards the gates of the underworld to prevent the dead from leaving.
-- So when it comes to Computer Science, Kerberos is a network authentication protocol, and is currently the default authentication technology used by Microsoft Active Directory to authenticate users to services within a local area network.
+- So when it comes to Computer Science, Kerberos is a network authentication protocol and is currently the default authentication technology used by Microsoft Active Directory to authenticate users to services within a local area network.
 
-- Kerberos uses symmetric key cryptography and requires trusted third-party authentication service to verify user identities. So they used the name of Kerberos for their computer network authentication protocol as the three heads of the Kerberos represent:
-  - a client : A user/ a service
-  - a server : Kerberos protected hosts reside
+- Kerberos uses symmetric-key cryptography and requires a trusted third-party authentication service to verify user identities. So they used the name of Kerberos for their computer network authentication protocol as the three heads of the Kerberos represent:
+  - a client: A user/ a service
+  - a server: Kerberos protected hosts reside
 
     ![image10](images/image10.png)
   - a Key Distribution Center (KDC), which acts as the trusted third-party authentication service.
 
-The KDC includes following two servers:
+The KDC includes the following two servers:
 
 - Authentication Server (AS) that performs the initial authentication and issues ticket-granting tickets (TGT) for users.
 - Ticket-Granting Server (TGS) that issues service tickets that are based on the initial ticket-granting tickets (TGT).
@@ -303,7 +303,7 @@ Certificate chain
 
 - The issuer line indicates it’s issued by Google Internet Authority G2, which also happens to be the subject of the second certificate, number 1
 - What the OpenSSL command line doesn’t show here is the trust store that contains the list of CA certificates trusted by the system OpenSSL runs on.
-- The public certificate of GlobalSign Authority must be present in the system’s trust store to close the verification chain. This is called a chain of trust, and figure below summarizes its behavior at a high level.
+- The public certificate of GlobalSign Authority must be present in the system’s trust store to close the verification chain. This is called a chain of trust, and the figure below summarizes its behaviour at a high level.
 
     ![image122](images/image122.png)
 
@@ -313,7 +313,7 @@ Certificate chain
 
 1. The client sends a HELLO message to the server with a list of protocols and algorithms it supports.
 2. The server says HELLO back and sends its chain of certificates. Based on the capabilities of the client, the server picks a cipher suite.
-3. If the cipher suite supports ephemeral key exchange, like ECDHE does(ECDHE is an algorithm known as the Elliptic Curve Diffie-Hellman Exchange), the server and the client negotiate a pre master key with the Diffie-Hellman algorithm. The pre master key is never sent over the wire.
+3. If the cipher suite supports ephemeral key exchange, like ECDHE does(ECDHE is an algorithm known as the Elliptic Curve Diffie-Hellman Exchange), the server and the client negotiate a pre-master key with the Diffie-Hellman algorithm. The pre-master key is never sent over the wire.
 4. The client and server create a session key that will be used to encrypt the data transiting through the connection.
 
 At the end of the handshake, both parties possess a secret session key used to encrypt data for the rest of the connection. This is what OpenSSL refers to as Master-Key
@@ -322,12 +322,12 @@ At the end of the handshake, both parties possess a secret session key used to e
 
 - There are 3 versions of TLS , TLS 1.0, 1.1 & 1.2
 - TLS 1.0 was released in 1999, making it a nearly two-decade-old protocol. It has been known to be vulnerable to attacks—such as BEAST and POODLE—for years, in addition to supporting weak cryptography, which doesn’t keep modern-day connections sufficiently secure.
-- TLS 1.1 is the forgotten “middle child.” It also has bad cryptography like its younger sibling. In most software it was leapfrogged by TLS 1.2 and it’s rare to see TLS 1.1 used.
+- TLS 1.1 is the forgotten “middle child.” It also has bad cryptography like its younger sibling. In most software, it was leapfrogged by TLS 1.2 and it’s rare to see TLS 1.1 used.
 
 ### “Perfect” Forward Secrecy
 
 - The term “ephemeral” in the key exchange provides an important security feature mis-named perfect forward secrecy (PFS) or just “Forward Secrecy”.
-- In a non-ephemeral key exchange, the client sends the pre-master key to the server by encrypting it with the server’s public key. The server then decrypts the pre-master key with its private key. If, at a later point in time, the private key of the server is compromised, an attacker can go back to this handshake, decrypt the pre-master key, obtain the session key, and decrypt the entire traffic. Non-ephemeral key exchanges are vulnerable to attacks that may happen in the future on recorded traffic. And because people seldom change their password, decrypting data from the past may still be valuable for an attacker.
+- In a non-ephemeral key exchange, the client sends the pre-master key to the server by encrypting it with the server’s public key. The server then decrypts the pre-master key with its private key. If at a later point in time, the private key of the server is compromised, an attacker can go back to this handshake, decrypt the pre-master key, obtain the session key, and decrypt the entire traffic. Non-ephemeral key exchanges are vulnerable to attacks that may happen in the future on recorded traffic. And because people seldom change their password, decrypting data from the past may still be valuable for an attacker.
 - An ephemeral key exchange like DHE, or its variant on elliptic curve, ECDHE, solves this problem by not transmitting the pre-master key over the wire. Instead, the pre-master key is computed by both the client and the server in isolation, using nonsensitive information exchanged publicly. Because the pre-master key can’t be decrypted later by an attacker, the session key is safe from future attacks: hence, the term perfect forward secrecy.
 - Keys are changed every X blocks along the stream. That prevents an attacker from simply sniffing the stream and applying brute force to crack the whole thing. "Forward secrecy" means that just because I can decrypt block M, does not mean that I can decrypt block Q
 - Downside:

courses/security/intro.md

@@ -9,7 +9,7 @@
 
 ## What to expect from this course
 
-The course covers fundamentals of information security along with touching on subjects of system security, network & web security. The aim of this course is to get familiar with the basics of information security in day to day operations & then as an SRE develop the mindset of ensuring that security takes a  front-seat while developing solutions. The course also serves as an introduction to common risks and best practices along with practical ways to find out vulnerable systems and loopholes which might become compromised if not secured.
+The course covers fundamentals of information security along with touching on subjects of system security, network & web security. This course aims to get you familiar with the basics of information security in day to day operations & then as an SRE develop the mindset of ensuring that security takes a  front-seat while developing solutions. The course also serves as an introduction to common risks and best practices along with practical ways to find out vulnerable systems and loopholes which might become compromised if not secured.
 
 
 ## What is not covered under this course

courses/security/network_security.md

@@ -1,9 +1,9 @@
-# Part II : Network Security
+# Part II: Network Security
 
 ## Introduction
 
 - TCP/IP is the dominant networking technology today. It is a five-layer architecture. These layers are, from top to bottom, the application layer, the transport layer (TCP), the network layer (IP), the data-link layer, and the physical layer. In addition to TCP/IP, there also are other networking technologies. For convenience, we use the OSI network model to represent non-TCP/IP network technologies. Different networks are interconnected using gateways. A gateway can be placed at any layer.
-- The OSI model is a seven-layer architecture. The OSI architecture is similar to the TCP/IP architecture, except that the OSI model specifies two additional layers between the application layer and the transport layer in the TCP/IP architecture. These two layers are the presentation layer and the session layer. Figure 5.1 shows the relation between the TCP/IP layers and the OSI layers. The application layer in TCP/IP corresponds to the application layer and the presentation layer in OSI. The transport layer in TCP/IP corresponds to the session layer and the transport layer in OSI. The remaining three layers in the TCP/IP architecture are one-to-one correspondent to the remaining three layers in the OSI model.
+- The OSI model is a seven-layer architecture. The OSI architecture is similar to the TCP/IP architecture, except that the OSI model specifies two additional layers between the application layer and the transport layer in the TCP/IP architecture. These two layers are the presentation layer and the session layer. Figure 5.1 shows the relationship between the TCP/IP layers and the OSI layers. The application layer in TCP/IP corresponds to the application layer and the presentation layer in OSI. The transport layer in TCP/IP corresponds to the session layer and the transport layer in OSI. The remaining three layers in the TCP/IP architecture are one-to-one correspondent to the remaining three layers in the OSI model.
 
     ![image14](images/image14.png)
     Correspondence between layers of the TCP/IP architecture and the OSI model. Also shown are placements of cryptographic algorithms in network layers, where the dotted arrows indicate actual communications of cryptographic algorithms
@@ -19,7 +19,7 @@ The functionalities of OSI layers are briefly described as follows:
 7. The physical layer is responsible for transmitting device-dependent frames through some physical media.
 
 - Starting from the application layer, data generated from an application program is passed down layer-by-layer to the physical layer. Data from the previous layer is enclosed in a new envelope at the current layer, where the data from the previous layer is also just an envelope containing the data from the layer before it. This is similar to enclosing a smaller envelope in a larger one. The envelope added at each layer contains sufficient information for handling the packet. Application-layer data are divided into blocks small enough to be encapsulated in an envelope at the next layer.
-- Application data blocks are “dressed up” in the TCP/IP architecture according to the following basic steps. At the sending side, an application data block is encapsulated in a TCP packet when it is passed down to the TCP layer. In other words, a TCP packet consists of a header and a payload, where the header corresponds to the TCP envelope and the payload is the application data block. Likewise, the TCP packet will be encapsulated in an IP packet when it is passed down to the IP layer. An IP packet consists of a header and a payload, which is the TCP packet passed down from the TCP layer. The IP packet will be encapsulated in a device-dependent frame (e.g., an Ethernet frame) when it is passed down to the data-link layer. A frame has a header, and it may also have a trailer. For example, in addition to having a header, an Ethernet frame also has a 32-bit cyclic redundancy check (CRC) trailer. When it is passed down to the physical layer, a frame will be transformed to a sequence of media signals for transmission
+- Application data blocks are “dressed up” in the TCP/IP architecture according to the following basic steps. At the sending side, an application data block is encapsulated in a TCP packet when it is passed down to the TCP layer. In other words, a TCP packet consists of a header and a payload, where the header corresponds to the TCP envelope and the payload is the application data block. Likewise, the TCP packet will be encapsulated in an IP packet when it is passed down to the IP layer. An IP packet consists of a header and a payload, which is the TCP packet passed down from the TCP layer. The IP packet will be encapsulated in a device-dependent frame (e.g., an Ethernet frame) when it is passed down to the data-link layer. A frame has a header, and it may also have a trailer. For example, in addition to having a header, an Ethernet frame also has a 32-bit cyclic redundancy check (CRC) trailer. When it is passed down to the physical layer, a frame will be transformed into a sequence of media signals for transmission
 
     ![image15](images/image15.png)
         Flow Diagram of a Packet Generation
@@ -28,7 +28,7 @@ The functionalities of OSI layers are briefly described as follows:
 
 ### Public Key Infrastructure
 
-- To deploy cryptographic algorithms in network applications, we need a way to distribute secret keys using open networks. Public-key cryptography is the best way to distribute these secret keys. In order to use public-key cryptography, we need to build a public-key infrastructure (PKI) to support and manage public-key certificates and certificate authority (CA) networks. In particular, PKIs are set up to perform the following functions:
+- To deploy cryptographic algorithms in network applications, we need a way to distribute secret keys using open networks. Public-key cryptography is the best way to distribute these secret keys. To use public-key cryptography, we need to build a public-key infrastructure (PKI) to support and manage public-key certificates and certificate authority (CA) networks. In particular, PKIs are set up to perform the following functions:
   - Determine the legitimacy of users before issuing public-key certificates to them.
   - Issue public-key certificates upon user requests.
   - Extend public-key certificates valid time upon user requests.
@@ -47,8 +47,8 @@ The functionalities of OSI layers are briefly described as follows:
 
 ### PGP & S/MIME : Email Security
 
-- There are a number of security protocols at the application layer. The most used of these protocols are email security protocols namely PGP and S/MIME.
-- SMTP (“Simple Mail Transfer Protocol”) is used for sending and delivering from a client to a server via port 25: it’s the outgoing server. On the contrary, POP (“Post Office Protocol”) allows the user to pick up the message and download it into his own inbox: it’s the incoming server. The latest version of the Post Office Protocol is named POP3, and it’s been used since 1996; it uses port 110
+- There are several security protocols at the application layer. The most used of these protocols are email security protocols namely PGP and S/MIME.
+- SMTP (“Simple Mail Transfer Protocol”) is used for sending and delivering from a client to a server via port 25: it’s the outgoing server. On the contrary, POP (“Post Office Protocol”) allows the user to pick up the message and download it into his inbox: it’s the incoming server. The latest version of the Post Office Protocol is named POP3, and it’s been used since 1996; it uses port 110
 
 PGP
 
@@ -61,11 +61,11 @@ GPG (GnuPG)
 - GnuPG is another free encryption standard that companies may use that is based on OpenPGP.
 - GnuPG serves as a replacement for Symantec’s PGP.
 - The main difference is the supported algorithms. However, GnuPG plays nice with PGP by design. Because GnuPG is open, some businesses would prefer the technical support and the user interface that comes with Symantec’s PGP.
-- It is important to note that there are some nuances between compatibility of GnuPG and PGP, such as the compatibility between certain algorithms, but in most applications such as email, there are workarounds. One such algorithm is the IDEA Module which isn’t included in GnuPG out of the box due to patent issues.
+- It is important to note that there are some nuances between the compatibility of GnuPG and PGP, such as the compatibility between certain algorithms, but in most applications such as email, there are workarounds. One such algorithm is the IDEA Module which isn’t included in GnuPG out of the box due to patent issues.
 
 S/MIME
 
-- SMTP can only handle 7-bit ASCII text (You can use UTF-8 extensions to alleviate this limitations, ) messages. While POP can handle other content types besides 7-bit ASCII, POP may, under a common default setting, download all the messages stored in the mail server to the user's local computer. After that, if POP removes these messages from the mail server. This makes it difficult for the user to read his messages from multiple computers.
+- SMTP can only handle 7-bit ASCII text (You can use UTF-8 extensions to alleviate these limitations, ) messages. While POP can handle other content types besides 7-bit ASCII, POP may, under a common default setting, download all the messages stored in the mail server to the user's local computer. After that, if POP removes these messages from the mail server. This makes it difficult for the user to read his messages from multiple computers.
 - The Multipurpose Internet Mail Extension protocol (MIME) was designed to support sending and receiving email messages in various formats, including nontext files generated by word processors, graphics files, sound files, and video clips. Moreover, MIME allows a single message to include mixed types of data in any combination of these formats.
 - The Internet Mail Access Protocol (IMAP), operated on TCP port 143(only for non-encrypted), stores (Configurable on both server & client just like PoP) incoming email messages in the mail server until the user deletes them deliberately. This allows the user to access his mailbox from multiple machines and download messages to a local machine without deleting it from the mailbox in the mail server.
 
@@ -74,8 +74,8 @@ SSL/TLS
 - SSL uses a PKI to decide if a server’s public key is trustworthy by requiring servers to use a security certificate signed by a trusted CA.
 - When Netscape Navigator 1.0 was released, it trusted a single CA operated by the RSA Data Security corporation.
 - The server’s public RSA keys were used to be stored in the security certificate, which can then be used by the browser to establish a secure communication channel. The security certificates we use today still rely on the same standard (named X.509) that Netscape Navigator 1.0 used back then.
-- Netscape’s intent was to train users(though this didn’t work out later) to differentiate secure communications from insecure ones, so they put a lock icon next to the address bar. When the lock is open, the communication is insecure. A closed lock means communication has been secured with SSL, which required the server to provide a signed certificate. You’re obviously familiar with this icon as it’s been in every browser ever since. The engineers at Netscape truly created a standard for secure internet communications.
-- A year after releasing SSL 2.0, Netscape fixed several security issues and released SSL 3.0, a protocol that, albeit being officially deprecated since June 2015, remains in use in certain parts of the world more than 20 years after its introduction. In an effort to standardize SSL, the Internet Engineering Task Force (IETF) created a slightly modified SSL 3.0 and, in 1999, unveiled it as Transport Layer Security (TLS) 1.0. The name change between SSL and TLS continues to confuse people today. Officially, TLS is the new SSL, but in practice, people use SSL and TLS interchangeably to talk about any version of the protocol.
+- Netscape intended to train users(though this didn’t work out later) to differentiate secure communications from insecure ones, so they put a lock icon next to the address bar. When the lock is open, the communication is insecure. A closed lock means communication has been secured with SSL, which required the server to provide a signed certificate. You’re obviously familiar with this icon as it’s been in every browser ever since. The engineers at Netscape truly created a standard for secure internet communications.
+- A year after releasing SSL 2.0, Netscape fixed several security issues and released SSL 3.0, a protocol that, albeit being officially deprecated since June 2015, remains in use in certain parts of the world more than 20 years after its introduction. To standardize SSL, the Internet Engineering Task Force (IETF) created a slightly modified SSL 3.0 and, in 1999, unveiled it as Transport Layer Security (TLS) 1.0. The name change between SSL and TLS continues to confuse people today. Officially, TLS is the new SSL, but in practice, people use SSL and TLS interchangeably to talk about any version of the protocol.
 
 - Must See:
   - <https://tls.ulfheim.net/>
@@ -91,13 +91,13 @@ Let us see how we keep a check on the perimeter i.e the edges, the first layer o
 - This is because IP packets, regardless of whether they are encrypted, can always be forwarded into an edge network.
 - Firewalls that were developed in the 1990s are important instruments to help restrict network access. A firewall may be a hardware device, a software package, or a combination of both.
 - Packets flowing into the internal network from the outside should be evaluated before they are allowed to enter. One of the critical elements of a firewall is its ability to examine packets without imposing a negative impact on communication speed while providing security protections for the internal network.
-- The packet inspection that is carried out by firewalls can be done using several different methods. On the basis of the particular method used by the firewall, it can be characterized as either a packet filter, circuit gateway, application gateway, or dynamic packet filter.
+- The packet inspection that is carried out by firewalls can be done using several different methods. Based on the particular method used by the firewall, it can be characterized as either a packet filter, circuit gateway, application gateway, or dynamic packet filter.
 
 ### Packet Filters
 
 - It inspects ingress packets coming to an internal network from outside and inspects egress packets going outside from an internal network
 - Packing filtering only inspects IP headers and TCP headers, not the payloads generated at the application layer
-- A packet filtering firewall uses a set of rules to determine whether a packet should be allowed or denied to pass through.
+- A packet-filtering firewall uses a set of rules to determine whether a packet should be allowed or denied to pass through.
 - 2 types:
   - Stateless
     - It treats each packet as an independent object, and it does not keep track of any previously processed packets. In other words, stateless filtering inspects a packet when it arrives and makes a decision without leaving any record of the packet being inspected.
@@ -114,21 +114,21 @@ Let us see how we keep a check on the perimeter i.e the edges, the first layer o
 ### Application Gateways(ALG)
 
 - Aka PROXY Servers
-- An Application Level Gateway (ALG) acts like a proxy for internal hosts, processing service requests from external clients.
+- An Application Level Gateway (ALG) acts as a proxy for internal hosts, processing service requests from external clients.
 - An ALG performs deep inspections on each IP packet (ingress or egress).
 - In particular, an ALG inspects application program formats contained in the packet (e.g., MIME format or SQL format) and examines whether its payload is permitted.
   - Thus, an ALG may be able to detect a computer virus contained in the payload. Because an ALG inspects packet payloads, it may be able to detect malicious code and quarantine suspicious packets, in addition to blocking packets with suspicious IP addresses and TCP ports. On the other hand, an ALG also incurs substantial computation and space overheads.
 
 ### Trusted Systems & Bastion Hosts
 
-- A Trusted Operating System (TOS) is an operating system that meets a particular set of security requirements. Whether an operating system can be trusted or not depends on a number of elements. For example, for an operating system on a particular computer to be certified trusted, one needs to validate that, among other things, the following four requirements are satisfied:
+- A Trusted Operating System (TOS) is an operating system that meets a particular set of security requirements. Whether an operating system can be trusted or not depends on several elements. For example, for an operating system on a particular computer to be certified trusted, one needs to validate that, among other things, the following four requirements are satisfied:
   - Its system design contains no defects;
   - Its system software contains no loopholes;
   - Its system is configured properly; and
   - Its system management is appropriate.
 
 - Bastion Hosts
-  - Bastion hosts are computers with strong defense mechanisms. They often serve as host computers for implementing application gateways, circuit gateways, and other types of firewalls. A bastion host is operated on a trusted operating system that must not contain unnecessary functionalities or programs. This measure helps to reduce error probabilities and makes it easier to conduct security checks. Only those network application programs that are absolutely necessary, for example, SSH, DNS, SMTP, and authentication programs, are installed on a bastion host.
+  - Bastion hosts are computers with strong defence mechanisms. They often serve as host computers for implementing application gateways, circuit gateways, and other types of firewalls. A bastion host is operated on a trusted operating system that must not contain unnecessary functionalities or programs. This measure helps to reduce error probabilities and makes it easier to conduct security checks. Only those network application programs that are necessary, for example, SSH, DNS, SMTP, and authentication programs, are installed on a bastion host.
   - Bastion hosts are also primarily used as controlled ingress points so that the security monitoring can focus more narrowly on actions happening at a single point closely.
 
 ---
@@ -137,8 +137,8 @@ Let us see how we keep a check on the perimeter i.e the edges, the first layer o
 
 ### Scanning Ports with Nmap
 
-- Nmap ("Network Mapper") is a free and open source (license) utility for network discovery and security auditing.  Many systems and network administrators also find it useful for tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime.
-- The best thing about Nmap is it’s free and open source and is very flexible and versatile
+- Nmap ("Network Mapper") is a free and open-source (license) utility for network discovery and security auditing.  Many systems and network administrators also find it useful for tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime.
+- The best thing about Nmap is it’s free and open-source and is very flexible and versatile
 - Nmap is often used to determine alive hosts in a network, open ports on those hosts, services running on those open ports, and version identification of that service on that port.
 - More at http://scanme.nmap.org/
 
@@ -152,7 +152,7 @@ Nmap uses 6 different port states:
 - **Open** — An open port is one that is actively accepting TCP, UDP or SCTP connections. Open ports are what interests us the most because they are the ones that are vulnerable to attacks. Open ports also show the available services on a network.
 - **Closed** — A port that receives and responds to Nmap probe packets but there is no application listening on that port. Useful for identifying that the host exists and for OS detection.
 - **Filtered** — Nmap can’t determine whether the port is open because packet filtering prevents its probes from reaching the port. Filtering could come from firewalls or router rules. Often little information is given from filtered ports during scans as the filters can drop the probes without responding or respond with useless error messages e.g. destination unreachable.
-- **Unfiltered** — Port is accessible but Nmap doesn’t know if its open or closed. Only used in ACK scan which is used to map firewall rulesets. Other scan types can be used to identify whether the port is open.
+- **Unfiltered** — Port is accessible but Nmap doesn’t know if it is open or closed. Only used in ACK scan which is used to map firewall rulesets. Other scan types can be used to identify whether the port is open.
 - **Open/filtered** — Nmap is unable to determine between open and filtered. This happens when an open port gives no response. No response could mean that the probe was dropped by a packet filter or any response is blocked.
 - **Closed/filtered** — Nmap is unable to determine whether a port is closed or filtered. Only used in the IP ID idle scan.
 
@@ -162,8 +162,8 @@ Nmap uses 6 different port states:
    - TCP Connect scan completes the 3-way handshake.
    - If a port is open, the operating system completes the TCP three-way handshake and the port scanner immediately closes the connection to avoid DOS. This is “noisy” because the services can log the sender IP address and might trigger Intrusion Detection Systems.
 2. UDP Scan
-   - This scan checks to see if there are any UDP ports listening.
-   - Since UDP does not respond with a positive acknowledgment like TCP and only responds to an incoming UDP packet when the port is closed,
+   - This scan checks to see if any UDP ports are listening.
+   - Since UDP does not respond with a positive acknowledgement like TCP and only responds to an incoming UDP packet when the port is closed,
 
 3. SYN Scan
    - SYN scan is another form of TCP scanning.
@@ -186,7 +186,7 @@ Nmap uses 6 different port states:
    - This special type of scan looks for machine answering to RPC (Remote Procedure Call) services
 9. IDLE Scan
    - It is a super stealthy method whereby the scan packets are bounced off an external host.
-   - You don’t need to have control over the other host but it does have to set up and meet certain requirements. You must input the IP address of our “zombie” host and what port number to use. It is one of the more controversial options in Nmap since it really only has a use for malicious attacks.
+   - You don’t need to have control over the other host but it does have to set up and meet certain requirements. You must input the IP address of our “zombie” host and what port number to use. It is one of the more controversial options in Nmap since it only has a use for malicious attacks.
 
 Scan Techniques
 
@@ -200,7 +200,7 @@ A couple of scan techniques which can be used to gain more information about a s
 - OpenVAS is made up of three main parts. These are:
   - a regularly updated feed of Network Vulnerability Tests (NVTs);
   - a scanner, which runs the NVTs; and
-  - a SQLite 3 database for storing both your test configurations and the NVTs’ results and configurations.
+  - an SQLite 3 database for storing both your test configurations and the NVTs’ results and configurations.
   - <https://www.greenbone.net/en/install_use_gce/>
 
 ### WireShark
@@ -209,7 +209,7 @@ A couple of scan techniques which can be used to gain more information about a s
 - This means Wireshark is designed to decode not only packet bits and bytes but also the relations between packets and protocols.
 - Wireshark understands protocol sequences.
 
-A simple demo of wireshark
+A simple demo of Wireshark
 
 1. Capture only udp packets:
    - Capture filter = “udp”
@@ -286,8 +286,8 @@ A simple demo of wireshark
 
 - Dumpcap is a network traffic dump tool. It captures packet data from a live network and writes the packets to a file. Dumpcap’s native capture file format is pcapng, which is also the format used by Wireshark.
 - By default, Dumpcap uses the pcap library to capture traffic from the first available network interface and writes the received raw packet data, along with the packets’ time stamps into a pcapng file. The capture filter syntax follows the rules of the pcap library.
-- The Wireshark command line utility called 'dumpcap.exe' can be used to capture LAN traffic over an extended period of time.
-- Wireshark itself can also be used, but dumpcap does not significantly utilize the computer's memory while capturing for long periods of time.
+- The Wireshark command-line utility called 'dumpcap.exe' can be used to capture LAN traffic over an extended period of time.
+- Wireshark itself can also be used, but dumpcap does not significantly utilize the computer's memory while capturing for long periods.
 
 ### DaemonLogger
 
@@ -301,7 +301,7 @@ A simple demo of wireshark
 
 - Netsniff-NG is a high-performance packet capture utility
 - While the utilities we’ve discussed to this point rely on Libpcap for capture, Netsniff-NG utilizes zero-copy mechanisms to capture packets. This is done with the intent to support full packet capture over high throughput links.
-- In order to begin capturing packets with Netsniff-NG, we have to specify an input and output. In most cases, the input will be a network interface, and the output will be a file or folder on disk.
+- To begin capturing packets with Netsniff-NG, we have to specify an input and output. In most cases, the input will be a network interface, and the output will be a file or folder on disk.
 
    `netsniff-ng –i eth1 –o data.pcap`
 
@@ -317,7 +317,7 @@ A simple demo of wireshark
 ### IDS
 
 A security solution that detects security-related events in your environment but does not block them.
-IDS sensors can be software and hardware based used to collect and analyze the network traffic. These sensors are available in two varieties, network IDS and host IDS.
+IDS sensors can be software and hardware-based used to collect and analyze the network traffic. These sensors are available in two varieties, network IDS and host IDS.
 
 - A host IDS is a server-specific agent running on a server with a minimum of overhead to monitor the operating system.
 - A network IDS can be embedded in a networking device, a standalone appliance, or a module monitoring the network traffic.
@@ -332,7 +332,7 @@ Signature Based IDS
 
 - ex: SNORT & SURICATA
 
-Policy Based IDS
+Policy-Based IDS
 
 - The policy-based IDSs (mainly host IDSs) trigger an alarm whenever a violation occurs against the configured policy.
 - This configured policy is or should be a representation of the security policies.
@@ -347,7 +347,7 @@ Anomaly Based IDS
   - Statistical anomaly detection learns the traffic patterns interactively over a period of time.
   - In the nonstatistical approach, the IDS has a predefined configuration of the supposedly acceptable and valid traffic patterns.
 
-Host Based IDS & Network Based IDS
+Host-Based IDS & Network-Based IDS
 
 - A host IDS can be described as a distributed agent residing on each server of the network that needs protection. These distributed agents are tied very closely to the underlying operating system.
 
@@ -355,7 +355,7 @@ Host Based IDS & Network Based IDS
 
 Honeypots 
 
-- The use of decoy machines to direct intruders' attention away from the machines under protection is a major technique to preclude intrusion attacks. Any device, system, directory, or file used as a decoy to lure attackers away from important assets and to collect intrusion or abusive behaviors is referred to as a honeypot.
+- The use of decoy machines to direct intruders' attention away from the machines under protection is a major technique to preclude intrusion attacks. Any device, system, directory, or file used as a decoy to lure attackers away from important assets and to collect intrusion or abusive behaviours is referred to as a honeypot.
 - A honeypot may be implemented as a physical device or as an emulation system. The idea is to set up decoy machines in a LAN, or decoy directories/files in a file system and make them appear important, but with several exploitable loopholes, to lure attackers to attack these machines or directories/files, so that other machines, directories, and files can evade intruders' attentions. A decoy machine may be a host computer or a server computer. Likewise, we may also set up decoy routers or even decoy LANs.
 
 ---
@@ -373,30 +373,30 @@ Honeypots
 IP Spoofing Detection Techniques
 
 - Direct TTL Probes
-  - In this technique we send a packet to a host of suspect spoofed IP that triggers reply and compare TTL with suspect packet; if the TTL in the reply is not the same as the packet being checked; it is a spoofed packet.
+  - In this technique we send a packet to a host of suspect spoofed IP that triggers reply and compares TTL with suspect packet; if the TTL in the reply is not the same as the packet being checked; it is a spoofed packet.
 
   - This Technique is successful when the attacker is in a different subnet from the victim.
   ![image19](images/image19.png)
 
 - IP Identification Number.
-  - Send a probe to the host of suspect spoofed traffic that triggers a reply and compare IP ID with suspect traffic.
+  - Send a probe to the host of suspect spoofed traffic that triggers a reply and compares IP ID with suspect traffic.
   - If IP IDs are not in the near value of packet being checked, suspect traffic is spoofed
 
 - TCP Flow Control Method
   - Attackers sending spoofed TCP packets will not receive the target’s SYN-ACK packets.
-  - Attackers cannot therefore be responsive to change in the congestion window size
+  - Attackers cannot, therefore, be responsive to change in the congestion window size
   - When the receiver still receives traffic even after a windows size is exhausted, most probably the packets are spoofed.
 
 ### Covert Channel
 
 - A covert or clandestine channel can be best described as a pipe or communication channel between two entities that can be exploited by a process or application transferring information in a manner that violates the system's security specifications.
 - More specifically for TCP/IP, in some instances, covert channels are established, and data can be secretly passed between two end systems.
-  - Ex: ICMP resides at the Internet layer of the TCP/IP protocol suite and is implemented in all TCP/IP hosts. Based on the specifications of the ICMP Protocol, an ICMP Echo Request message should have an 8-byte header and a 56-byte payload. The ICMP Echo Request packet should not carry any data in the payload. However, these packets are often used to carry secret information. The ICMP packets are altered slightly to carry secret data in the payload. This makes the size of the packet larger, but no control exists in the protocol stack to defeat this behavior. The alteration of ICMP packets gives intruders the opportunity to program specialized client-server pairs. These small pieces of code export confidential information without alerting the network administrator.
+  - Ex: ICMP resides at the Internet layer of the TCP/IP protocol suite and is implemented in all TCP/IP hosts. Based on the specifications of the ICMP Protocol, an ICMP Echo Request message should have an 8-byte header and a 56-byte payload. The ICMP Echo Request packet should not carry any data in the payload. However, these packets are often used to carry secret information. The ICMP packets are altered slightly to carry secret data in the payload. This makes the size of the packet larger, but no control exists in the protocol stack to defeat this behaviour. The alteration of ICMP packets allows intruders to program specialized client-server pairs. These small pieces of code export confidential information without alerting the network administrator.
   - ICMP can be leveraged for more than data exfiltration. For eg. some C&C tools such as Loki used ICMP channel to establish encrypted interactive session back in 1996.
   
-  - Deep packet inspection has since come a long way. A lot of IDS/IPS detect ICMP tunneling.
+  - Deep packet inspection has since come a long way. A lot of IDS/IPS detect ICMP tunnelling.
     - Check for echo responses that do not contain the same payload as  request
-    - Check for volume of ICMP traffic specially for volumes beyond an acceptable threshold
+    - Check for the volume of ICMP traffic especially for volumes beyond an acceptable threshold
 
 ### IP Fragmentation Attack
 
@@ -410,9 +410,9 @@ IP Spoofing Detection Techniques
 
 TCP Flags
 
-- Data exchange using TCP does not happen until a three-way handshake has been successfully completed. This handshake uses different flags to influence the way TCP segments are processed.
+- Data exchange using TCP does not happen until a three-way handshake has been completed. This handshake uses different flags to influence the way TCP segments are processed.
 - There are 6 bits in the TCP header that are often called flags. Namely:
-  - 6 different flags are part of the TCP header: Urgent pointer field (URG), Acknowledgment field (ACK), Push function (PSH), Reset the connection (RST), Synchronize sequence numbers (SYN), and sender is finished with this connection (FIN).
+  - 6 different flags are part of the TCP header: Urgent pointer field (URG), Acknowledgment field (ACK), Push function (PSH), Reset the connection (RST), Synchronize sequence numbers (SYN), and the sender is finished with this connection (FIN).
   ![image20](images/image20.png)
 
   - Abuse of the normal operation or settings of these flags can be used by attackers to launch DoS attacks. This causes network servers or web servers to crash or hang.
@@ -426,14 +426,14 @@ TCP Flags
 | 1    |1     |1      |1     |Illegal Combination
 ```
 
-- The attacker's ultimate goal is to write special programs or pieces of code that are able to construct these illegal combinations resulting in an efficient DoS attack.
+- The attacker's ultimate goal is to write special programs or pieces of code that can construct these illegal combinations resulting in an efficient DoS attack.
 
 SYN FLOOD
 
 - The timers (or lack of certain timers) in 3 way handshake are often used and exploited by attackers to disable services or even to enter systems.
-- After step 2 of the three-way handshake, no limit is set on the time to wait after receiving a SYN. The attacker initiates many connection requests to the web server of Company XYZ (almost certainly with a spoofed IP address).
-- The SYN+ACK packets (Step 2) sent by the web server back to the originating source IP address are not replied to. This leaves a TCP session half-open on the web server. Multiple packets cause multiple TCP sessions to stay open.
-- Based on the hardware limitations of the server, a limited number of TCP sessions can stay open, and as a result, the web server refuses further connection establishments attempts from any host as soon as a certain limit is reached. These half-open connections need to be completed or timed out before new connections can be established.
+- After step 2 of the three-way handshake, no limit is set on the time to wait after receiving a SYN. The attacker initiates many connection requests to the webserver of Company XYZ (almost certainly with a spoofed IP address).
+- The SYN+ACK packets (Step 2) sent by the web server back to the originating source IP address are not replied to. This leaves a TCP session half-open on the webserver. Multiple packets cause multiple TCP sessions to stay open.
+- Based on the hardware limitations of the server, a limited number of TCP sessions can stay open, and as a result, the webserver refuses further connection establishments attempts from any host as soon as a certain limit is reached. These half-open connections need to be completed or timed out before new connections can be established.
 
 FIN Attack
 
@@ -446,9 +446,9 @@ FIN Attack
 
   ![image22](images/image22.png)
 
-- An authorized user (Employee X) sends HTTP requests over a TCP session with the web server.
-- The web server accepts the packets from Employee X only when the packet has the correct SEQ/ACK numbers. As seen previously, these numbers are important for the web server to distinguish between different sessions and to make sure it is still talking to Employee X. Imagine that the cracker starts sending packets to the web server spoofing the IP address of Employee X, using the correct SEQ/ACK combination. The web server accepts the packet and increments the ACK number.
-- In the meantime, Employee X continues to send packets but with incorrect SEQ/ACK numbers. As a result of sending unsynchronized packets, all data from Employee X is discarded when received by the web server. The attacker pretends to be Employee X using the correct numbers. This finally results in the cracker hijacking the connection, whereby Employee X is completely confused and the web server replies assuming the cracker is sending correct synchronized data.
+- An authorized user (Employee X) sends HTTP requests over a TCP session with the webserver.
+- The web server accepts the packets from Employee X only when the packet has the correct SEQ/ACK numbers. As seen previously, these numbers are important for the webserver to distinguish between different sessions and to make sure it is still talking to Employee X. Imagine that the cracker starts sending packets to the web server spoofing the IP address of Employee X, using the correct SEQ/ACK combination. The web server accepts the packet and increments the ACK number.
+- In the meantime, Employee X continues to send packets but with incorrect SEQ/ACK numbers. As a result of sending unsynchronized packets, all data from Employee X is discarded when received by the webserver. The attacker pretends to be Employee X using the correct numbers. This finally results in the cracker hijacking the connection, whereby Employee X is completely confused and the webserver replies assuming the cracker is sending correct synchronized data.
 
 STEPS:
 
@@ -457,7 +457,7 @@ STEPS:
 3. Employee X acknowledges the packet.
 4. The cracker launches a spoofed packet to the server.
 5. The web server responds to the cracker. The cracker starts verifying SEQ/ACK numbers to double-check success. At this time, the cracker takes over the session from Employee X, which results in a session hanging for Employee X.
-6. The cracker can start sending traffic to the web server.
+6. The cracker can start sending traffic to the webserver.
 7. The web server returns the requested data to confirm delivery with the correct ACK number.
 8. The cracker can continue to send data (keeping track of the correct SEQ/ACK numbers) until eventually setting the FIN flag to terminate the session.
 
@@ -465,7 +465,7 @@ STEPS:
 
 - A buffer is a temporary data storage area used to store program code and data.
 - When a program or process tries to store more data in a buffer than it was originally anticipated to hold, a buffer overflow occurs.
-- Buffers are temporary storage locations in memory (memory or buffer sizes are often measured in bytes) that are able to store a fixed amount of data in bytes. When more data is retrieved than can be stored in a buffer location, the additional information must go into an adjacent buffer, resulting in overwriting the valid data held in them.
+- Buffers are temporary storage locations in memory (memory or buffer sizes are often measured in bytes) that can store a fixed amount of data in bytes. When more data is retrieved than can be stored in a buffer location, the additional information must go into an adjacent buffer, resulting in overwriting the valid data held in them.
 
 
 Mechanism:
@@ -477,7 +477,7 @@ Mechanism:
 CounterMeasure:
 
 - The most important approach is to have a concerted focus on writing correct code.
-- A second method is to make the data buffers (memory locations) address space of the program code non executable. This type of address space makes it impossible to execute code, which might be infiltrated in the program's buffers during an attack.
+- A second method is to make the data buffers (memory locations) address space of the program code non-executable. This type of address space makes it impossible to execute code, which might be infiltrated in the program's buffers during an attack.
 
 ### More Spoofing
 

courses/security/threats_attacks_defences.md

@@ -5,22 +5,22 @@
 ### Cache Poisoning Attack
 
 - Since DNS responses are cached, a quick response can be provided for repeated translations.
-DNS negative queries are also cached, e.g., misspelled words, and all cached data periodically times out.
+DNS negative queries are also cached, e.g., misspelt words, and all cached data periodically times out.
 Cache poisoning is an issue in what is known as pharming. This term is used to describe a hacker’s attack in which a website’s traffic is redirected to a bogus website by forging the DNS mapping. In this case, an attacker attempts to insert a fake address record for an Internet domain into the DNS.
 If the server accepts the fake record, the cache is poisoned and subsequent requests for the address of the domain are answered with the address of a server controlled by the attacker. As long as the fake entry is cached by the server, browsers or e-mail servers will automatically go to the address provided by the compromised DNS server.
 the typical time to live (TTL) for cached entries is a couple of hours, thereby permitting ample time for numerous users to be affected by the attack.
 
 ### DNSSEC (Security Extension)
 
-- The long-term solution to these DNS problems is authentication. If a resolver cannot distinguish between valid and invalid data in a response, then add source authentication to verify that the data received in a response is equal to the data entered by the zone administrator
-- DNS Security Extensions (DNSSEC) protects against data spoofing and corruption, and provides mechanisms to authenticate servers and requests, as well as mechanisms to establish authenticity and integrity.
+- The long-term solution to these DNS problems is authentication. If a resolver cannot distinguish between valid and invalid data in a response, then add source authentication to verify that the data received in response is equal to the data entered by the zone administrator
+- DNS Security Extensions (DNSSEC) protects against data spoofing and corruption and provides mechanisms to authenticate servers and requests, as well as mechanisms to establish authenticity and integrity.
 - When authenticating DNS responses, each DNS zone signs its data using a private key. It is recommended that this signing be done offline and in advance. The query for a particular record returns the requested resource record set (RRset) and signature (RRSIG) of the requested resource record set. The resolver then authenticates the response using a public key, which is pre-configured or learned via a sequence of key records in the DNS hierarchy.
 - The goals of DNSSEC are to provide authentication and integrity for DNS responses without confidentiality or DDoS protection.
 
 ### BGP
 
 - BGP stands for border gateway protocol. It is a routing protocol that exchanges routing information among multiple Autonomous Systems (AS)
-  - An Autonomous system is a collection of routers or networks with the same network policy usually under a single administrative control.
+  - An Autonomous System is a collection of routers or networks with the same network policy usually under single administrative control.
 - BGP tells routers which hop to use in order to reach the destination network.
 - BGP is used for both communicating information among routers in an AS (interior) and between multiple ASes (exterior).
 
@@ -29,11 +29,11 @@ the typical time to live (TTL) for cached entries is a couple of hours, thereby
 ## How BGP Works
 
 - BGP is responsible for finding a path to a destination router & the path it chooses should be the shortest and most reliable one.
-- This decision is done through a protocol known as Link state. With the link state protocol each router broadcasts to all other routers in the network the state of its links and IP subnets. Each router then receives information from the other routers and constructs a complete topology view of the entire network. The next hop routing table is based on this topology view.
-- The link state protocol uses a famous algorithm in the field of computer science, Dijkstra’s shortest path algorithm:
-  - We start from our router considering the path cost to all our direct neighbors.
+- This decision is done through a protocol known as Link state. With the link-state protocol, each router broadcasts to all other routers in the network the state of its links and IP subnets. Each router then receives information from the other routers and constructs a complete topology view of the entire network. The next-hop routing table is based on this topology view.
+- The link-state protocol uses a famous algorithm in the field of computer science, Dijkstra’s shortest path algorithm:
+  - We start from our router considering the path cost to all our direct neighbours.
   - The shortest path is then taken
-  - We then re-look at all our neighbors that we can reach and update our link state table with the cost information. We then continue taking the shortest path until every router has been visited.
+  - We then re-look at all our neighbours that we can reach and update our link state table with the cost information. We then continue taking the shortest path until every router has been visited.
 
 ## BGP Vulnerabilities
 
@@ -41,34 +41,34 @@ the typical time to live (TTL) for cached entries is a couple of hours, thereby
 - Injecting bogus route advertising information into the BGP-distributed routing database by malicious sources, accidentally or routers can disrupt Internet backbone operations. 
 - Blackholing traffic:
   - Blackhole route is a network route, i.e., routing table entry, that goes nowhere and packets matching the route prefix are dropped or ignored. Blackhole routes can only be detected by monitoring the lost traffic.
-  - Blackhole routes are best defence against many common viral attacks where the traffic is dropped from infected machines to/from command & control masters.
+  - Blackhole routes are the best defence against many common viral attacks where the traffic is dropped from infected machines to/from command & control masters.
   - Infamous BGP Injection attack on Youtube
 
-- EX: In 2008, Pakistan decided to block YouTube by creating a BGP route that led into a black hole. Instead this routing information got transmitted to a hong kong ISP and from there accidentally got propagated to the rest of the world meaning millions were routed through to this black hole and therefore unable to access YouTube.
+- EX: In 2008, Pakistan decided to block YouTube by creating a BGP route that led into a black hole. Instead, this routing information got transmitted to a hong kong ISP and from there accidentally got propagated to the rest of the world meaning millions were routed through to this black hole and therefore unable to access YouTube.
 - Potentially, the greatest risk to BGP occurs in a denial of service attack in which a router is flooded with more packets than it can handle. Network overload and router resource exhaustion happen when the network begins carrying an excessive number of BGP messages, overloading the router control processors, memory, routing table and reducing the bandwidth available for data traffic.
-- Refer : <https://medium.com/bugbountywriteup/bgp-the-weak-link-in-the-internet-what-is-bgp-and-how-do-hackers-exploit-it-d899a68ba5bb>
-- Router flapping is another type of attack. Route flapping refers to repetitive changes to the BGP routing table, often several times a minute. Withdrawing and re-advertising at a high-rate can cause a serious problem for routers, since they propagate the announcements of routes. If these route flaps happen fast enough, e.g., 30 to 50 times per second, the router becomes overloaded, which eventually prevents convergence on valid routes. The potential impact for Internet users is a slowdown in message delivery, and in some cases packets may not be delivered at all.
+- Refer: <https://medium.com/bugbountywriteup/bgp-the-weak-link-in-the-internet-what-is-bgp-and-how-do-hackers-exploit-it-d899a68ba5bb>
+- Router flapping is another type of attack. Route flapping refers to repetitive changes to the BGP routing table, often several times a minute. Withdrawing and re-advertising at a high-rate can cause a serious problem for routers since they propagate the announcements of routes. If these route flaps happen fast enough, e.g., 30 to 50 times per second, the router becomes overloaded, which eventually prevents convergence on valid routes. The potential impact for Internet users is a slowdown in message delivery, and in some cases, packets may not be delivered at all.
 
 BGP Security
 
-- Border Gateway Protocol Security recommends the use of BGP peer authentication, since it is one of the strongest mechanisms for preventing malicious activity.
+- Border Gateway Protocol Security recommends the use of BGP peer authentication since it is one of the strongest mechanisms for preventing malicious activity.
   - The authentication mechanisms are Internet Protocol Security (IPsec) or BGP MD5.
-- Another method, known as prefix limits, can be used to avoid filling router tables. In this approach, routers should be configured to disable or terminate a BGP peering session, and issue warning messages to administrators, when a neighbor sends in excess of a preset number of prefixes.
+- Another method, known as prefix limits, can be used to avoid filling router tables. In this approach, routers should be configured to disable or terminate a BGP peering session, and issue warning messages to administrators when a neighbour sends in excess of a preset number of prefixes.
 - IETF is currently working on improving this space
 
-## Web Based Attacks
+## Web-Based Attacks
 
 ### HTTP Response Splitting Attacks
 
 - HTTP response splitting attack may happen where the server script embeds user data in HTTP response headers without appropriate sanitation.
 - This typically happens when the script embeds user data in the redirection URL of a redirection response (HTTP status code 3xx), or when the script embeds user data in a cookie value or name when the response sets a cookie.
 - HTTP response splitting attacks can be used to perform web cache poisoning and cross-site scripting attacks.
-- HTTP response splitting is the attacker’s ability to send a single HTTP request that forces the web server to form an output stream, which is then interpreted by the target as two HTTP responses instead of one response.
+- HTTP response splitting is the attacker’s ability to send a single HTTP request that forces the webserver to form an output stream, which is then interpreted by the target as two HTTP responses instead of one response.
 
 ### Cross-Site Request Forgery (CSRF or XSRF)
 
 - A Cross-Site Request Forgery attack tricks the victim’s browser into issuing a command to a vulnerable web application.
-- Vulnerability is caused by browsers automatically including user authentication data, session ID, IP address, Windows domain credentials, etc  with each request.
+- Vulnerability is caused by browsers automatically including user authentication data, session ID, IP address, Windows domain credentials, etc. with each request.
 - Attackers typically use CSRF to initiate transactions such as transfer funds, login/logout user, close account, access sensitive data, and change account details.
 - The vulnerability is caused by web browsers that automatically include credentials with each request, even for requests caused by a form, script, or image on another site. CSRF can also be dynamically constructed as part of a payload for a cross-site scripting attack
 - All sites relying on automatic credentials are vulnerable. Popular browsers cannot prevent cross-site request forgery. Logging out of high-value sites as soon as possible can mitigate CSRF risk. It is recommended that a high-value website must require a client to manually provide authentication data in the same HTTP request used to perform any operation with security implications. Limiting the lifetime of session cookies can also reduce the chance of being used by other malicious sites.
@@ -77,20 +77,20 @@ BGP Security
 ### Cross-Site Scripting (XSS) Attacks
 
 - Cross-Site Scripting occurs when dynamically generated web pages display user input, such as login information, that is not properly validated, allowing an attacker to embed malicious scripts into the generated page and then execute the script on the machine of any user that views the site.
-- If successful, Cross-Site Scripting vulnerabilities can be exploited to manipulate or steal cookies, create requests that can be mistaken for those of a valid user, compromise confidential information, or execute malicious code on end user systems.
+- If successful, Cross-Site Scripting vulnerabilities can be exploited to manipulate or steal cookies, create requests that can be mistaken for those of a valid user, compromise confidential information, or execute malicious code on end-user systems.
 - Cross-Site Scripting (XSS or CSS) attacks involve the execution of malicious scripts on the victim’s browser. The victim is simply a user’s host and not the server. XSS results from a failure to validate user input by a web-based application.
 
 ### Document Object Model (DOM) XSS Attacks
 
-- The Document Object Model (DOM) based XSS does not require the web server to receive the XSS payload for a successful attack. The attacker abuses the runtime by embedding their data on the client side. An attacker can force the client (browser) to render the page with parts of the DOM controlled by the attacker. 
-- When the page is rendered and the data is processed by the page, typically by a client side HTML-embedded script such as JavaScript, the page’s code may insecurely embed the data in the page itself, thus delivering the cross-site scripting payload. There are several DOM objects which can serve as an attack vehicle for delivering malicious script to victims browser.
+- The Document Object Model (DOM) based XSS does not require the webserver to receive the XSS payload for a successful attack. The attacker abuses the runtime by embedding their data on the client-side. An attacker can force the client (browser) to render the page with parts of the DOM controlled by the attacker. 
+- When the page is rendered and the data is processed by the page, typically by a client-side HTML-embedded script such as JavaScript, the page’s code may insecurely embed the data in the page itself, thus delivering the cross-site scripting payload. There are several DOM objects which can serve as an attack vehicle for delivering malicious script to victims browser.
 
 ### Clickjacking
 
 - The technique works by hiding malicious link/scripts under the cover of the content of a legitimate site.
-- Buttons on a website actually contain invisible links, placed there by the attacker. So, an individual who clicks on an object they can visually see, is actually being duped into visiting a malicious page or executing a malicious script.
+- Buttons on a website actually contain invisible links, placed there by the attacker. So, an individual who clicks on an object they can visually see is actually being duped into visiting a malicious page or executing a malicious script.
 - When mouseover is used together with clickjacking, the outcome is devastating. Facebook users have been hit by a clickjacking attack, which tricks people into “liking” a particular Facebook page, thus enabling the attack to spread since Memorial Day 2010.
-- There is not yet effective defense against clickjacking, and disabling JavaScript is the only viable method
+- There is not yet effective defence against clickjacking, and disabling JavaScript is the only viable method
 
 ## DataBase Attacks & Defenses
 
@@ -107,7 +107,7 @@ Here the username & password is the input provided by the user. Suppose an attac
 
 SELECT USERNAME,PASSWORD from USERS where USERNAME='' OR '1'='1' AND PASSOWRD='' OR '1'='1';
 
-This query results in a true statement & user gets logged in. This example depicst the bost basic type of SQL injection
+This query results in a true statement & the user gets logged in. This example depicts the bost basic type of SQL injection
 ```
 
 
@@ -130,13 +130,13 @@ In spite of the most aggressive steps to protect computers from attacks, attacke
 
 ### Denial of Service Attacks
 
-- Denial of service (DoS) attacks result in downtime or inability of a user to access a system. DoS attacks impact the availability tenet of information systems security. A DoS attack is a coordinated attempt to deny service by occupying a computer to perform large amounts of unnecessary tasks. This excessive activity makes the system unavailable to perform legitimate operations
+- Denial of service (DoS) attacks result in downtime or inability of a user to access a system. DoS attacks impact the availability of tenet of information systems security. A DoS attack is a coordinated attempt to deny service by occupying a computer to perform large amounts of unnecessary tasks. This excessive activity makes the system unavailable to perform legitimate operations
 - Two common types of DoS attacks are as follows:
   - Logic attacks—Logic attacks use software flaws to crash or seriously hinder the performance of remote servers. You can prevent many of these attacks by installing the latest patches to keep your software up to date.
   - Flooding attacks—Flooding attacks overwhelm the victim computer’s CPU, memory, or network resources by sending large numbers of useless requests to the machine.
 - Most DoS attacks target weaknesses in the overall system architecture rather than a software bug or security flaw
 - One popular technique for launching a packet flood is a SYN flood.
-- One of the best defenses against DoS attacks is to use intrusion prevention system (IPS) software or devices to detect and stop the attack.
+- One of the best defences against DoS attacks is to use intrusion prevention system (IPS) software or devices to detect and stop the attack.
 
 ### Distributed Denial of Service Attacks
 
@@ -160,14 +160,14 @@ In spite of the most aggressive steps to protect computers from attacks, attacke
 
 ### Birthday Attack
 
-- Once an attacker compromises a hashed password file, a birthday attack is performed. A birthday attack is a type of cryptographic attack that is used to make brute-force attack of one-way hashes easier. It is a mathematical exploit that is based on the birthday problem in probability theory.
+- Once an attacker compromises a hashed password file, a birthday attack is performed. A birthday attack is a type of cryptographic attack that is used to make a brute-force attack of one-way hashes easier. It is a mathematical exploit that is based on the birthday problem in probability theory.
 - Further Reading:
   - <https://www.sciencedirect.com/topics/computer-science/birthday-attack>
   - <https://www.internetsecurity.tips/birthday-attack/>
 
 ### Brute-Force Password Attacks
 
-- In a brute-force password attack, the attacker tries different passwords on a system until one of them is successful. Usually the attacker employs a software program to try all possible combinations of a likely password, user ID, or security code until it locates a match. This occurs rapidly and in sequence. This type of attack is called a brute-force password attack because the attacker simply hammers away at the code. There is no skill or stealth involved—just brute force that eventually breaks the code.
+- In a brute-force password attack, the attacker tries different passwords on a system until one of them is successful. Usually, the attacker employs a software program to try all possible combinations of a likely password, user ID, or security code until it locates a match. This occurs rapidly and in sequence. This type of attack is called a brute-force password attack because the attacker simply hammers away at the code. There is no skill or stealth involved—just brute force that eventually breaks the code.
 - Further Reading:
   - <https://owasp.org/www-community/attacks/Brute_force_attack>
   - <https://owasp.org/www-community/controls/Blocking_Brute_Force_Attacks>
@@ -187,7 +187,7 @@ https://capec.mitre.org/data/definitions/16.html
 ### Man-in-the-Middle Attacks
 
 - A man-in-the-middle attack takes advantage of the multihop process used by many types of networks. In this type of attack, an attacker intercepts messages between two parties before transferring them on to their intended destination.
-- Web spoofing is a type of man-in-the-middle attack in which the user believes a secure session exists with a particular web server. In reality, the secure connection exists only with the attacker, not the web server. The attacker then establishes a secure connection with the web server, acting as an invisible go-between. The attacker passes traffic between the user and the web server. In this way, the attacker can trick the user into supplying passwords, credit card information, and other private data.
+- Web spoofing is a type of man-in-the-middle attack in which the user believes a secure session exists with a particular web server. In reality, the secure connection exists only with the attacker, not the webserver. The attacker then establishes a secure connection with the webserver, acting as an invisible go-between. The attacker passes traffic between the user and the webserver. In this way, the attacker can trick the user into supplying passwords, credit card information, and other private data.
 - Further Reading:
   - <https://owasp.org/www-community/attacks/Man-in-the-middle_attack>
 
@@ -199,7 +199,7 @@ https://capec.mitre.org/data/definitions/16.html
 
 ### Eavesdropping
 
-- Eavesdropping, or sniffing, occurs when a host sets its network interface on promiscuous mode and copies packets that pass by for later analysis. Promiscuous mode enables a network device to intercept and read each network packet(ofcourse given some conditions) given sec, even if the packet’s address doesn’t match the network device. It is possible to attach hardware and software to monitor and analyze all packets on that segment of the transmission media without alerting any other users. Candidates for eavesdropping include satellite, wireless, mobile, and other transmission methods.
+- Eavesdropping, or sniffing, occurs when a host sets its network interface on promiscuous mode and copies packets that pass by for later analysis. Promiscuous mode enables a network device to intercept and read each network packet(of course given some conditions) given sec, even if the packet’s address doesn’t match the network device. It is possible to attach hardware and software to monitor and analyze all packets on that segment of the transmission media without alerting any other users. Candidates for eavesdropping include satellite, wireless, mobile, and other transmission methods.
 
 ### Social Engineering
 

courses/security/writing_secure_code.md

@@ -17,7 +17,7 @@ The first and most important step in reducing security and reliability issues is
 
  Try to keep your code clean and simple.
 
-### Avoid Multi Level Nesting
+### Avoid Multi-Level Nesting
 
 - Multilevel nesting is a common anti-pattern that can lead to simple mistakes. If the error is in the most common code path, it will likely be captured by the unit tests. However, unit tests don’t always check error handling paths in multilevel nested code. The error might result in decreased reliability (for example, if the service crashes when it mishandles an error) or a security vulnerability (like a mishandled authorization check error).
 
@@ -42,7 +42,7 @@ The first and most important step in reducing security and reliability issues is
 
 ### Fuzz Testing
 
-- Fuzz testing is a technique that complements the previously mentioned testing techniques. Fuzzing involves using a fuzz engine to generate a large number of candidate inputs that are then passed through a fuzz driver to the fuzz target. The fuzzer then analyzes how the system handles the input. Complex inputs handled by all kinds of software are popular targets for fuzzing - for example file parsers, compression algo, network protocol implementation and audio codec.
+- Fuzz testing is a technique that complements the previously mentioned testing techniques. Fuzzing involves using a fuzzing engine to generate a large number of candidate inputs that are then passed through a fuzz driver to the fuzz target. The fuzzer then analyzes how the system handles the input. Complex inputs handled by all kinds of software are popular targets for fuzzing - for example, file parsers, compression algorithms, network protocol implementation and audio codec.
 
 ### Integration Testing
 

mkdocs.yml

@@ -39,7 +39,7 @@ nav:
         - Conclusion:  databases_nosql/further_reading.md
     - Big Data:
         - Introduction: big_data/intro.md
-        - Evolution and Architecure of Hadoop: big_data/evolution.md
+        - Evolution and Architecture of Hadoop: big_data/evolution.md
         - Conclusion: big_data/tasks.md
 - Systems Design:
     - Introduction: systems_design/intro.md