Encryption: The process of converting plaintext into ciphertext which can’t be understood without the key to decrypt it.

Plaintext: Data in human-readable form

Ciphertext: Data that has been encrypted

SKC (Secret Key Cryptography) → Symmetric Key

RSA (An asymmetric cryptography algorithm) → Cert

HMAC (Hash-based message authentication code) → Salt

CBC (Cipher Block Chaining) → Initialisation Vector (Salt)

PKC (Public Key Cryptography) → Asymmetric Key

PKI (Public Key Infrastructure) → Trent (Trusted Third Party)

Secret Key Cryptography is also called Symmetric Cryptography because the same key is used to both encrypt and decrypt the data.

e.g. RC2, RC4, DES, AES, Blowfish

Public Key Cryptography is also called Asymmetric Cryptography as it is a method of encrypting or signing data with two different keys and making one of the keys, the public key, available for anyone to use.

e.g. RSA, DSA (factoring prime numbers), ElGamal (Eliptic curve)

Hashing is a mathematical algorithm that converts plaintext to a unique text string or a ciphertext.

e.g. MD5, SHA-1, etc.

The old existing protocols were typically text-based and insecure but the new protocols involve encryption (and typically authentication).

A major problem in encryption is playback where an intruder can copy an encrypted message and play it back, as the same plain text will always give the same cipher text.

The solution is to add salt, introducing some variation in the input, even if the actual message is always the same. In the context of ciphers, the salt is called Initialisation Vector while for hash functions is called salt.

A salt can be as simple as a message sequence number. The right salting mechanisms provide Freshness and Semantic Security.

Freshness: To ensure that the received ciphertext is not a malicious replay of previously transmitted communication. 

i.e. to ensure that the received message is ‘fresh’

Semantic Security: To disallow an attacker from being able to answer any yes/no question about the ciphertext. For example, “Is this message the same as the previous?” or “Is this message an “OK” or an “ERROR” message?”

Where the attacker knows part of the ciphertext and the corresponding plaintext. The known ciphertext and plaintext can then be used to decrypt the rest of the ciphertext.

A replay attack occurs when an attacker eavesdrops on a secure network communication, intercepts it, and then delays or resends it to misdirect the receiver into doing what the hacker wants.

Active attacks are attacks in which the hacker attempts to change or transform the content of messages or information.

Where the intruder mixes parts of two different encrypted messages and, sometimes, is able to create a new message. This message is likely to make no

sense, but may trick the receiver into doing something that helps the intruder.

Where the intruder sends a message to the target, this is then encrypted with the target’s private-key and the intruder then analyses the encrypted message. For example, an intruder may send an e-mail to the encryption file server and the intruder spies on the delivered message.

Where the intruder uses brute force to decrypt the ciphertext and tries every possible key.

A man-in-the-middle attack is a type of cyber attack in which the attacker secretly intercepts and relays messages between two parties who believe they are communicating directly with each other.

Cipher Block Chaining (CBC): Each cipher text block is decrypted in a process that requires observing the blocks that have already been processed. (Suitable for encrypting large messages)

Electronic Code Block (ECB): A simple mode of operation with a block cipher that's mostly used with symmetric key encryption. (Suitable for encrypting small messages)

Stream Cipher: A method of encrypting text (to produce ciphertext) in which a cryptographic key and algorithm are applied to each binary digit in a data stream, one bit at a time.

Pseudo-Random Number Generators (PRNGs): Repeat after a given time. Fast. Deterministic. Periodic.

True Random Number Generators (TRNGs): True random e.g. keystroke analysis. Slow. Nondeterministic. Aperiodic.

Prime numbers are used extensively in encryption, such as in Diffie-Hellman

Greatest Common Divisor (GCD), or Greatest Common Factor (gcf) is used in many encryption algorithms and is the largest positive integer that divides into two numbers without a remainder. 

For example, the GCD of 9 and 15 is 3.