2: Symmetric Key Cryptography

Cryptography

The process of transferring messages between participants without anyone else being able to read or modify them.

Issue with unsecured channels

It can be infiltrated by a malicious actor to either listen to messages or even change them altogether.

Code

A way to represent data, e.g. morse code, ASCII, hex and base64.

Caeser Cipher

A cipher that replaces reach letter with one three to the right, e.g. A becomes D.

Kerckhoff’s Principle

A cipher should be secure even if the attacker knows everything other than the key.

Frequency Analysis

Counts the number of times a symbol and pair of symbols appear in an attempt to crack a cipher.

Symmetric Cryptography

Cryptography where the sender and receiver have the same keys.

Arithmetic Modulo N

Where you can count up to n - 1 and then loop back to 0.

XOR

Binary addition modulo 2.

XOR properties

Associative and commutative, for all bitstrings M, M XOR 0 = M and M XOR M = 0.

One-Time Pad

A key that XOR/Adding to the ciphertext produces the plaintext.

One-Time Pad Properties

The key needs to be as long as the message and only used once.

Advanced Encryption Standard (AES)

A state-of-the-art block cipher that works on blocks of 128 bits.

AES Properties

Generates 10 round keys from a single 128-bit key, uses one permutation (shifting rows) and uses three substitutions (substitute bytes, mix columns and add round key).

Substitute Bytes

Substitutes bytes using finite field arithmetic.

Shift Rows

Shifting rows moves the 2nd row by one byte to the left, 3rd row two bytes to the left and the 4th row three bytes to the left.

Add Round Key

Applies XOR to the block and the 128-bit round key, generated from the main key.

Data Encryption Standard

A previous standard of symmetric encryption. It was designed by IBM in the early 70’s before the NSA fixed the key length to 56 bytes and added S-boxes.

S-Boxes

A type of substitution in DES that makes it resistant to differential cryptanalysis - without them, DES would’ve been broken in the 90’s.

Triple DES

DES that takes three keys - encrypts a message with the first one, decrypts it with the second key, and encrypts it with the third key. All keys being equal is just DES.

Padding

Extra bytes added to the end of a message if a message is not the right size for block ciphers to work with. The bytes must not be random, and must not just be 0’s.

PKCS 5/7 Padding

Used to indicate the padding in a message, i.e. if there is one byte of space to fill, write 01, two bytes, write 0202, three bytes, 030303… if the message goes to the end of the block, add a new block of 16161616…

PKCS 7 Padding

Covers 16 bytes of space.

PKCS 5 Padding

Covers 8 bytes of space.

Electronic Codebook Mode (ECB)

Each block is individually encrypted. They are encrypted in the same order as plaintext blocks. Thus, repeated blocks are revealed by the ciphertext.

Cipher Block Chaining Mode (CBC)

Each block is XOR’d with the previous block - start with a random initialisation vector (IV).

CBC Encryption

1. Split up the plaintext into blocks.
2. XOR the first block with the IV.
3. Encrypt using the key.
4. Take that ciphertext block and XOR it into the next plaintext block.
5. Encrypt that block using the key.
6. Repeat from step 4 until all of the plaintext blocks are encrypted.

CBC Decryption

1. Split up the ciphertext into blocks.
2. Decrypt the first block with the key.
3. XOR result with the IV.
4. Take next block and decrypt it with the key.
5. XOR result with previous result.
6. Repeat until all blocks are decrypted.

Probabilistic Encryption

Uses random events to make every encryption different, e.g. a random event.

Non-Random IVs

Non-Random IVs can easily allow encryption to be cracked - e.g. the Zerologon vulnerability in Windows servers. With that information, authentication could be bypassed and domain controller passwords reset.

Counter Mode (CTR)

Where each block in plaintext is encrypted with the key then XOR’d with the IV added to the value of the counter (e.g. 0 for the first block, 1 for the second block, etc.).

CTR Decryption

1. Split the ciphertext into blocks
2. Take the nonce
3. Encrypt it with the key
4. XOR it with the first ciphertext block
5. Add 1 to the counter
6. Encrypt the nonce and counter added together
7. XOR the next ciphertext block
8. Repeat from step 5 until all blocks are decrypted

CTR Plaintext Attack

If you know the plaintext, you can change the encrypted message. This is done by XOR’ing the plaintext message with the message you want, then XOR’ing the encrypted message with that.