Untitled

• Feistel ciphers have an interesting property, their round function can be inverted regardless of the function choice in box F.

Encryption

• L(i) = R(i-1)
• R(i) = L(i-1) XOR F(k(i), R(i-1))

Decryption

• R(i) = L(i-1)
• L(i-1) = R(i) XOR F(k(i), L(i))
• A round key is generated every round.
• Any function F can be chosen and still achieve the same effect.
• The same code is used for encryption and decryption.
• A secure cipher needs proper round key generation, the right number of rounds, and a solid function F.

Properties of DES

• DES is a variant of the Feistel design.
• It has 16 rounds (r = 16).
• Its block length is 64 bits.
• Key length is 56 bits.
• Round keys k(i) are each 48 bits.
• With only 56 bits, its key length is insufficient for modern applications, hence the common use of triple DES.

Triple DES (3DES)

• Employs 3 keys: K1, K2, K3.
• Key length: 56 + 56 + 56 = 168 bits.
• Another version of 3DES uses only 2 keys.

2-key 3DES

• Uses keys K1, K2.
• Key length is 112 bits.
• Is less secure than 3 key DES, and can be broken.

DES Operation

• It operates on 64 bits of plaintext.
• Begins with an initial permutation.
• Blocks split into left and right halves.
• 16 identical rounds.
• Half blocks are joined back together.
• Ends with a final inverse operation.
• Key schedule provides 16 round keys of 48 bits using selection from the main 56-bit key.

Function F

• A round consists of 6 stages.

Expansion Permutation

• Expands and permutes 32 bits to 48 bits.

Round Key Addition

• The 48-bit output is XOR'd with the round key (4 bit).

Splitting

• Splits the value into 8 lots of 6-bit values.

S-Boxes

• Each 6-bit value goes into a different S-box to produce a 4-bit result.
• S-boxes are the non-linear part of DES.
• Each S-box has a lookup table with 4 rows and 16 columns.
• Bits 1 and 6 generate the row number, bits 2, 3, 4, and 5 specify the column number.

P-Box

• Combines 8 lots of 4 bit outputs into a 32-bit value.
• This is permuted to form the output of function F.

DES Key Schedule

• Takes a 64-bit key, acting as a 56-bit key with 8 parity bits (error detection).
• Parity bits are in positions 8, 16, 24, 32, 40, 48, 56, 64, ensuring each key byte has bits set as one.

AES

• AES is a block cipher that does not rely on the Feistel cipher, instead using a substitution-permutation network.
• Block ciphers consist of rounds, each containing a key addition, a substitution, and a permutation phase.
• The permutation phase aims to maximize diffusion.
• Substitution introduces non-linearity.
• Uses avalanche effect.

Rijndael

• This algorithm can operate on block sizes of 128, 192, or 256 bits.
• AES standard fixed the block at 128 bits.
• AES supports key sizes of bits 128, 192, or 256.
• Each of these sizes have a different number of rounds specified.

US Federal Standard

• Released 4 recommended ways of using DES for data encryption.
• Simple to use, but insertion alters the plaintext.
• One bit error in the ciphertext means one whole block error in the decrypted plaintext.

CBC Mode

• Probably the best of the original operation modes.
• One bit error in the ciphertext gives an error in the corresponding plaintext, but also a one bit error in the next decrypted plaintext block.

OFB Mode

• This mode turns a block cipher into a stream cipher.
• This mode doesn't provide data integrity.
• It has bit flip property.
• A one-bit flip of the ciphertext will generate a one-bit error in the plaintext.

CFB Mode

• Turns a block cipher into a stream cipher.
• A single bit error in the ciphertext propagates, just as in CBC mode.

ECB Mode

• Simplest way to use a blockcipher.
• The encrypted data is divided into blocks of bits (n-bits).
• The last block is padded if needed.

ECB Problems

• If two input blocks are equal, they're mapped them to the same output blocks.
• Message beginnings and endings are often stereotyped.
• Blocks can be deleted from the message.
• Encrypted blocks are used in a replay attack.
• ECB mode is deterministic.

CBC (Cipher Block Chaining)

• Counters the problems with ECB mode.
• Adds context of each ciphertext block.

Encryption

• c(0) = E(k)(m(i) XOR c(i)), for 1 < i < l
• Output is IV || c1, c2,...

Decryption

• m(i) = d(k)(c(i)) XOR c(i-1), for 1 < i < l

ECB vs. CBC

• A single bit error in the ciphertext results in a whole block decrypted wrongly, whilist in CBC mode the error will also affect a single bit of the next block.
• An IV is required.

OFB (Output Feedback) Mode

• It enables a block cipher to be used as a stream cipher.
• It uses the block cipher to create the keystream, n bits at a time, where n is the block size.
• The plaintext is divided into blocks, that are m bits long.

Encryption

• c(i) = Y(i) XOR m(i)

Decryption

• Performed similarly.

CFB (Cipher Feedback) Mode

• The keystream output is produced by encryption of the ciphertext.

CTR (Counter) Mode

• Combines the advantages of ECB mode, but without the disadvantages.
• It requires a public IV, or counter, which is chosen differently for each message encrypted under the fixed key k.

Encryption

• c(i) = m(i) XOR E(k)(IV + i)
• n = bit representation of i

Stream vs Block Cipher

Definition

• Stream Cipher: Encrypts data one bit or byte at a time in a continuous stream.
• Block Cipher: Encrypts data in fixed-size blocks (64-bit, 128-bit) at a time.

Working Mechanism

• Stream Cipher: Converts plaintext into ciphertext bit by bit (or byte by byte) using a keystream.
• Block Cipher: Divides plaintext into fixed-size blocks and encrypts each block separately, using the same key.

Examples

• Stream Cipher: RC4
• Block Cipher: AES, DES, 3DES

Vulnerability

• Stream Ciphers: More vulnerable to attacks if the same key stream is used multiple times