Symmetric Block Cipher Algorithms Flashcards
Feistel Function
Larger Block sizes increase security, Larger Key sizes increase security, and if the round function is secure, then more rounds increase security.
Key Space
refers to the number of possible keys. For examples, DES uses a 56-bit key soit has a key space of 2^56. It is the set of possible keys for a given algorithm. Contrast this with the Caesar cipher, which had a key space of 26. With only 26 letters in the alphabet, there were only 26 possible keys.
Key Schedule
Refers to the generation of sub keys from a single key. In many ciphers, the encryption and decryption is done in rounds. Each round takes the algorithm’s key, alters it to some degree, then applies it to that round. In other words, in each round a slightly different key is used, but that key is derived from the original key. A key schedule is an algorithim that, given the key, calculates the subkeys for these rounds.
Cipher (formal definition)
The algorithms(s) needed to encrypt and decrypt a message.
Key (formal definition)
The random bits used in encrypting a message.
Algorithm (formal definition)
The mathematical process used to alter a message and read it unintelligible by any but the intended party.
Horst Feistel
Invented the Feistel Function. Also known as the Feistel Network or Feistel cipher.
Splits the block of plain text data (often 64 bits) into two parts(traditionally termed L0 R0).
The round function is applied to 1 of the halves then the output is XOR’d with the other half
This process is repeated many times, and the main differences between cryptography algorithims is the exact nature of the round function F, AND the number of iterations it goes through.
Unbalanced Feistel Cipher
Variation of the Feistel Cipher where L0 and R0 are not of equal lengths. L0 might be 32 bits and R0 might be 64 bits.(this is used in the skipjack algorithim).
DES (Data Encryption Standard)
56-bit key applied to a 64 bit block cipher(note a 64 bit key is generated, 8 bits are used for error corrections).
S-Box
means substitution box and is basically a look up table.
3DES
It does DES three times with three different keys. K1, K2, AND K3.
DESx
Variation of DES that uses a technique called Key Whitening, Just XOR a key with the text either before the round function, after the round function, or both.
AES
AKA Rijndael block cipher. Was chosen as the replacement for DES in 2001.
3 Different key sizes 128, 192, 256. AKA AES 128, AES 192, AES 256.
Uses a Substitution-Permutation matrix. Operates on a 4x4 column-major matrix called the state.
Joan Daeman and Vincent Rijmen.
Invented the AES.
Blowfish
Symmetric Block Cipher. Like DES it is a 16 bit round Feistel cipher working on 64 bit blocks AND has varying key sizes ranging from 32 bits to 448 bits.
