- also known as the hill cipher
- choose an integer n, the key is an n x n matrix
- M =
- (1 2 3)
- (4 5 6)
- (11 9 8)
- Message is written as a series of row vectors
- abc becomes (0 1 2) which gets
- (012)(1 2 3)
- (4 5 6)
- (11 9 8)
- mod 26
- (0,23,22)
- In order to decrypt we need the determinant of matrix M to satisfy
- gcd(det(M),26)=1
- Find the inverse of matrix M
- (-14 11 -3)
- -1/3(34 -25 6)
- (-19 13 -3)
- Since 17 is the inverse of -3 mod 26
- (22 5 1 )
- (6 17 24)
- (15 13 1)
- can return plaintext by mulitplying inverse matrix by (0,23,22) mod 26
- In order to perform the block cipher we just divide the plaintext into blocks of n characters and if the ciphertext does not divide evenly, the blanks are left as 0 but the matrix multiplication is still done
- changing one letter changes n letters of plaintext therefore frequency decryption is very difficult
- Claude Shannon
- the fundamental foundations of cryptography include
- Diffusion
- means that changing one character of the plaintext, then several characters
- Confusion
- means that key does not relate in a simple way to the ciphertext
- each ciphertext should depend on several parts of the key

## Tuesday, October 1, 2013

### Notes - Block Cipher

The following are notes from Introduction to Cryptography with Coding Theory.

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