Vigeniere Cipher key t characters where t is in 1 t max key space 26 t max same characters in plaintext map to different characters in the ciphertext to decrypt do it in reverse if t length of the key is known do frequency analysis on each stream of every t characters p i freq of ith character in plaintext q i freq of ith char in one particular stream of ciphertext I j sum i 0 25 p i q i j 26 find j that gives I j 0 046 repeat for each iteration running time 26 t summations if t is not known try the attack for all possible t running time 26 t t max 26 t max 2 To identify t for i 1 to t max generate a stream of ciphertext assuming the period is i Calculate q i values Check if sum j 0 25 q j 2 0 065 When i t the q i values are going to be p i permuted running time t max 26 t t max 26 t max Modern Cryptography Formal definitions Precise mathematical assumptions proofs of security Security guarentee goal Threat model i e the assumed abilities of the attacker An encryption algorithm is secure if the only way to break it is to take a long time it is infeasible for the attacker to get the message from the ciphertext the ciphertext should not rebeal information about the key