To communicate between them, Anaïs and Bastien use an alphabet of only ten letters: 

a = 0, b = 1, c = 2, d = 3, e = 4, f = 5, g = 6, h = 7, i = 8, j = 9.

To encipher their messages, they decide to use the generalised Vigenère cipher on bigrams.

Bastien receives from Anaïs the ciphertext CHFE

Decipher the message knowing that the enciphering key is

A : 

 4  1 

 5  2 

B : 

 4 

 4 

Your answer will be a four letters word written in capital letters with no spaces.

Again with the same data:

• B wishes to send the message 12345 to A. What is the ciphertext?

• Moreover, A receives the ciphertext 12345 from C. What is the corresponding plaintext?

Your answer will be a couple of integers (x,y).

C receives the ciphered signature  2632215 but does not know who sent it (A, B or none).

To check if it comes from , needs to compute , then and finally compare with the expected result .

What are the values of ,,, and (in that order)?