R implementation of a Vigniere cypher.

Contains four functions:

• encrypt(plaintext, key) Encrypts the string plaintext using the key key. key is recycled as many times as is necessary to match the length of plaintext.

• decrypt(cyphertext, key) Decrypts the string cyphertext using the key key.

• modulo_add(i, m, n) and modulo_subtract(i, m, n) Adds (subtracts) m to i modulo n. Differs from the default R %% implentation in that zero is excluded, so modulo_subtract(1, 1) returns 26, not 0 (compare with base R (1 - 1) %% 26. n defaults to 26 as it is intended for use with the alphabet.