def analyse(cypher):
# Figure out the most common character in cypher text. Since this is just
# a statistical analysis, there is no guarantee, that the most common
# character in the cypher matches the most common character in the language
# of the plain text. So let's assume that the correct match is somewhere
# among the 3 most common characters. Return those and let the caller
# decide what to do with this problem.
most_common = Counter(cypher).most_common(3)
# But calculate the key as distance of common characters
possible_keys = list()
for m in most_common:
distance = ALPHABET.index(PLAIN_MOST_COMMON) - ALPHABET.index(m[0])
possible_keys.append(distance)
return possible_keys
def decrypt_character(c, key):
# Calculate the new index for the character lookup
plain_index = ALPHABET.index(c) + key
# Make sure we can substitute in both directions
if plain_index < 0:
plain_index += len(ALPHABET)
elif plain_index >= len(ALPHABET):
plain_index -= len(ALPHABET)
return ALPHABET[plain_index]