pyJustinBieberDecryptionExercise

# Encoded Text Decyphering Project

# The encryption process is done like this. Each letter in the alphabet is given a number based on its offset from "a",
# plus some additional offset. So for example if the additional offset is 10, then 'a' is numbered 10, 'b' is 11 etc.
# The resulting number is doubled. Each character in an odd position within the text, is replaced with this 2-digit number.
# Then each character at an even position within the text, is similary replaced by the 2-digit number, but these are inserted in reverse order,
# so for example for an 8-character piece of text, the 2nd character is replaced with the number derived from the 8th character in the text,
# the 3rd is replaced by the number from the 6th character etc.

intercept = "1242263422322626121816404638362648184618341226364644"          # Encoded text

LetterDistr = "?qjzxvkwyfbghmpduclsntoirae"         # letter distribution in English. Right characters are commoner

fwd = []
back = []
# build two lists, one with odd numbered pairs going forwards, and 2nd with even numbered pairs going backwards
for x in range(0, len(intercept), 4):
    f = intercept[x: x+2]
    b = intercept[x+2: x + 4]
    fwd.append(int(f) // 2)         # divide by 2
    back = [int(b) // 2] + back     # arrange integers backwards

combo = []
# Combine the 2 lists
for x in range(len(fwd)):
    combo.append(fwd[x])
    combo.append(back[x])

bestscore = 0
bestoffset = 0
bestresult = ""
# For each possible offset 0 thru 25, decode the test, and decide which offset gives an output most like English
for offset in range(26):
    res = ""
    score = 0
    for i in combo:
        i -= offset
        if i < 0:                   # wrap around the alphabet
            i += 26
        ch = chr(i + ord('a'))      # add offset to the ascii value of 'a'
        res += ch
        if ch in LetterDistr:
            score += LetterDistr.index(ch)      # accumulate a score based on letter frequency in English
    if score > bestscore:
        bestscore = score
        bestoffset = offset
        bestresult = res
print(f"The best offset was {bestoffset} which gave the following text\n{bestresult}")

# Output:-
#
# The best offset was 5 which gave the following text
# bringbiebedisonptesimlimsq