A Puzzle in Seven Parts

3 minutes to read

We are given this string:

AbCDeFGAbCDEFGAbCDEFGABCdEFGaBCdeFGAbCDEFGAbCDEFGaBCdefgAbCDeFGABCdeFGAbCDeFGABCdefgaBCdeFGAbCDeFGABCdefgABCDEFGAbCDeFGAbCDeFGAbCDeFGABCDEFGAbCDEFGaBCdeFGaBCdeFGAbcDEFGAbCDeFGAbCDEFGAbCDeFGABCdefgAbCDeFGABcDEfGABCdefgaBCdefgAbCDeFGAbCDeFGABCDefGABCDEFgAbCDeFGABCdEFGAbCDEFGaBCdefgAbCDEFGABCDefGAbCDEFGAbcDEFgAbCDeFGAbCDeFGABCdefgABCdefgAbCDEFGaBCdefgaBCdeFGAbCDeFGaBCdeFGAbcDEFGAbCDEFGABCdEFGAbCDeFGABcDEfGAbCDEFGAbcDEFgaBCdeFGABCdEFGAbCDeFGAbCDEFGAbCDeFGAbCDeFGAbCDeFGABCdefgABCDefGAbCDeFGAbCDEFGabCDEFGAbCDeFGaBCdeFGABCDefGaBCdefgAbCDeFGABCdEFGABCdefgAbCDeFGAbCDeFGaBCdefgAbCDEFGAbcDEFgAbCDEFGABCDEFGAbCDeFGABCdefgaBCdeFGaBCDEfGAbCDeFGAbCDEFGaBCdeFGABCdEFGAbCDeFGABCDEFGAbCDeFGABCdeFGAbCDeFGaBCdeFGaBCdeFGAbCDEFGaBCdeFGABCdEFGAbCDEFGaBCdeFGABCDefGABcDEfGaBCdeFGABCdEFGaBCdeFGAbCDeFGAbCDeFGABCdefgAbCDEFGAbcDEFgAbCDEFGaBCdeFGAbCDeFGABCDefGaBCdeFGaBCDEfGABCDefGABCDEFgaBCdeFGABCdEFGaBCdeFGAbCDeFGAbCDeFGAbCDEFGAbCDEFGABCdEFGaBCdeFGAbCDEFGAbCDeFGAbCDEFGAbCDEFGAbCDeFGaBCdeFGABCdefgaBCdeFGAbcDEFGAbCDeFGABCDEFGAbCDeFGABCDefGAbCDEFGabCDEFGAbCDEFGAbcDEFgAbCDeFGAbCDEFGAbCDEFGABcDEfGaBCdeFGABCdefgAbCDEFGABCDEFGAbCDeFGaBCdeFGAbCDeFGAbCDeFGAbCDEFGAbcDEFgAbCDeFGABcDEfGAbCDeFGABCdefgAbCDeFGAbCDEFGAbCDeFGAbCDEFGAbCDEFGaBCdeFGAbCDeFGABCDEFGAbCDEFGABCDefGaBCdeFGAbCDeFGAbCDEFGaBCdeFGAbCDEFGabCDEFGaBCdeFGaBCDEfGAbCDeFGAbCDEFGAbCDEFGAbcDEFgABCDefGABCDefGAbCDeFGaBCdeFGABCDefGaBCdefgAbCDeFGABCdEFGABCdefgABCDefGAbCDeFGABCdeFGAbCDeFGAbCDEFGaBCdeFGABCdEFGABCdefgAbCDeFGAbCDeFGaBCdefgAbCDEFGAbcDEFgAbCDEFGAbcDEFgAbCDeFGAbCDEFGAbCDEFGABcDEfGaBCdeFGABCdefgaBCdeFGaBCDEfGABCDefGaBCdefgAbCDeFGaBCdeFGAbCDeFGaBCdeFGaBCdeFGAbCDEFGAbCDEFGabCDEFGAbCDeFGABCdefgAbCDeFGABCdefgaBCdeFGAbcDEFGaBCdeFGAbCDEFGAbCDeFGABCdefgAbCDEFGabCDEFGABCDefGABCdeFGAbCDeFGABCdefgaBCdeFGaBCDEfGaBCdeFGAbCDEFGABCdefgABCDEFgaBCdeFGABCdEFGAbCDeFGAbCDEFGAbCDEFGabCDEFGAbCDEFGABCDefGABCDefGaBCdefgAbCDeFGAbCDeFGABCDefGaBCdefgAbCDeFGABCdeFGABCdefgABCdeFGAbCDeFGABCDefGAbCDEFGAbcDEFgAbCDEFGaBCdeFGAbCDEFGABCdEFGAbCDeFGABCDefGaBCdeFGAbCDeFGABCdefgABCDEFgAbCDeFGABCdefgAbCDeFGaBCdeFGAbCDeFGAbCDeFGAbCDeFGABCdEFGAbCDEFGaBCdefgaBCdeFGaBCDEfGABCDefGaBCdefgABCdefgABCDEFgaBCdeFGAbCDeFGAbCDeFGABcDEfGAbCDEFGAbcDEFGAbCDEFGaBCdeFGAbCDeFGaBCdeFGAbCDeFGABcDEfGAbCDeFGaBCdeFGaBCdeFGAbCDEFGAbCDeFGABCdEFGAbCDeFGAbCDEFGaBCdeFGABCdefgABCDefGaBCdefgAbCDEFGAbcdEFGAbCDeFGAbCDeFGABCDefGABCDEFgABCDefGaBCdefgaBCdeFGABCdeFGAbCDeFGaBCdefgAbCDEFGabCDEFGABCdefgaBCdeFGAbCDeFGABCdefgAbCDeFGABcDEfGAbCDEFGAbcDEFgAbCDeFGABCdEFGAbCDeFGABCDefGAbCDeFGABCdEFGaBCdeFGaBCdeFGaBCdeFGAbCDEFGaBCdeFGABCDefGAbCDeFGAbCDeFGAbCDEFGAbcDEFgaBCdeFGABcDEfGAbCDeFGAbCDeFGaBCdeFGaBCDEfGaBCdeFGaBCdeFGABCDefGABCDEFgABCDefGaBCDEfG

Seven-segment display

As the challenge suggests, the above string must be related to a seven-segment display:

A Puzzle in Seven Parts 1

For example, the first 7 letters are AbCDeFG, which form a 5 (uppercase letters are on and lowercase letters are off). Let’s use Python to split the whole string in chunks of 7 letters and remove lowercase letters:

$ python3 -q
>>> import re
>>> m = 'AbCDeFGAbCDEFGAbCDEFGABCdEFGaBCdeFGAbCDEFGAbCDEFGaBCdefgAbCDeFGABCdeFGAbCDeFGABCdefgaBCdeFGAbCDeFGABCdefgABCDEFGAbCDeFGAbCDeFGAbCDeFGABCDEFGAbCDEFGaBCdeFGaBCdeFGAbcDEFGAbCDeFGAbCDEFGAbCDeFGABCdefgAbCDeFGABcDEfGABCdefgaBCdefgAbCDeFGAbCDeFGABCDefGABCDEFgAbCDeFGABCdEFGAbCDEFGaBCdefgAbCDEFGABCDefGAbCDEFGAbcDEFgAbCDeFGAbCDeFGABCdefgABCdefgAbCDEFGaBCdefgaBCdeFGAbCDeFGaBCdeFGAbcDEFGAbCDEFGABCdEFGAbCDeFGABcDEfGAbCDEFGAbcDEFgaBCdeFGABCdEFGAbCDeFGAbCDEFGAbCDeFGAbCDeFGAbCDeFGABCdefgABCDefGAbCDeFGAbCDEFGabCDEFGAbCDeFGaBCdeFGABCDefGaBCdefgAbCDeFGABCdEFGABCdefgAbCDeFGAbCDeFGaBCdefgAbCDEFGAbcDEFgAbCDEFGABCDEFGAbCDeFGABCdefgaBCdeFGaBCDEfGAbCDeFGAbCDEFGaBCdeFGABCdEFGAbCDeFGABCDEFGAbCDeFGABCdeFGAbCDeFGaBCdeFGaBCdeFGAbCDEFGaBCdeFGABCdEFGAbCDEFGaBCdeFGABCDefGABcDEfGaBCdeFGABCdEFGaBCdeFGAbCDeFGAbCDeFGABCdefgAbCDEFGAbcDEFgAbCDEFGaBCdeFGAbCDeFGABCDefGaBCdeFGaBCDEfGABCDefGABCDEFgaBCdeFGABCdEFGaBCdeFGAbCDeFGAbCDeFGAbCDEFGAbCDEFGABCdEFGaBCdeFGAbCDEFGAbCDeFGAbCDEFGAbCDEFGAbCDeFGaBCdeFGABCdefgaBCdeFGAbcDEFGAbCDeFGABCDEFGAbCDeFGABCDefGAbCDEFGabCDEFGAbCDEFGAbcDEFgAbCDeFGAbCDEFGAbCDEFGABcDEfGaBCdeFGABCdefgAbCDEFGABCDEFGAbCDeFGaBCdeFGAbCDeFGAbCDeFGAbCDEFGAbcDEFgAbCDeFGABcDEfGAbCDeFGABCdefgAbCDeFGAbCDEFGAbCDeFGAbCDEFGAbCDEFGaBCdeFGAbCDeFGABCDEFGAbCDEFGABCDefGaBCdeFGAbCDeFGAbCDEFGaBCdeFGAbCDEFGabCDEFGaBCdeFGaBCDEfGAbCDeFGAbCDEFGAbCDEFGAbcDEFgABCDefGABCDefGAbCDeFGaBCdeFGABCDefGaBCdefgAbCDeFGABCdEFGABCdefgABCDefGAbCDeFGABCdeFGAbCDeFGAbCDEFGaBCdeFGABCdEFGABCdefgAbCDeFGAbCDeFGaBCdefgAbCDEFGAbcDEFgAbCDEFGAbcDEFgAbCDeFGAbCDEFGAbCDEFGABcDEfGaBCdeFGABCdefgaBCdeFGaBCDEfGABCDefGaBCdefgAbCDeFGaBCdeFGAbCDeFGaBCdeFGaBCdeFGAbCDEFGAbCDEFGabCDEFGAbCDeFGABCdefgAbCDeFGABCdefgaBCdeFGAbcDEFGaBCdeFGAbCDEFGAbCDeFGABCdefgAbCDEFGabCDEFGABCDefGABCdeFGAbCDeFGABCdefgaBCdeFGaBCDEfGaBCdeFGAbCDEFGABCdefgABCDEFgaBCdeFGABCdEFGAbCDeFGAbCDEFGAbCDEFGabCDEFGAbCDEFGABCDefGABCDefGaBCdefgAbCDeFGAbCDeFGABCDefGaBCdefgAbCDeFGABCdeFGABCdefgABCdeFGAbCDeFGABCDefGAbCDEFGAbcDEFgAbCDEFGaBCdeFGAbCDEFGABCdEFGAbCDeFGABCDefGaBCdeFGAbCDeFGABCdefgABCDEFgAbCDeFGABCdefgAbCDeFGaBCdeFGAbCDeFGAbCDeFGAbCDeFGABCdEFGAbCDEFGaBCdefgaBCdeFGaBCDEfGABCDefGaBCdefgABCdefgABCDEFgaBCdeFGAbCDeFGAbCDeFGABcDEfGAbCDEFGAbcDEFGAbCDEFGaBCdeFGAbCDeFGaBCdeFGAbCDeFGABcDEfGAbCDeFGaBCdeFGaBCdeFGAbCDEFGAbCDeFGABCdEFGAbCDeFGAbCDEFGaBCdeFGABCdefgABCDefGaBCdefgAbCDEFGAbcdEFGAbCDeFGAbCDeFGABCDefGABCDEFgABCDefGaBCdefgaBCdeFGABCdeFGAbCDeFGaBCdefgAbCDEFGabCDEFGABCdefgaBCdeFGAbCDeFGABCdefgAbCDeFGABcDEfGAbCDEFGAbcDEFgAbCDeFGABCdEFGAbCDeFGABCDefGAbCDeFGABCdEFGaBCdeFGaBCdeFGaBCdeFGAbCDEFGaBCdeFGABCDefGAbCDeFGAbCDeFGAbCDEFGAbcDEFgaBCdeFGABcDEfGAbCDeFGAbCDeFGaBCdeFGaBCDEfGaBCdeFGaBCdeFGABCDefGABCDEFgABCDefGaBCDEfG'  
>>> ms = [m[i : i + 7] for i in range(0, len(m), 7)]
>>> ms = [re.sub('[a-g]', '', mss) for mss in ms]
>>> ' '.join(ms)
'ACDFG ACDEFG ACDEFG ABCEFG BCFG ACDEFG ACDEFG BC ACDFG ABCFG ACDFG ABC BCFG ACDFG ABC ABCDEFG ACDFG ACDFG ACDFG ABCDEFG ACDEFG BCFG BCFG ADEFG ACDFG ACDEFG ACDFG ABC ACDFG ABDEG ABC BC ACDFG ACDFG ABCDG ABCDEF ACDFG ABCEFG ACDEFG BC ACDEFG ABCDG ACDEFG ADEF ACDFG ACDFG ABC ABC ACDEFG BC BCFG ACDFG BCFG ADEFG ACDEFG ABCEFG ACDFG ABDEG ACDEFG ADEF BCFG ABCEFG ACDFG ACDEFG ACDFG ACDFG ACDFG ABC ABCDG ACDFG ACDEFG CDEFG ACDFG BCFG ABCDG BC ACDFG ABCEFG ABC ACDFG ACDFG BC ACDEFG ADEF ACDEFG ABCDEFG ACDFG ABC BCFG BCDEG ACDFG ACDEFG BCFG ABCEFG ACDFG ABCDEFG ACDFG ABCFG ACDFG BCFG BCFG ACDEFG BCFG ABCEFG ACDEFG BCFG ABCDG ABDEG BCFG ABCEFG BCFG ACDFG ACDFG ABC ACDEFG ADEF ACDEFG BCFG ACDFG ABCDG BCFG BCDEG ABCDG ABCDEF BCFG ABCEFG BCFG ACDFG ACDFG ACDEFG ACDEFG ABCEFG BCFG ACDEFG ACDFG ACDEFG ACDEFG ACDFG BCFG ABC BCFG ADEFG ACDFG ABCDEFG ACDFG ABCDG ACDEFG CDEFG ACDEFG ADEF ACDFG ACDEFG ACDEFG ABDEG BCFG ABC ACDEFG ABCDEFG ACDFG BCFG ACDFG ACDFG ACDEFG ADEF ACDFG ABDEG ACDFG ABC ACDFG ACDEFG ACDFG ACDEFG ACDEFG BCFG ACDFG ABCDEFG ACDEFG ABCDG BCFG ACDFG ACDEFG BCFG ACDEFG CDEFG BCFG BCDEG ACDFG ACDEFG ACDEFG ADEF ABCDG ABCDG ACDFG BCFG ABCDG BC ACDFG ABCEFG ABC ABCDG ACDFG ABCFG ACDFG ACDEFG BCFG ABCEFG ABC ACDFG ACDFG BC ACDEFG ADEF ACDEFG ADEF ACDFG ACDEFG ACDEFG ABDEG BCFG ABC BCFG BCDEG ABCDG BC ACDFG BCFG ACDFG BCFG BCFG ACDEFG ACDEFG CDEFG ACDFG ABC ACDFG ABC BCFG ADEFG BCFG ACDEFG ACDFG ABC ACDEFG CDEFG ABCDG ABCFG ACDFG ABC BCFG BCDEG BCFG ACDEFG ABC ABCDEF BCFG ABCEFG ACDFG ACDEFG ACDEFG CDEFG ACDEFG ABCDG ABCDG BC ACDFG ACDFG ABCDG BC ACDFG ABCFG ABC ABCFG ACDFG ABCDG ACDEFG ADEF ACDEFG BCFG ACDEFG ABCEFG ACDFG ABCDG BCFG ACDFG ABC ABCDEF ACDFG ABC ACDFG BCFG ACDFG ACDFG ACDFG ABCEFG ACDEFG BC BCFG BCDEG ABCDG BC ABC ABCDEF BCFG ACDFG ACDFG ABDEG ACDEFG ADEFG ACDEFG BCFG ACDFG BCFG ACDFG ABDEG ACDFG BCFG BCFG ACDEFG ACDFG ABCEFG ACDFG ACDEFG BCFG ABC ABCDG BC ACDEFG AEFG ACDFG ACDFG ABCDG ABCDEF ABCDG BC BCFG ABCFG ACDFG BC ACDEFG CDEFG ABC BCFG ACDFG ABC ACDFG ABDEG ACDEFG ADEF ACDFG ABCEFG ACDFG ABCDG ACDFG ABCEFG BCFG BCFG BCFG ACDEFG BCFG ABCDG ACDFG ACDFG ACDEFG ADEF BCFG ABDEG ACDFG ACDFG BCFG BCDEG BCFG BCFG ABCDG ABCDEF ABCDG BCDEG'

Now we can use dcode.fr to decode the above digits:

A Puzzle in Seven Parts 2

More decoding

Now we have a string in hexadecimal encoding (notice that q are 9):

>>> enc = '566A46615q5745785558644E5657527155305A61636C557761454E6A526C4A565557356b54315A75516C68574d564A585q54464A64324A45576C64534d304A45566A465665474E58536b6C5662476854556C5257565664586345646b4d566C3354315A735q564A75516C6C5662474d315454466b57574E46576b3q574d46704A566b633155315q7q536C646A5345705754555A614d317045526E64545254465A5647316F5530314q516b7457526C5A535A444643556C42554d44303d'  
>>> enc = enc.replace('q', '9')
>>> bytes.fromhex(enc)
b'VjFaYWExUXdNVWRqU0ZaclUwaENjRlJVUW5kT1ZuQlhWMVJXYTFJd2JEWldSM0JEVjFVeGNXSklVbGhTUlRWVVdXcEdkMVl3T1ZsYVJuQllVbGM1TTFkWWNFWk9WMFpJVkc1U1YySldjSEpWTUZaM1pERndTRTFZVG1oU01IQktWRlZSZDFCUlBUMD0='

And then we have Base64… multiple times:

>>> b64decode(bytes.fromhex(enc))
b'V1Zaa1QwMUdjSFZrU0hCcFRUQndOVnBXV1RWa1IwbDZWR3BDV1UxcWJIUlhSRTVUWWpGd1YwOVlaRnBYUlc5M1dYcEZOV0ZIVG5SV2JWcHJVMFZ3ZDFwSE1YTmhSMHBKVFVRd1BRPT0='

Flag

After all the decoding process, we get the flag:

>>> b64decode(b64decode(bytes.fromhex(enc)))
b'WVZkT01GcHVkSHBpTTBwNVpWWTVkR0l6VGpCWU1qbHRXRE5TYjFwV09YZFpXRW93WXpFNWFHTnRWbVprU0Vwd1pHMXNhR0pJTUQwPQ=='  
>>> b64decode(b64decode(b64decode(bytes.fromhex(enc))))
b'YVdOMFpudHpiM0p5ZVY5dGIzTjBYMjltWDNSb1pWOXdZWEowYzE5aGNtVmZkSEpwZG1saGJIMD0='
>>> b64decode(b64decode(b64decode(b64decode(bytes.fromhex(enc)))))
b'aWN0Zntzb3JyeV9tb3N0X29mX3RoZV9wYXJ0c19hcmVfdHJpdmlhbH0='
>>> b64decode(b64decode(b64decode(b64decode(b64decode(bytes.fromhex(enc))))))
b'ictf{sorry_most_of_the_parts_are_trivial}'