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+--- Thread: Aes padding (/thread-10448.html)
Aes padding - Hairy_Ape - May-21-2018
I have a question about padding when encrypting using aes.
Killerrex had previously helped me develop a code for decrypting an AES encrypted string where only only part of the key was know (we were tasked with creating a loop to iterate through possible key values as it was all numeric).
The code decrypts in ECB mode, so there is no IV.
To strip the padding I used:
unpad = lambda s : s [0:-ord (s [-1])]This worked for an encrypted string with a known key. But for this puzzle, I'm unable to get any readable output.
I was wondering what other padding methods may have been used. Any thoughts?
RE: Aes padding - scidam - May-23-2018
Some time ago I used the following functions for padding/unpadding operations:
def padding(s, bs=16):
if len(s) % bs == 0:
return s + ''.join(random.SystemRandom().choice(string.ascii_lowercase + string.digits) for _ in range(bs - 1)) + chr(96 - bs)
if len(s) % bs > 0 and len(s) > bs:
res = s + ''.join(random.SystemRandom().choice(string.ascii_lowercase + string.digits) for _ in range(bs - len(s) % bs -1)) + chr(96 + len(s) % bs - bs)
else:
res = s + ''.join(random.SystemRandom().choice(string.ascii_lowercase + string.digits) for _ in range(bs - len(s) - 1)) + chr(96 + len(s) - bs)
return res
def unpadding(s, bs=16):
return s[:ord(s[-1])-96] if len(s) % bs == 0 else ''