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Simple chosen-plaintext attack on AES-CTR given NONCE and IV re-use for multiple ciphertexts. Basically just a OTP chosen-plaintext attack implementation.
#!/usr/bin/env python
"""
Simple chosen-plaintext attack on AES-CTR given NONCE and IV re-use for
multiple ciphertexts
Copyleft 2011 Ian Gallagher <crash@neg9.org>
"""
import sys
def decrypt(keystream, ciphertext):
"""
Given an ordinal list of keystream bytes, and an ordinal list of
ciphertext, return the binary plaintext string after decryption
(standard XOR - applicable for AES-CTR mode, etc.)
"""
pt = ''
for pos in xrange(len(ciphertext)):
if pos >= len(keystream):
print >>sys.stderr, "Ran out of keystream material at pos = %d" % pos
break
else:
pt += chr(ciphertext[pos] ^ keystream[pos])
return(pt)
def derivekeystream(chosen_ciphertext, chosen_plaintext):
"""
Given an ordinal list of a chosen plaintext and the corrosponding chosen
ciphertext, derive AES-CTR keystream and return it as an ordinal list
"""
return map(lambda x: x[0] ^ x[1], zip(map(ord, chosen_ciphertext), map(ord, chosen_plaintext)))
def main():
"""
chosen_ciphertext and target_ciphertext should be in the binary encrypted
format, so prepare it by base64 decoding it, or whatever.
chosen_plaintext should be in the resulting binary/ASCII format of the
origial data.
"""
chosen_plaintext = ''
chosen_ciphertext = ''
keystream = derivekeystream(chosen_ciphertext, chosen_plaintext)
target_ciphertext = sys.argv[1]
print decrypt(keystream, map(ord, target_ciphertext))
if '__main__' == __name__:
main()
# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4
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