def aes128_cbc_decrypt(iv, key, ciphertext):
init_global_sbox()
plaintext = ""
cblocks = [
ciphertext[i:i + BLKSZ] for i in xrange(0, len(ciphertext), BLKSZ)
]
for i in xrange(len(cblocks) - 1, 0, -1):
t = aes128_decrypt_block(key, cblocks[i])
pb = xor_str(t, cblocks[i - 1])
plaintext = pb + plaintext
t = aes128_decrypt_block(key, cblocks[0])
pb = xor_str(t, iv)
plaintext = pb + plaintext
return plaintext
def decrypt(self, ciphertext, nonce):
if not ciphertext:
return ""
keystream = self._gen_keystream(len(ciphertext), nonce)
assert len(keystream) == len(ciphertext)
plaintext = xor_str(keystream, ciphertext)
return plaintext
def encrypt(self, plaintext, nonce):
if not plaintext:
return ""
keystream = self._gen_keystream(len(plaintext), nonce)
assert len(keystream) == len(plaintext)
ciphertext = xor_str(keystream, plaintext)
return ciphertext
def aes128_cbc_encrypt(iv, key, msg):
init_global_sbox()
ciphertext = ""
mblocks = [msg[i:i + BLKSZ] for i in xrange(0, len(msg), BLKSZ)]
assert (len(iv) == BLKSZ)
prev = iv
for mb in mblocks:
xmb = xor_str(prev, mb)
cb = aes128_encrypt_block(key, xmb)
prev = cb
ciphertext += cb
return ciphertext
def edit(self, ciphertext, nonce, offset, newtext):
unmodified = ciphertext
block_offset = (offset / BLKSZ)
block_count = (len(newtext) / BLKSZ) + 1
keystream = ""
for i in xrange(block_offset, block_offset + block_count):
keystream += self.ECB.encrypt(struct.pack('<QQ', nonce, i))
keystream = keystream[(offset % BLKSZ):(
offset +
len(newtext))] # only need enough keystream for new ciphertext
new_ciphertext = xor_str(keystream, newtext)
ciphertext = ciphertext[:offset] + new_ciphertext + ciphertext[
offset + len(new_ciphertext):]
assert (len(unmodified) == len(ciphertext))
return ciphertext
