python aes encrypt/decrypt does not return the same results

Question

below code sample does not return the original text after encrypt/decrypt operation and I am trying to figure it out why

from Crypto.Cipher import AES

text = """This is plain text 
to use.
It should be exqctly 128 characters long to avoid padding and it is split
with new lines as in 
file"""

password = "password........"

block = 32
mode = AES.MODE_CBC

enc = AES.new(password, mode)

encrypted = enc.encrypt(text)

print "ORIGINAL: " + text

print "ENCRYPTED: " + str(encrypted)

print "DECRYPTED: " + str(enc.decrypt(encrypted))

can one tell why first part of the text is malformed ?

MartinStettner · Accepted Answer · 2012-02-22 12:36:47Z

7

I think, you need to reset the initialisation vector (IV), in order to get the desired result. The easist way might be to create a new AES object for decrypting:

enc = AES.new(password, mode)
encrypted = enc.encrypt(text)
print "ORIGINAL: " + text
print "ENCRYPTED: " + str(encrypted)
dec = AES.new(password, mode)
print "DECRYPTED: " + str(dec.decrypt(encrypted))

answered Feb 22, 2012 at 12:36

MartinStettner

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2 Comments

President James K. Polk Over a year ago

can't you explicitly set and get the IV? Better to get the IV of the encrypter and set the IV for the decrypter.

MartinStettner Over a year ago

You can set it in the constructor (third parameter to AES.new). I think the current implementation uses a default IV (not a random one), so it's not strictly necessary to copy it from the encrypter to the decrypter (note, this would have to be done before the encryption!). For security reasons, it would be best to use a different (random) IV for each encryption, but you need to pass it somehow to the decrypter, so it might not be worth the additional effort ... See en.wikipedia.org/wiki/Initialization_vector for more info.

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