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+ <H1>[pycrypto] DES3 problem</H1>

+ Legrandin

+ <A HREF="mailto:pycrypto%40lists.dlitz.net?Subject=Re%3A%20%5Bpycrypto%5D%20DES3%20problem&In-Reply-To=%3CCAGfyce0oB2vFOj0p6U30RdZ2Xks9KyEjuNLyc_R_Q47f3mYQXg%40mail.gmail.com%3E"

+ TITLE="[pycrypto] DES3 problem">helderijs at gmail.com

+ </A>

+ Fri Nov 29 12:16:47 PST 2013

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+<PRE>Hi,

+Since a cipher object is stateful, it can be used for either

+encryption or decryption but not both.

+In order to simulate both ends of a communication, you need two cipher objects.

+Your code should be:

+>>> from Crypto.Cipher import DES3

+>>> from Crypto import Random

+>>> key = b'Sixteen byte key'

+>>> iv = Random.new().read(DES3.block_size)

+>>> cipher = DES3.new(key, DES3.MODE_OFB, iv)

+>>> plaintext = b'sona si latine loqueris '

+>>> msg = iv + cipher.encrypt(plaintext)

+>>>

+>>> iv = msg[:16]

+>>> cipher = DES3.new(key, DES3.MODE_OFB, iv)

+>>> p = cipher.decrypt(msg[16:])

+>>> print(p)

+

+The only exception is the ECB mode. Being it stateless, it lets you

+intermix encryption and decryption.

+However, that mode should be avoided when possible because it's very

+tricky to get right.

+A nonce (sometimes called IV) is critical a value required by most

+modes (ECB again being an exception).

+It is typically required to be unique per each combination of

+key/message. In some cases - like for CBC - it must also be

+unpredictable to an adversary. The nonce/IV does not need to kept

+secret but it needs to be delivered to the receiver somehow, otherwise

+it wouldn't be able to perform decryption.

+One common choice is to generate the nonce/IV randomly and prepend it

+to the ciphertext (that is, the result of a call to .encrypt() ) but

+nothing stops you from sending it afterwards.

+2013/11/29 Dave Pawson <<A HREF="http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto">dave.pawson at gmail.com</A>>:

+> On 29 November 2013 15:16, Legrandin <<A HREF="http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto">helderijs at gmail.com</A>> wrote:

+>

+>> The DES3 example you are looking for is actually here:

+>>

+>> <A HREF="https://www.dlitz.net/software/pycrypto/api/current/Crypto.Cipher.DES-module.html">https://www.dlitz.net/software/pycrypto/api/current/Crypto.Cipher.DES-module.html</A>

+>

+> I can't get that working.

+>

+> Request please?

+> Assuming I'm not unusual in wanting to both encrypt and then decrypt.

+> It would be very helpful to show the decrypt after the encrypt?

+> the oddities I'm finding,

+> 1. Why is it sometimes (I don't know why) shown creating two ciphers,

+> one for encrypt, one for decrypt.

+> 2. The use of a nonce (as per above)

+> Is it normal to decrypt using

+>

+> ciphertext=iv + ciphere.encrypt(plaintext)

+> plain = cipherd.decrypt(ciphertext[16:])

+>

+> Using the example....

+>

+>>>> from Crypto.Cipher import DES3

+>>>> from Crypto import Random

+>>>> key = b'Sixteen byte key'

+>>>> iv = Random.new().read(DES3.block_size)

+>>>> cipher = DES3.new(key, DES3.MODE_OFB, iv)

+>>>> plaintext = b'sona si latine loqueris '

+>>>> msg = iv + cipher.encrypt(plaintext)

+>>>> p = cipher.decrypt(msg[16:])

+>>>> print(p)

+> b'\xc0/)~\xc1\xa4\xb0\xb3\x0c\x92y_\x9a\xaa\xe3\xa0'

+>

+> Any ideas please?

+>

+> TiA

+>

+> --

+> Dave Pawson

+> XSLT XSL-FO FAQ.

+> Docbook FAQ.

+> <A HREF="http://www.dpawson.co.uk">http://www.dpawson.co.uk</A>

+> _______________________________________________

+> pycrypto mailing list

+> <A HREF="http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto">pycrypto at lists.dlitz.net</A>

+> <A HREF="http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto">http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto</A>

+</PRE>

+

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