Re: DES ECB Encryption cannot decrypt

From: I.Charitopoulos (i.charitopoulos_at_mellon.com.gr)
Date: 08/26/03

  • Next message: Darren Mombourquette: "Enterprise Policy URL Condition" 
    Date: Tue, 26 Aug 2003 18:55:44 +0300

    I have come across the same problem, though so far I truncated the padded 0s
    at the end of the ciphertext, believing it to be "garbage" that somehow
    ended up there, probably due to some imaginary glitch. Thanx for the *very
    insightfull* post Ivan.

    You should note though, in some cases like communicating with legacy systems
    (as I have unfortunately been attempting to lately) requires that we lower
    the security standards and use low-security algorithms. I am currently
    trying to pass and receive encrypted data to an old embedded system that the
    only encryption it offers is a really awkward looking C implementation of
    DES (still trying to make sure its in ECB mode) written back in '96,
    provided "as is" in its operating system... and finding it quite difficult.

    ----- Original Message -----
    From: "Ivan Medvedev [MS]" <ivanmed@online.microsoft.com>
    Newsgroups: microsoft.public.dotnet.security
    Sent: Friday, May 09, 2003 6:31 PM
    Subject: Re: DES ECB Encryption cannot decrypt

    > Per -
    > there is a thing in crypto called "padding" - when the data is
    encrypted
    > with DES in .NET classes by default it is padded with up to 8 bytes of
    > padding, that is why you are getting 16 bytes of ciphertext. Now, the
    > decryptor by default expects the cipher text to have that padding and
    > attempts to remove it, and because you only give it the first 8 bytes of
    the
    > ciphertext it fails. Padding must be used if you need to process data of
    > various lengths. If your data size is always a multiple of 8 you don't
    have
    > to use padding and you can turn it off by setting DES.Padding =
    > PaddingMode.None for both encrypting and decrypting algorithm object.
    > I would highly recommend that you read some books or online resources
    on
    > cryptography in general. I have seen companies having big problems because
    > people implementing crypto didn't exactly know what they were doing. For
    > example, I wouldn't recommend using ECB mode unless you know for sure that
    > it is the right choice, instead I would recommend sticking to the default.
    > In out prior conversation I have posted a modified version of your
    code
    > that worked without problems and could roundtrip any data. As I understood
    > you could not use that code because the encryptor returned a byte array
    and
    > for some reason you only can pass around strings (please correct me if I
    > understood it incorrectly). Therefore I suggested that you use Base64; use
    > ToBase64 in the encrypting method to get a string out of the encrypted
    > bytes, pass the string around and use FromBase64 in the decrypting method
    to
    > get bytes out of the string. ToBase64/FromBase64 guarantees that you get
    the
    > same byte array out as you put in.
    > Please let me know if this approach does not work for you or you have
    > trouble making the program work, and I will post another version of the
    > program that does exactly what you want.
    > --Ivan
    >
    > This posting is provided "AS IS" with no warranties, and confers no
    rights.

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