Re: DES ECB Encryption cannot decrypt
From: Ivan Medvedev [MS] (ivanmed_at_online.microsoft.com)
Date: 08/26/03
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Date: Tue, 26 Aug 2003 09:50:00 -0700
One possible gotcha when trying to interoperate with unmanaged crypto
libraries is endianess (little-endian vs big-endian). Sometimes you need to
reverse the byte order in the key material to get the same results from
unmanaged code.
--Ivan
This posting is provided "AS IS" with no warranties, and confers no rights.
"I.Charitopoulos" <i.charitopoulos@mellon.com.gr> wrote in message
news:OY2W7x%23aDHA.1816@TK2MSFTNGP09.phx.gbl...
> 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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