🚀 go-pugleaf

In article <1992Nov16.055949.1294@seanews.akita.com> billmcc@seanews.akita.com (Bill McCormick) writes:
>I've been hacking together a little program that does encryption using
>    key, decryption using another, then re-encryption using the
>    first key.  Is this correct?  i.e., E3(P,K) = E(D(E(P,K1),K2),K1)

Yes.

2.  Has triple DES actually been proven to be stronger than a single
>    DES encryption?  I guess this boils down to whether DES has been
>    proven to be a group.

DES is not a group.  See Keith Cambell and Michael Weiner's paper in the
Crypto 92 proceedings, whenever Springer-Verlag gets around to publishing
them.

>3.  I've been using MD5 to hash an arbitary user-entered string into
>    a key for the encryption process.  Is MD5 really suitable for the
>    task?  It appears to be a particularly good hash for long strings
>    of text, but is it suitable for short password-length text?

Yes.

>4.  In order to provide fast user-feedback if an incorrect decryption
>    key is entered, I'm storing a "checksum" of the first block of
>    plaintext in the encrypted file.  I was originally using a
>    simple checksum, but that might give away too much information
>    about the first block.  Now I apply MD5 to the block and take
>    a byte of the message digest as the check digit.  Is this
>    reasonable or would it be better to simply use the first
>    8 bytes of the file as a DES/IDEA key to encrypt a constant and store
>    a byte of that constant?

Encrypting a constant is always dangerous, but both DES and (so far) IDEA
are resistant to that kind of known plaintext attack.  Going with a checksum
just seems safer.

>5.  Which is stronger, CBC or CFB?  Is one preferred over the other in
>    cases where byte-per-byte output isn't important?

Both are secure.  CBC is more efficient, since you are encrypting data in
64-bit blocks.

Bruce