🚀 go-pugleaf

In an earlier article, I asked if encryption by XOR with a pseudo-random
sequence R generated with a secure one-way hash function like MD5:

  R[i+1] = MD5(R[i] XOR password)
  ciphertext = plaintext XOR R

was secure.  The reponse I got was that it was secure, although not
immune to forgery if part of the plaintext was known.  Also, appending
the password to R[i] instead of XOR might be prefereable.

I later discovered a weakness.  If two files encrypted with the same
password are XORed together, then the result is the XOR of the plaintext,
which can be easily broken.

One solution would be to use a salt which would be different for
each file, e.g.

  salt = MD5(time-of-day + filename + process id + ...)
  R[i+1] = MD5(R[i] + password + salt)
  ciphertext = salt + (plaintext XOR R)

where + means concatenation.  Does this weaken the encryption?

--------------------------------        _\/_
Matt Mahoney, mvm@epg.harris.com       |(TV)|  Drug of the Nation
#include <disclaimer.h>                |____|