Encryption with Python

[Kamilche]

Peter Hansen <peter at engcorp.com> wrote in message news:<TdqdnZROdf28kUTdRVn-hQ at powergate.ca>...

> Besides, what you say is not possible.  On my machine,
> which is about a P4 2500MHz, scanning an array.array('c') with
> 22MB of data in it, doing nothing but reading each byte and
> ignoring it, takes about 8 seconds.  So does converting the
> array to a list, which is pretty much all C code.  Strings
> are immutable, so you can't be working with one of them...
> 
> Doing anything meaningful with real data, no matter how trivial
> the algorithm, would definitely take longer.

But I DON'T manipulate the data byte by byte, only the encryption
tables.
Ah, the power of algorithms!  ;-) 
Take a look at this:

def encrypt(s, offset = 0, _tables = []):
    ' Encrypt or decrypt a string'
    cnt = len(_tables)
    if cnt == 0:
        # Initialize static variable _tables
        # Increase 'cnt' to prevent recurring patterns.
        import random
        cnt = 10 
        random.seed(257)
        for i in range(cnt):
            marked, table, max = [], [], 256
            for j in range(max):
                table.append(chr(j))
                marked.append(False)
            marked[0] = True
            marked[255] = True
            for j in range(max):
                if not marked[j]:
                    while 1:
                        c = random.randint(0, 255)
                        if marked[c] == False:
                            table[j], table[c] = table[c], table[j]
                            marked[c] = True
                            marked[j] = True
                            break
            _tables.append(''.join(table))
            
    # Tables initialized - encrypt the data.
    return s.translate(_tables[offset % cnt])

s = "This is a standard length string to encrypt."
print "\nYou need to specify an offset to avoid patterns."
print "The string is:", s
print "\nEncrypted with offsets, you get:"
for i in range(5):
    print "\t", encrypt(s, i)
print "\nEncrypted without offsets, you get:"
for i in range(5):
    print "\t", encrypt(s)


Most XOR algorithms could benefit from using this technique. There's
never a need to XOR a byte more than once, while building the table.
>From then on, it's a simple substitution problem, which the
'translate' function accomplishes quite nicely.