Working with bytes.

[Anton Vredegoor]

Jason Harper <JasonHarper at pobox.com> wrote:

>Anton Vredegoor wrote:
>> I wonder whether it would be possible to use more than six bits per
>> byte but less than seven? There seem to be some character codes left
>> and these could be used too?
>
>Look up Base85 coding (a standard part of PostScript) for an example of
>how this can be done - 4 bytes encoded per 5 characters of printable ASCII.

Thanks to you and Piet for mentioning this. I found some other
interesting application of Base85 encoding. It's used for a scheme to
encode ipv6 addresses (which use 128 bits). Since a md5 digest is 16
bytes (== 128 bits) there's a possibility to use this scheme. See 

http://www.faqs.org/rfcs/rfc1924.html 

for the details. 

Anton

from string import digits, ascii_letters

_rfc1924_chars = digits+ascii_letters+'!#$%&()*+-;<=>?@^_`{|}~'
_rfc1924_table = dict([(c,i) for i,c in enumerate(_rfc1924_chars)])
_rfc1924_bases = [85L**i for i in range(20)]

def bytes_to_rfc1924(sixteen):
    res = []
    i = 0L
    for byte in sixteen:
        i <<= 8
        i |= ord(byte)
    for j in range(20):
        i,k = divmod(i,85)
        res.append(_rfc1924_chars[k])
    return "".join(res)

def rfc1924_to_bytes(twenty):
    res = []
    i = 0L
    for b,byte in zip(_rfc1924_bases,twenty):
        i +=  b*_rfc1924_table[byte]
    for j in range(16):
        k = i & 255
        res.append(chr(k))
        i >>= 8
    res.reverse()
    return "".join(res)

def test():
    import md5
    
    #md5.digest returns 16 bytes == 128 bits, an ipv6 address
    #also uses 128 bits (I don't know which format so I'm using md5
    #as a dummy placeholder to get 16 bytes of 'random' data)
    
    bytes = md5.new('9034572345asdf').digest()
    r = bytes_to_rfc1924(bytes)
    print r
    check = rfc1924_to_bytes(r)
    assert bytes == check
    
if __name__=='__main__':
    test()

output:

k#llNFNo4sYFxKn*J<lB