md5.txt

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MD5 in 8 lines of perl5
The MD5 algorithm (Message Digest 5) is a cryptographic message digest 
algorithm. It is the message digest algorithm used by PGP 2.x. 
MD5 was designed by Ron Rivest, who is also the `R' in `RSA'. MD5 is described 
in rfc1321. C source code is included with the RFC. 
John Allen <[e-mail]> wrote this implementation of MD5 optimised for 
size, in 8 lines of perl5: 



#!/bin/perl -iH9T4C`>_-JXF8NMS^$#)4=@<,$18%"0X4!`L0%P8*#Q4``04``04#!P`` ~JLA
@A=unpack N4C24,unpack u,$^I;@K=map{int abs 2**32*sin$_}1..64;sub L{($x=pop)
<<($n=pop)|2**$n-1&$x>>32-$n}sub M{($x=pop)-($m=1+~0)*int$x/$m}do{$l+=$r=read
STDIN,$_,64;$r++,$_.="\x80"if$r<64&&!$p++;@W=unpack V16,$_."\0"x7;$W[14]=$l*8
if$r<57;($a,$b,$c,$d)=@A;for(0..63){$a=M$b+L$A[4+4*($_>>4)+$_%4],M&{(sub{$b&$c
|$d&~$b},sub{$b&$d|$c&~$d},sub{$b^$c^$d},sub{$c^($b|~$d)})[$z=$_/16]}+$W[($A[
20+$z]+$A[24+$z]*($_%16))%16]+$K[$_]+$a;($a,$b,$c,$d)=($d,$a,$b,$c)}$v=a;for(
@A[0..3]){$_=M$_+${$v++}}}while$r>56;print unpack H32,pack V4,@A # RSA's MD5



MD5 creates a 128 bit hash (4 32 bit ints), which is designed so that it should 
be computationaly expensive to find a text which matches a given hash. ie if you 
have a hash for document A, H(A), it is difficult to find a document B which has 
the same hash, and even more difficult to arrange that document B says what you 
want it to say. 
Example
For example here is how to use John's SHA implementation: 
% echo squeamish ossifrage | md5
9469c6c14b5ed78b8aef396d2f9f96d7
output is in hex. As expected the hash of a message differing by only one letter 
produces an entirely different hash: 
% echo squeamish ossifrag3 | md5
39a5af4479676e60ccb090dff16a4a8f
Hashes are used (amongst other things) in digital signatures, it is common to 
digitally sign a hash of a message rather than the whole message. This is what 
the block of text at the bottom of a PGP clearsigned document is, the MD5 digest 
of the message, encrypted with the authors secret key. 
Actually somewhat amusingly, it is possible to use the above md5 implementation 
together with the rsa in 3 lines of perl implementation, and Mark Shoulson's 
pgpacket util (for extracting pgp keys in hex) to produce a PGP compatible 
signature. 
Well it made me chuckle when I got it to work, after I realised I now had enough 
components to put it together. 
John also wrote a SHA in few lines of perl5. 


Comments, html bugs to me (Adam Back) at <[e-mail]>