SHA in 8 lines of perl5
SHA1 (Secure Hash Algorithm 1) is a cryptographic message digest
algorithm. It is considered to be more secure than MD5, as it has
longer hash size, and because Hans Dobbertin discovered a partial
weakness in the MD5 collision function. (There was a SHA0 but it
had some security problems which were fixed in SHA1).
SHA1 was designed by NIST and the NSA. SHA1 is specified in
NIST Federal Information Processing standard
FIPS 180-1.
John Allen
<allen@grumman.com>
wrote this implementation of SHA optimised for size, in 8 lines of
perl5:
<![CDATA[
#!/usr/bin/perl -iD9T4C`>_-JXF8NMS^$#)4=L/2X?!:@GF9;MGKH8\;O-S*8L'6
@A=unpack"N*",unpack u,$^I;@K=splice@A,5,4;sub M{($x=pop)-($m=1+~0)*int$x/$m};
sub L{$n=pop;($x=pop)<<$n|2**$n-1&$x>>32-$n}@F=(sub{$b&($c^$d)^$d},$S=sub{$b^$c
^$d},sub{($b|$c)&$d|$b&$c},$S);do{$l+=$r=read STDIN,$_,64;$r++,$_.="\x80"if$r<
64&&!$p++;@W=unpack N16,$_."\0"x7;$W[15]=$l*8 if$r<57;for(16..79){push@W,L$W[$_
-3]^$W[$_-8]^$W[$_-14]^$W[$_-16],1}($a,$b,$c,$d,$e)=@A;for(0..79){$t=M&{$F[$_/
20]}+$e+$W[$_]+$K[$_/20]+L$a,5;$e=$d;$d=$c;$c=L$b,30;$b=$a;$a=$t}$v='a';@A=map{
M$_+${$v++}}@A}while$r>56;printf'%.8x'x5 ."\n",@A
]]>
SHA creates a 160 bit hash (five 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:
<![CDATA[
% echo -n abc | sha1
a9993e364706816aba3e25717850c26c9cd0d89d
]]>
output is in hex.
(The SHA1 hash of string "abc" is one of the standard test vectors.)
As expected the hash of a message differing by only one letter
produces an entirely different hash:
<![CDATA[
% echo -n abd | sha
cb4cc28df0fdbe0ecf9d9662e294b118092a5735
]]>
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/GPG
clearsigned document is when DSA is used, the SHA1 digest of the message,
signed with the authors private DSA key.
John also wrote an MD5 in few lines of perl5.
Comments, html bugs to me
(Adam Back) at
<adam@cypherspace.org>