Mixmaster

Users of Type-I Pseudonyms often terminate them on the Usenet group
alt.anonymous.messages. Many of these are being lost due to news servers
running default Cleanfeed installations that perform 'Fuzzy MD5' to filter
Excessive Multi-Posting.

Cleanfeed's Fuzzy MD5 performs the following filtering prior to hashing the
message payload:
$mbody = lc $hdr{__BODY__};
$mbody =~ s/^(?!http)\S{7,70}\r?$//mg;
$mbody =~ s/\r{3}.*$//mg;
$mbody =~ s/\s+$//;
$mbody =~ s/^[^\n]*\Z//m if $lines > 5;
$mbody =~ tr/a-z0-9//cd;
This results in virtually all PGP encrypted messages having the same MD5
hash and subsequently being dropped.

Resolution:
During encryption, add a PGP Comments header that contains 8 blocks of 6
random a-z characters separated by a whitespace character. This will
ensure that each PGP payload will generate a different Fuzzy MD5 hash.