As shown earlier, at each remailer your reply passes on its way home, the net data of the message have to be encrypted with a symmetric passphrase
the holder of the account has to keep a record of. So with a 4 hop chain plus the nym server itself plus a passphrase for the subject encryption
at the exit remailer 6 random numbers are at work. And if you use 3 chains for reliability reasons, the number increases to 16 (not 18, as the esub
mantra along with the subject itself is the same for all chains). So it was necessary to find a way to shield the user from that complexity without
giving up any security.
Pseudo-random-sequence generators (PRNGs) allow the creation of an unlimited amount of random numbers, which, originating from a specific seed
value, always apear in a definite series, which is why they're called 'pseudo' (OmniMix uses a PRNG based on AES CTR mode with 128 bit key and 128
bit IV range originally written by Wolfgang Ehrhardt according to
http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf).
Now you only have to create and store one single 'seed' value for an entire chain. OmniMix assigns a unique 'slot number' to every reply chain of
a nym account, which even after the deletion of the chain never is reused. In order to produce individual series of passphrases for every reply
chain, the seed is XORed by the according slot number, so that one seed value is sufficient to do the job for all reply block chains of a single
nym account.
