hascash.txt

To: cypherpunks@toad.com
Subject: [ANNOUNCE] hash cash postage implementation
From: Adam Back <aba@dcs.ex.ac.uk>
Date: Fri, 28 Mar 1997 16:52:26 GMT
Sender: owner-cypherpunks@toad.com

A partial hash collision based postage scheme

I've been talking about a partial hash collision based postage scheme
on the crypto lists for the last few days. The idea of using partial
hashes is that they can be made arbitrarily expensive to compute (by
choosing the desired number of bits of collision), and yet can be
verified instantly.

A first cut implementation of these ideas can be fetched from here:

   * hashcash.tgz
   * PGP signature of hashcash.tgz

I will describe what the implementation allows by example, using the program
so you can follow along if you wish.

There is an integrated partial hash collision generator (hashcash mint) and
remailer plug-in. The remailer plug-in should be easy to hook into typeI
remailers. typeII or nymserver would require modifying the mixmaster
client/remailer, the code has been designed to make this relatively easy to
do, and link directly into mixmaster, or alpha or newnym.

Compiling

        % gcc -O6 -c *.c
        % gcc -o hashcash hashcash.o sha1.o
        % gcc -o sha1 sha1file.o sha1.o
        % gcc -o sha1test sha1test.o sha1.o
        %

The functions provided by the program are

Run with no arguments and it prints a list of flags and terse usage
instructions.

Speed test:

        % hashcash -t
        speed: 7218 hashes per sec
        %

This tells you the number of hashes it can search per second. (Note, the
implementation of sha1 it is using is not optimised, it is my reference
implementation. I have another speed freak sha1 implementation which needs
1/2 hrs work, and has the same interface, so I'll plug it in later. It's
commented in sha1.c).

Estimate time required to find 17-bit partial hash collision:

        % hashcash -t -17
        speed: 7274 hashes per sec
        find: 17 bit partial sha1 collision
        estimate: 9 seconds
        %

(varies quite widely from estimated time)

Calculate a 17 bit collision on string "flame"

(flame is a now dead remailer):

        % hashcash -17 flame
        speed: 7274 hashes per sec
        find: 17 bit partial sha1 collision
        collision: 09948flame356018443
        tries: 57450
        %

The collision is actually found on the hash of the date since Jan 1 1970 in
days (5 digit leading zero filled) and string given.

So the collision is found on:

        % echo -n 09948flame | sha1
        fbbea210abafe3e72afe7849eaea2052e309e017
        %

The collision that was found can be seen manually as the collision is
constrained to be the MSbits of the digest:

        % echo -n 09948flame356018443 | sha1
        fbbead76da651cf856f7466fed9624d3ada061d9
        %

You can manually see that the first 20 bits match. (Note we asked for a 17
bit hash, but it generates a 17 bit or larger hash. We just got lucky and
got an extra 3 bits, which would happen about 1 time in 8).

The hashcash client can also report on a collision:

        % hashcash flame 09948flame356018443
        collision: 20 bits
        %

You can check on the validity period as compared to todays date:

        % hashcash flame 09948flame356018443 28
        validity: 28 days remaining
        collision: 20 bits
        %

You can check that a hash has a requested number of bits:

        % hashcash -25 flame 09948flame356018443
        collision: 20 bits
        required: 25 bits
        rejected: too few bits
        %

The exit status is set to failure if any of the tests fail: ie if there are
too few bits, or if you do a validity check and the hash has expired, or
isn't yet in it's validity period.

Double spending protection

You can also ask the hashcash client to remember collisions within a
selected validity period.

        % hashcash -d -25 flame 09948flame356018443 28
        validity: 28 days remaining
        collision: 20 bits
        required: 20 bits
        database: not double spent
        adding: 28 09948flame356018443
        %

The collision will only be added if all the tests pass (in validity period,
required number of bits). The exit status also tells you if the hash was ok.

The database would grow indefinately as the mailer accepted new payments, so
the validity period is stored in the database, and at the next use of the
database after the validity period expires, the collision will be discarded.
There is no need to keep expired collisions around because we wouldn't
accetp it anyway because it's out of date, so who cares if we've previously
accepted it. A validity period of 0 means valid forever, and it will never
be discarded from the database.

An example of double spending

A new test now is whether a hash has been presented before, so we the above
command and expect it to be rejected as already spent, because it is in the
database:

        % hashcash -d -25 flame 09948flame356018443 28
        validity: 28 days remaining
        collision: 20 bits
        required: 25 bits
        rejected: too few bits
        database: double spent
        %

(exit status is set to failure, due to double spent cash)

That's it

It's very lightly tested, so if anything breaks let me know.

It's got some inefficiencies in places, but working code comes first,
efficiency later.

(Also I have not tested my SHA1 implementation on a big endian machine, it
auto-detects byte endian-ness, theoretically).

Remailer applications discussed so far

   * exit remailer postage per post
   * spam filter on mail you receive, if it hasn't got a 20 bit hash, or 1c
     digicash you have a program which bounces it with a notice explaining
     the required postage, and where to obtain software from. This would put
     spammers out of business overnight, as 1,000,000 x 20 = 100 MIP years
     which is going to be more compute than they've got, and 1,000,000 x 1c
     = $10,000 is going to be more than they are likely to make through
     sales interest from the spam.
   * good behaviour bond for nymserver users. The nymserver user pays the
     nymserver (in a largish hash collision, or $25 digicash) for a
     replyable nymserver account. They agree an contract with the penalty
     clause that breaking the contract means the nymserver keeps the
     digicash/collision, and disables the account. They user's identity
     isn't known, but to join in again they have to pay up another large
     hash collision or more digicash.
   * there are lots of other ideas to play with.

How does this fit in with digicash

Digicash postage on remailers, and mail would be useful, however there are a
number of problems with digicash:

   * It is more onerous to set up an account (form filling etc)
   * Not many people have accounts
   * It's only anonymous for the payer anonymous (and not anonymous for the
     seller)

So my suggestion is that we have remailers which accept either hash
collision postage, or digicash postage. The advantages of this approach are:

   * Hashcash may provide a stop gap measure until digicash becomes more
     widely used
   * Hashcash is free, all you've got to do is burn some cycles on your PC.
     It is in keeping with net culture of free discourse, where the
     financially challenged can duke it out with millionaires, retired
     government officials, etc on equal terms.
   * Hashcash may provide us with a fall back method for controling spam if
     digicash goes sour (gets outlawed or required to escrow user
     identities).

Any comments, code, etc gratefully received.  A couple of remailers
alpha testing it would be nice also.

Adam