# MATH: Brands' cash, Hal's post #2

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This post gives numerical examples to go along with Hal Finney's excellent
description of Brands' digital cash, posted earlier.  If the math is too
much, just remember the whole point:

> Blind signatures are, IMO, the key to anonymous digital cash, and in fact
> to many forms of anonymity.  The ability to engage in mutual information
> manipulation with another person, while guaranteeing that no linkage will
> later be possible between the data exchanged and the results of that
> calculation, is the foundation for interacting in a complex way without
> losing any privacy...
> Vicki wants to end up with a non-interactive signature on m', which is a
> special transformation of m.  To do this, she engages in an interactive
> signature protocol with Paul, getting him to sign m... the result is
> that she ends up with a non-interactive signature on m' because Paul was
> willing to participate in an interactive signature session on m

Continuing along:

> Now for the mathematics.  Recall the g is the "generator" of the group,
> the base of all of the powers.  x is Paul's secret key, and GX=g^x is his
> public key.

I will use g = 10, n = 17389 as in the previous example.  Paul will choose
x = 351 to be his secret key, so GX = 10^351 mod 17389 = 16987 is his public
key.  In addition, the message is m = 1994.

> As the first step of the interactive protocol, Paul chooses a random w
> and sends Vicki MX = m^x, GW = g^w, and MW = m^w.

Paul chooses a random w = 666

MX = 1994^351 mod 17389 = 11740
GW = 10^666 mod 17389 = 7115
MW = 1994^666 mod 17389 = 13262

> The relationship between m', which is what Vicki will end up
> with a signature on, and m, which is the number that Paul sees, is
>       m' = (m^s)*(g^t).

Vicki chooses s = 3694, t = 1243

m' = (1994^3694)*(10^1243) mod 17389 = 10313

> the challenge c is calculated as the hash of (m,MX,GW,MW).  Vicki
> must transform these numbers so that Paul will not recognize them, but in
> such a way that the mathematical relationships are maintained.
> To do this, Vicki chooses two (more) random numbers, u and v (along with
> s and t above).

Vicki chooses u = 5192, v = 100

>   MX' = m'^x = ((m^s)*(g^t))^x = (m^(s*x))*(g^(t*x)) = (MX^s)*(GX^t)
>   GW' = g^w' = g^(u*w+v) = (g^(u*w))*(g^v) = (GW^u)*(g^v)
>   MW' = m'^w' = ((m^s)*(g^t))^(u*w+v) = [...] =
>                                       (GW^(u*t))*(MW^(u*s))*(m'^v)

MX' = (MX^s)*(GX^t) = (11740^3694)*(16987^1243) mod 17389 = 10710
GW' = (GW^u)*(g^v) = (7115^5192)*(10^100) mod 17389 = 12113
MW' = (7115^(5192 1243))*(11740^(5192 3694))*(10313^100) mod 17389 = 9314

> Using these, Vicki calculates her hash c'= Hash(m',MX',GW',MW').

c' = hash(10313,10710,12113,9314) = 7672 (some hash function I made up)

> Now, the c she sends to Paul...
> c = c'/u

c = (7672/5192) mod 17389 = 323

[     5192 c = 7672 mod 17389
--> 5192 c" = 1 mod 17389
--> c" = 3520
==> c = c" 7672 mod 17389 = 323

check: (323 5192) mod 17389 = 7672
]

> Paul will ... calculate r = c*x+w.

r = (323 351 + 666) mod 17388 = 9711

> [Vicki calculates] r' = u*r + v

r' = (5192 9711 + 100) mod 17388 = 11800

> The resulting signature on m' is (MX',GW',MW',r')

So the resulting signature is (10710,12113,9314,11800)

Okay, that should be an actual example of the protocol, unless I messed
up somewhere ;)  I hope to finish going through Hal's third post soon.

Karl Barrus
[email protected]

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