tor/src/ext/curve25519_donna

See http://code.google.com/p/curve25519-donna/ for details.

BUILDING:

If you run `make`, two .a archives will be built, similar to djb's curve25519
code. Alternatively, read on:

The C implementation is contained within curve25519-donna.c. It has no external
dependancies and is BSD licenced. You can copy/include/link it directly in with
your program. Recommended C flags: -O2

The x86-64 bit implementation is contained within curve25519-donna-x86-64.c and
curve25519-donna-x86-64.s. Build like this:

% cpp curve25519-donna-x86-64.s > curve25519-donna-x86-64.s.pp
% as -o curve25519-donna-x86-64.s.o curve25519-donna-x86-64.s.pp
% gcc -O2 -c curve25519-donna-x86-64.c

Then the two .o files can be linked in

USAGE:

The usage is exactly the same as djb's code (as described at
http://cr.yp.to/ecdh.html) expect that the function is called curve25519_donna.

In short,

To generate a private key, generate 32 random bytes and:

mysecret[0] &= 248;
mysecret[31] &= 127;
mysecret[31] |= 64;

To generate the public key, just do

static const uint8_t basepoint[32] = {9};
curve25519_donna(mypublic, mysecret, basepoint);

To generate an agreed key do:
uint8_t shared_key[32];
curve25519_donna(shared_key, mysecret, theirpublic);

And hash the shared_key with a cryptographic hash function before using.