ed25519/src/add_scalar.c

#include "ed25519.h"
#include "sha512.h"
#include "ge.h"
#include "sc.h"


/* see http://crypto.stackexchange.com/a/6215/4697 */
void ed25519_add_scalar(unsigned char *public_key, unsigned char *private_key, const unsigned char *scalar) {
    const unsigned char SC_1[32] = {1}; /* scalar with value 1 */
    
    unsigned char n[32];
	unsigned char salt[64];
    ge_p3 nB;
    ge_p1p1 A_p1p1;
    ge_p3 A;
    ge_p3 public_key_unpacked;
    ge_cached T;

    int i;

    /* copy the scalar and clear highest bit */
    for (i = 0; i < 31; ++i) {
        n[i] = scalar[i];
    }
    n[31] = scalar[31] & 127;

    /* private key: a = n + t */
	/* we also create a new salt, just to be sure */
    if (private_key) {
        sc_muladd(private_key, SC_1, n, private_key);

		/* generate new salt as the last 32 bytes of SHA512(new_private_key, old_salt) */
		sha512(private_key, 64, salt);
		for (i = 0; i < 32; ++i) {
			private_key[i + 32] = salt[i + 32];
		}
    }

	/* public key: A = nB + T */
    if (public_key) {
		/* if we know the private key we don't need a point addition, which is faster */
		/* using a "timing attack" you could find out wether or not we know the private
		   key, but this information seems rather useless - if this is important pass
		   public_key and private_key seperately in 2 function calls */
		if (private_key) {
			ge_scalarmult_base(&A, private_key);
		} else {
			/* unpack public key into T */
			ge_frombytes_negate_vartime(&public_key_unpacked, public_key);
			fe_neg(public_key_unpacked.X, public_key_unpacked.X); // undo negate
			fe_neg(public_key_unpacked.T, public_key_unpacked.T); // undo negate
			ge_p3_to_cached(&T, &public_key_unpacked);

			/* calculate n*B */
			ge_scalarmult_base(&nB, n);

			/* A = n*B + T */
			ge_add(&A_p1p1, &nB, &T);
			ge_p1p1_to_p3(&A, &A_p1p1);
		}
			
		/* pack public key */
		ge_p3_tobytes(public_key, &A);
    }
}