48 lines
1.2 KiB
C
48 lines
1.2 KiB
C
#include "ed25519.h"
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#include "ge.h"
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#include "sc.h"
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/* see http://crypto.stackexchange.com/a/6215/4697 */
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void ed25519_add_scalar(unsigned char *public_key, unsigned char *private_key, const unsigned char *scalar) {
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const unsigned char SC_1[32] = {1}; /* scalar with value 1 */
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unsigned char n[32];
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ge_p3 nB;
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ge_p1p1 A_p1p1;
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ge_p2 A;
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ge_p3 public_key_unpacked;
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ge_cached T;
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int i;
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/* copy the scalar and clear highest bit */
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for (i = 0; i < 31; ++i) {
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n[i] = scalar[i];
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}
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n[31] = scalar[31] & 127;
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/* private key: a = n + t */
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if (private_key) {
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sc_muladd(private_key, SC_1, n, private_key);
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}
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if (public_key) {
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/* unpack public key into T */
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ge_frombytes_negate_vartime(&public_key_unpacked, public_key);
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fe_neg(public_key_unpacked.X, public_key_unpacked.X); // undo negate
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fe_neg(public_key_unpacked.T, public_key_unpacked.T); // undo negate
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ge_p3_to_cached(&T, &public_key_unpacked);
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/* calculate n*B */
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ge_scalarmult_base(&nB, n);
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/* A = n*B + T */
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ge_add(&A_p1p1, &nB, &T);
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ge_p1p1_to_p2(&A, &A_p1p1);
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/* pack public key */
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ge_tobytes(public_key, &A);
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}
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}
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