diff --git a/ChangeLog b/ChangeLog index 7202afc52eab93d3379ef44b9845ec7d925bab12..a8a888bc21c2ba4dac8721308dd9c2038ccdf8fc 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,9 @@ +2015-12-10 Niels Möller + + * ecc-256.c (ecc_256_modp): Fixed carry propagation bug. Problem + reported by Hanno Böck. + (ecc_256_modq): Fixed another carry propagation bug. + 2015-11-23 Niels Möller * nettle.texinfo: Document rsa_encrypt, rsa_decrypt and diff --git a/ecc-256.c b/ecc-256.c index 4fd186e3db5edf947d3018d493371ac2d5872206..e757985c2e1e82d08526446f0e0cfd272528d71c 100644 --- a/ecc-256.c +++ b/ecc-256.c @@ -113,8 +113,19 @@ ecc_256_modp (const struct ecc_modulo *p, mp_limb_t *rp) assert (q2 < 2); - /* We multiply by two low limbs of p, 2^96 - 1, so we could use - shifts rather than mul. */ + /* + n-1 n-2 n-3 n-4 + +---+---+---+---+ + | u1| u0| u low | + +---+---+---+---+ + - | q1(2^96-1)| + +-------+---+ + |q2(2^.)| + +-------+ + + We multiply by two low limbs of p, 2^96 - 1, so we could use + shifts rather than mul. + */ t = mpn_submul_1 (rp + n - 4, p->m, 2, q1); t += cnd_sub_n (q2, rp + n - 3, p->m, 1); t += (-q2) & 0xffffffff; @@ -124,7 +135,10 @@ ecc_256_modp (const struct ecc_modulo *p, mp_limb_t *rp) u0 -= t; t = (u1 < cy); u1 -= cy; - u1 += cnd_add_n (t, rp + n - 4, p->m, 3); + + cy = cnd_add_n (t, rp + n - 4, p->m, 2); + u0 += cy; + u1 += (u0 < cy); u1 -= (-t) & 0xffffffff; } rp[2] = u0; @@ -211,7 +225,7 @@ ecc_256_modq (const struct ecc_modulo *q, mp_limb_t *rp) /* Conditional add of p */ u1 += t; - u2 += (t<<32) + (u0 < t); + u2 += (t<<32) + (u1 < t); t = cnd_add_n (t, rp + n - 4, q->m, 2); u1 += t;