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Added optimised implementation of P-256 that uses 32->64 multiplications (MUL31).

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This commit is contained in:
Thomas Pornin 2017-01-16 18:04:40 +01:00
parent 89ea3b1876
commit bd3036844b
9 changed files with 1700 additions and 27 deletions
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32
src/ec/ec_c25519_i31.c
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@ -202,11 +202,27 @@ api_mul(unsigned char *G, size_t Glen,
*/
byteswap(G);
/*
* Decode the point ('u' coordinate). This should be reduced
* modulo p, but we prefer to avoid the dependency on
* br_i31_decode_reduce(). Instead, we use br_i31_decode_mod()
* with a synthetic modulus of value 2^255 (this must work
* since G was truncated to 255 bits), then use a conditional
* subtraction. We use br_i31_decode_mod() and not
* br_i31_decode(), because the ec_prime_i31 implementation uses
* the former but not the latter.
* br_i31_decode_reduce(a, G, 32, C255_P);
*/
br_i31_zero(b, 0x108);
b[9] = 0x0100;
br_i31_decode_mod(a, G, 32, b);
a[0] = 0x107;
br_i31_sub(a, C255_P, NOT(br_i31_sub(a, C255_P, 0)));
/*
* Initialise variables x1, x2, z2, x3 and z3. We set all of them
* into Montgomery representation.
*/
br_i31_decode_reduce(a, G, 32, C255_P);
br_i31_montymul(x1, a, C255_R2, C255_P, P0I);
memcpy(x3, x1, sizeof x1);
br_i31_zero(z2, C255_P[0]);
@ -308,8 +324,18 @@ api_mul(unsigned char *G, size_t Glen,
c255_mul(b, z2, b);
}
}
c255_mul(x2, x2, b);
br_i31_from_monty(x2, C255_P, P0I);
c255_mul(b, x2, b);
/*
* To avoid a dependency on br_i31_from_monty(), we use
* a Montgomery multiplication with 1.
* memcpy(x2, b, sizeof b);
* br_i31_from_monty(x2, C255_P, P0I);
*/
br_i31_zero(a, C255_P[0]);
a[1] = 1;
br_i31_montymul(x2, a, b, C255_P, P0I);
br_i31_encode(G, 32, x2);
byteswap(G);
return 1;