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Use CMOV on x86_64

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CMOV has been constant time on all generations of x86_64 CPUs, even when
reading from memory.
This commit is contained in:
Frank Denis 2020-02-25 09:22:47 +01:00
parent 178359ddf9
commit afae623190
1 changed files with 31 additions and 0 deletions
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31
src/libsodium/include/sodium/private/ed25519_ref10_fe_51.h
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@ -106,6 +106,36 @@ fe25519_neg(fe25519 h, const fe25519 f)
Preconditions: b in {0,1}.
*/
#ifdef HAVE_AMD64_ASM
static void
fe25519_cmov(fe25519 f, const fe25519 g, unsigned int b)
{
uint64_t t64_0, t64_1, t64_2, t64_3, t64_4;
__asm__ __volatile__(
"cmpl $0, %[b] ;\n"
"movq 0(%[f]), %[t64_0] ;\n"
"movq 8(%[f]), %[t64_1] ;\n"
"movq 16(%[f]), %[t64_2] ;\n"
"movq 24(%[f]), %[t64_3] ;\n"
"movq 32(%[f]), %[t64_4] ;\n"
"cmovnzq 0(%[g]), %[t64_0] ;\n"
"cmovnzq 8(%[g]), %[t64_1] ;\n"
"cmovnzq 16(%[g]), %[t64_2] ;\n"
"cmovnzq 24(%[g]), %[t64_3] ;\n"
"cmovnzq 32(%[g]), %[t64_4] ;\n"
"movq %[t64_0], 0(%[f]) ;\n"
"movq %[t64_1], 8(%[f]) ;\n"
"movq %[t64_2], 16(%[f]) ;\n"
"movq %[t64_3], 24(%[f]) ;\n"
"movq %[t64_4], 32(%[f]) ;\n"
: [t64_0] "=&r"(t64_0), [t64_1] "=&r"(t64_1),
[t64_2] "=&r"(t64_2), [t64_3] "=&r"(t64_3),
[t64_4] "=&r"(t64_4)
: [f] "r"(f), [g] "r"(g), [b] "rm"(b)
: "memory", "flags", "cc");
}
#else
static void
fe25519_cmov(fe25519 f, const fe25519 g, unsigned int b)
{
@ -135,6 +165,7 @@ fe25519_cmov(fe25519 f, const fe25519 g, unsigned int b)
f[3] = f3 ^ x3;
f[4] = f4 ^ x4;
}
#endif
/*
Replace (f,g) with (g,f) if b == 1;