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tls: P256: add comment on logic in sp_512to256_mont_reduce_8, no code changes

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Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2021-11-28 12:55:20 +01:00
parent 00b5051cd2
commit 832626227e
1 changed files with 23 additions and 10 deletions
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33
networking/tls_sp_c32.c
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@ -850,6 +850,20 @@ static int sp_256_mul_add_8(sp_digit* r /*, const sp_digit* a, sp_digit b*/)
* a Double-wide number to reduce. Clobbered.
* m The single precision number representing the modulus.
* mp The digit representing the negative inverse of m mod 2^n.
*
* Montgomery reduction on multiprecision integers:
* Montgomery reduction requires products modulo R.
* When R is a power of B [in our case R=2^128, B=2^32], there is a variant
* of Montgomery reduction which requires products only of machine word sized
* integers. T is stored as an little-endian word array a[0..n]. The algorithm
* reduces it one word at a time. First an appropriate multiple of modulus
* is added to make T divisible by B. [In our case, it is p256_mp_mod * a[0].]
* Then a multiple of modulus is added to make T divisible by B^2.
* [In our case, it is (p256_mp_mod * a[1]) << 32.]
* And so on. Eventually T is divisible by R, and after division by R
* the algorithm is in the same place as the usual Montgomery reduction was.
*
* TODO: Can conditionally use 64-bit (if bit-little-endian arch) logic?
*/
static void sp_512to256_mont_reduce_8(sp_digit* r, sp_digit* a/*, const sp_digit* m, sp_digit mp*/)
{
@ -941,15 +955,6 @@ static void sp_256_mont_sqr_8(sp_digit* r, const sp_digit* a
* r Inverse result. Must not coincide with a.
* a Number to invert.
*/
#if 0
//p256_mod - 2:
//ffffffff 00000001 00000000 00000000 00000000 ffffffff ffffffff ffffffff - 2
//Bit pattern:
//2 2 2 2 2 2 2 1...1
//5 5 4 3 2 1 0 9...0 9...1
//543210987654321098765432109876543210987654321098765432109876543210...09876543210...09876543210
//111111111111111111111111111111110000000000000000000000000000000100...00000111111...11111111101
#endif
static void sp_256_mont_inv_8(sp_digit* r, sp_digit* a)
{
int i;
@ -957,7 +962,15 @@ static void sp_256_mont_inv_8(sp_digit* r, sp_digit* a)
memcpy(r, a, sizeof(sp_digit) * 8);
for (i = 254; i >= 0; i--) {
sp_256_mont_sqr_8(r, r /*, p256_mod, p256_mp_mod*/);
/*if (p256_mod_2[i / 32] & ((sp_digit)1 << (i % 32)))*/
/* p256_mod - 2:
* ffffffff 00000001 00000000 00000000 00000000 ffffffff ffffffff ffffffff - 2
* Bit pattern:
* 2 2 2 2 2 2 2 1...1
* 5 5 4 3 2 1 0 9...0 9...1
* 543210987654321098765432109876543210987654321098765432109876543210...09876543210...09876543210
* 111111111111111111111111111111110000000000000000000000000000000100...00000111111...11111111101
*/
/*if (p256_mod_minus_2[i / 32] & ((sp_digit)1 << (i % 32)))*/
if (i >= 224 || i == 192 || (i <= 95 && i != 1))
sp_256_mont_mul_8(r, r, a /*, p256_mod, p256_mp_mod*/);
}