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Add a second round of carry reduction for P192 fast reduction

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The first round of carry reduction can generate a carry so a
second round is needed.

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
This commit is contained in:
Gabor Mezei 2023-03-28 15:03:20 +02:00
parent da018175de
commit 514806bbe9
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GPG Key ID: F072ACA227ACD71D
1 changed files with 12 additions and 2 deletions
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14
library/ecp_curves.c
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@ -4897,7 +4897,7 @@ static inline void carry64(mbedtls_mpi_uint *dst, mbedtls_mpi_uint *carry)
#define A(i) Np + (i) * WIDTH #define A(i) Np + (i) * WIDTH
#define ADD(i) add64(p, A(i), &c) #define ADD(i) add64(p, A(i), &c)
#define NEXT p += WIDTH; carry64(p, &c) #define NEXT p += WIDTH; carry64(p, &c)
#define LAST p += WIDTH; *p = c; while (++p < end) *p = 0 #define LAST p += WIDTH; do *p = 0; while (++p < end)
#define RESET last_carry[0] = c; c = 0; p = Np #define RESET last_carry[0] = c; c = 0; p = Np
#define ADD_LAST add64(p, last_carry, &c) #define ADD_LAST add64(p, last_carry, &c)
@ -4934,13 +4934,23 @@ int mbedtls_ecp_mod_p192_raw(mbedtls_mpi_uint *Np, size_t Nn)
RESET; RESET;
/* Use the reduction for the carry as well:
* 2^192 * last_carry = 2^64 * last_carry + last_carry mod P192
* It can generate a carry. */
ADD_LAST; NEXT; // A0 += last_carry
ADD_LAST; NEXT; // A1 += last_carry
// A2 += carry
RESET;
/* Use the reduction for the carry as well: /* Use the reduction for the carry as well:
* 2^192 * last_carry = 2^64 * last_carry + last_carry mod P192 * 2^192 * last_carry = 2^64 * last_carry + last_carry mod P192
*/ */
ADD_LAST; NEXT; // A0 += last_carry ADD_LAST; NEXT; // A0 += last_carry
ADD_LAST; NEXT; // A1 += last_carry ADD_LAST; NEXT; // A1 += last_carry
// A2 += carry
LAST; // A2 += carry LAST;
return 0; return 0;
} }