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Document some internal bignum functions

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Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2020-06-04 15:00:49 +02:00
parent 8ff7cc9911
commit d108d07050
1 changed files with 27 additions and 3 deletions
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30
library/bignum.c
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@ -1090,7 +1090,8 @@ cleanup:
} }
/* /*
* Helper for mbedtls_mpi subtraction * Helper for mbedtls_mpi subtraction:
* d -= s where d and s have the same size and d >= s.
*/ */
static void mpi_sub_hlp( size_t n, static void mpi_sub_hlp( size_t n,
const mbedtls_mpi_uint *s, const mbedtls_mpi_uint *s,
@ -1698,8 +1699,27 @@ static void mpi_montg_init( mbedtls_mpi_uint *mm, const mbedtls_mpi *N )
*mm = ~x + 1; *mm = ~x + 1;
} }
/* /** Montgomery multiplication: A = A * B * R^-1 mod N (HAC 14.36)
* Montgomery multiplication: A = A * B * R^-1 mod N (HAC 14.36) *
* \param[in,out] A One of the numbers to multiply.
* It must have at least one more limb than N
* (A->n >= N->n + 1).
* On successful completion, A contains the result of
* the multiplication A * B * R^-1 mod N where
* R = (2^ciL)^n.
* \param[in] B One of the numbers to multiply.
* It must be nonzero and must not have more limbs than N
* (B->n <= N->n).
* \param[in] N The modulo. N must be odd.
* \param mm The value calculated by `mpi_montg_init(&mm, N)`.
* This is -N^-1 mod 2^ciL.
* \param[in,out] T A bignum for temporary storage.
* It must be at least twice the limb size of N plus 2
* (T->n >= 2 * (N->n + 1)).
* Its initial content is unused and
* its final content is indeterminate.
* Note that unlike the usual convention in the library
* for `const mbedtls_mpi*`, the content of T can change.
*/ */
static void mpi_montmul( mbedtls_mpi *A, const mbedtls_mpi *B, const mbedtls_mpi *N, mbedtls_mpi_uint mm, static void mpi_montmul( mbedtls_mpi *A, const mbedtls_mpi *B, const mbedtls_mpi *N, mbedtls_mpi_uint mm,
const mbedtls_mpi *T ) const mbedtls_mpi *T )
@ -1729,6 +1749,8 @@ static void mpi_montmul( mbedtls_mpi *A, const mbedtls_mpi *B, const mbedtls_mpi
memcpy( A->p, d, ( n + 1 ) * ciL ); memcpy( A->p, d, ( n + 1 ) * ciL );
/* If A >= N then A -= N. Do the subtraction unconditionally to prevent
* timing attacks. Modify T as a side effect. */
if( mbedtls_mpi_cmp_abs( A, N ) >= 0 ) if( mbedtls_mpi_cmp_abs( A, N ) >= 0 )
mpi_sub_hlp( n, N->p, A->p ); mpi_sub_hlp( n, N->p, A->p );
else else
@ -1738,6 +1760,8 @@ static void mpi_montmul( mbedtls_mpi *A, const mbedtls_mpi *B, const mbedtls_mpi
/* /*
* Montgomery reduction: A = A * R^-1 mod N * Montgomery reduction: A = A * R^-1 mod N
*
* See mpi_montmul() regarding constraints and guarantees on the parameters.
*/ */
static void mpi_montred( mbedtls_mpi *A, const mbedtls_mpi *N, mbedtls_mpi_uint mm, const mbedtls_mpi *T ) static void mpi_montred( mbedtls_mpi *A, const mbedtls_mpi *N, mbedtls_mpi_uint mm, const mbedtls_mpi *T )
{ {