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Document some internal bignum functions
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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@ -1328,7 +1328,8 @@ cleanup:
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}
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/*
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* Helper for mbedtls_mpi subtraction
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* Helper for mbedtls_mpi subtraction:
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* d -= s where d and s have the same size and d >= s.
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*/
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static void mpi_sub_hlp( size_t n,
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const mbedtls_mpi_uint *s,
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@ -1977,8 +1978,27 @@ static void mpi_montg_init( mbedtls_mpi_uint *mm, const mbedtls_mpi *N )
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*mm = ~x + 1;
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}
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/*
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* Montgomery multiplication: A = A * B * R^-1 mod N (HAC 14.36)
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/** Montgomery multiplication: A = A * B * R^-1 mod N (HAC 14.36)
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*
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* \param[in,out] A One of the numbers to multiply.
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* It must have at least one more limb than N
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* (A->n >= N->n + 1).
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* On successful completion, A contains the result of
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* the multiplication A * B * R^-1 mod N where
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* R = (2^ciL)^n.
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* \param[in] B One of the numbers to multiply.
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* It must be nonzero and must not have more limbs than N
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* (B->n <= N->n).
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* \param[in] N The modulo. N must be odd.
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* \param mm The value calculated by `mpi_montg_init(&mm, N)`.
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* This is -N^-1 mod 2^ciL.
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* \param[in,out] T A bignum for temporary storage.
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* It must be at least twice the limb size of N plus 2
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* (T->n >= 2 * (N->n + 1)).
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* Its initial content is unused and
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* its final content is indeterminate.
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* Note that unlike the usual convention in the library
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* for `const mbedtls_mpi*`, the content of T can change.
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*/
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static void mpi_montmul( mbedtls_mpi *A, const mbedtls_mpi *B, const mbedtls_mpi *N, mbedtls_mpi_uint mm,
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const mbedtls_mpi *T )
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@ -2008,6 +2028,8 @@ static void mpi_montmul( mbedtls_mpi *A, const mbedtls_mpi *B, const mbedtls_mpi
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memcpy( A->p, d, ( n + 1 ) * ciL );
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/* If A >= N then A -= N. Do the subtraction unconditionally to prevent
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* timing attacks. Modify T as a side effect. */
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if( mbedtls_mpi_cmp_abs( A, N ) >= 0 )
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mpi_sub_hlp( n, N->p, A->p );
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else
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@ -2017,6 +2039,8 @@ static void mpi_montmul( mbedtls_mpi *A, const mbedtls_mpi *B, const mbedtls_mpi
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/*
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* Montgomery reduction: A = A * R^-1 mod N
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*
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* See mpi_montmul() regarding constraints and guarantees on the parameters.
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*/
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static void mpi_montred( mbedtls_mpi *A, const mbedtls_mpi *N,
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mbedtls_mpi_uint mm, const mbedtls_mpi *T )
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