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Update comments following code review

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Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
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Tom Cosgrove
2022-08-30 14:31:49 +01:00
parent b496486cdc
commit 5dd97e60d5
3 changed files with 16 additions and 9 deletions
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9
library/bignum_core.c
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@ -333,6 +333,11 @@ mbedtls_mpi_uint mbedtls_mpi_core_mla( mbedtls_mpi_uint *d, size_t d_len,
mbedtls_mpi_uint b )
{
mbedtls_mpi_uint c = 0; /* carry */
/*
* It is a documented precondition of this function that d_len >= s_len.
* If that's not the case, we swap these round: this turns what would be
* a buffer overflow into an incorrect result.
*/
if( d_len < s_len )
s_len = d_len;
size_t excess_len = d_len - s_len;
@ -418,9 +423,7 @@ void mbedtls_mpi_core_montmul( mbedtls_mpi_uint *X,
* i.e. (carry, borrow) of (0, 0) => return X
* (0, 1) => return T
*
* We've confirmed that the unit tests exercise this function with all 3 of
* the valid (carry, borrow) combinations (listed above), and that we don't
* see (carry, borrow) = (1, 0).
* (carry, borrow) = (1, 0) can't happen.
*
* So the correct return value is already in X if (carry ^ borrow) = 0,
* but is in (the lower AN_limbs limbs of) T if (carry ^ borrow) = 1.