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Make mbedtls_mpi_lt_mpi_ct more portable

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The code relied on the assumptions that CHAR_BIT is 8 and that unsigned
does not have padding bits.

In the Bignum module we already assume that the sign of an MPI is either
-1 or 1. Using this, we eliminate the above mentioned dependency.
This commit is contained in:
Janos Follath 2019-10-28 12:12:15 +00:00
parent 9741fa6e2b
commit 51ed14e20f
1 changed files with 4 additions and 5 deletions
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9
library/bignum.c
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@ -967,12 +967,11 @@ int mbedtls_mpi_lt_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y,
return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
/*
* Get sign bits of the signs.
* Set sign_N to 1 if N >= 0, 0 if N < 0.
* We know that N->s == 1 if N >= 0 and N->s == -1 if N < 0.
*/
sign_X = X->s;
sign_X = sign_X >> ( sizeof( unsigned ) * 8 - 1 );
sign_Y = Y->s;
sign_Y = sign_Y >> ( sizeof( unsigned ) * 8 - 1 );
sign_X = ( X->s & 2 ) >> 1;
sign_Y = ( Y->s & 2 ) >> 1;
/*
* If the signs are different, then the positive operand is the bigger.