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mbedtls_mpi_gcd: fix the case B==0
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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ChangeLog.d/mpi_gcd-0.txt
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4
ChangeLog.d/mpi_gcd-0.txt
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@ -0,0 +1,4 @@
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Bugfix
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* Fix mbedtls_mpi_gcd(G,A,B) when the value of B is zero. This had no
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effect on Mbed TLS's internal use of mbedtls_mpi_gcd(), but may affect
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applications that call mbedtls_mpi_gcd() directly. Fixes #4642.
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@ -2391,6 +2391,16 @@ int mbedtls_mpi_gcd( mbedtls_mpi *G, const mbedtls_mpi *A, const mbedtls_mpi *B
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lz = mbedtls_mpi_lsb( &TA );
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lzt = mbedtls_mpi_lsb( &TB );
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/* The loop below gives the correct result when A==0 but not when B==0.
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* So have a special case for B==0. Leverage the fact that we just
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* calculated the lsb and lsb(B)==0 iff B is odd or 0 to make the test
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* slightly more efficient than cmp_int(). */
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if( lzt == 0 && mbedtls_mpi_get_bit( &TB, 0 ) == 0 )
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{
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ret = mbedtls_mpi_copy( G, A );
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goto cleanup;
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}
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if( lzt < lz )
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lz = lzt;
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