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Use branch-free size comparison for the padding size

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In mbedtls_rsa_rsaes_pkcs1_v15_decrypt, use size_greater_than (which
is based on bitwise operations) instead of the < operator to compare
sizes when the values being compared must not leak. Some compilers
compile < to a branch at least under some circumstances (observed with
gcc 5.4 for arm-gnueabi -O9 on a toy program).
This commit is contained in:
Gilles Peskine 2018-10-04 21:24:21 +02:00
parent a1af5c8ea2
commit 8c9440a2cb
1 changed files with 4 additions and 5 deletions
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9
library/rsa.c
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@ -1541,7 +1541,7 @@ int mbedtls_rsa_rsaes_pkcs1_v15_decrypt( mbedtls_rsa_context *ctx,
}
/* There must be at least 8 bytes of padding. */
bad |= ( pad_count < 8 );
bad |= size_greater_than( 8, pad_count );
/* If the padding is valid, set plaintext_size to the number of
* remaining bytes after stripping the padding. If the padding
@ -1555,10 +1555,9 @@ int mbedtls_rsa_rsaes_pkcs1_v15_decrypt( mbedtls_rsa_context *ctx,
(unsigned) ( ilen - ( p - buf ) ) );
/* Set output_too_large to 0 if the plaintext fits in the output
* buffer and to 1 otherwise. This is the sign bit (1 for negative)
* of (output_max_len - plaintext_size). */
output_too_large = ( ( output_max_len - plaintext_size ) >>
( ( sizeof( output_max_len ) * 8 - 1 ) ) );
* buffer and to 1 otherwise. */
output_too_large = size_greater_than( plaintext_size,
plaintext_max_size );
/* Set ret without branches to avoid timing attacks. Return:
* - INVALID_PADDING if the padding is bad (bad != 0).