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Base64 decoding: don't use the table for '='

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Base64 decoding uses equality comparison tests for characters that don't
leak information about the content of the data other than its length, such
as whitespace. Do this with '=' as well, since it only reveals information
about the length. This way the table lookup can focus on character validity
and decoding value.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-07-28 11:33:04 +02:00
parent 82c04329bf
commit 6b541a026b
1 changed files with 25 additions and 39 deletions
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64
library/base64.c
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@ -54,7 +54,7 @@ static const unsigned char base64_dec_map[128] =
127, 127, 127, 127, 127, 127, 127, 127, 127, 127,
127, 127, 127, 62, 127, 127, 127, 63, 52, 53,
54, 55, 56, 57, 58, 59, 60, 61, 127, 127,
127, 64, 127, 127, 127, 0, 1, 2, 3, 4,
127, 127, 127, 127, 127, 0, 1, 2, 3, 4,
5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
25, 127, 127, 127, 127, 127, 127, 26, 27, 28,
@ -90,31 +90,6 @@ static void mbedtls_base64_cond_assign_uchar( unsigned char * dest, const unsign
*dest = ( ( *src ) & mask ) | ( ( *dest ) & ~mask );
}
/*
* Constant flow conditional assignment to uint_32
*/
static void mbedtls_base64_cond_assign_uint32( uint32_t * dest, const uint32_t src,
uint32_t condition )
{
/* MSVC has a warning about unary minus on unsigned integer types,
* but this is well-defined and precisely what we want to do here. */
#if defined(_MSC_VER)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
/* Generate bitmask from condition, mask will either be 0xFFFFFFFF or 0 */
uint32_t mask = ( condition | -condition );
mask >>= 31;
mask = -mask;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
*dest = ( src & mask ) | ( ( *dest ) & ~mask );
}
/*
* Constant flow check for equality
*/
@ -273,17 +248,22 @@ int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
if( x != 0 )
return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
if( src[i] == '=' && ++j > 2 )
return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
dec_map_lookup = mbedtls_base64_table_lookup( base64_dec_map, sizeof( base64_dec_map ), src[i] );
if( src[i] > 127 || dec_map_lookup == 127 )
return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
if( dec_map_lookup < 64 && j != 0 )
if( src[i] > 127 )
return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
if( src[i] == '=' )
{
if( ++j > 2 )
return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
}
else
{
if( j != 0 )
return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
dec_map_lookup = mbedtls_base64_table_lookup( base64_dec_map, sizeof( base64_dec_map ), src[i] );
if( dec_map_lookup == 127 )
return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
}
n++;
}
@ -311,10 +291,16 @@ int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
if( *src == '\r' || *src == '\n' || *src == ' ' )
continue;
dec_map_lookup = mbedtls_base64_table_lookup( base64_dec_map, sizeof( base64_dec_map ), *src );
mbedtls_base64_cond_assign_uint32( &j, j - 1, mbedtls_base64_eq( dec_map_lookup, 64 ) );
x = ( x << 6 ) | ( dec_map_lookup & 0x3F );
if( *src == '=' )
{
--j;
x = x << 6;
}
else
{
dec_map_lookup = mbedtls_base64_table_lookup( base64_dec_map, sizeof( base64_dec_map ), *src );
x = ( x << 6 ) | ( dec_map_lookup & 0x3F );
}
if( ++n == 4 )
{