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Attempt to make Base64 table access constant flow

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Add constant flow table access code, and use that exclusively to access
the base64 lookup table

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
This commit is contained in:
Paul Elliott 2021-02-05 17:49:23 +00:00
parent bcfa41753d
commit 738d2310a7
1 changed files with 61 additions and 10 deletions
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71
library/base64.c
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@ -96,6 +96,44 @@ static const unsigned char base64_dec_map[128] =
#define BASE64_SIZE_T_MAX ( (size_t) -1 ) /* SIZE_T_MAX is not standard */
static void mbedtls_base64_cond_assign(unsigned char * dest, const unsigned char * const src,
unsigned char condition)
{
/* make sure assign is 0 or 1 in a time-constant manner */
condition = (condition | (unsigned char)-condition) >> 7;
*dest = ( *dest ) * ( 1 - condition ) + ( *src ) * condition;
}
/*
* Constant time check for equality
*/
static unsigned char mbedtls_base64_eq(uint32_t in_a, uint32_t in_b)
{
uint32_t difference = in_a ^ in_b;
difference |= -difference;
difference >>= 31;
return (unsigned char) ( 1 ^ difference );
}
/*
* Constant time lookup into table.
*/
static unsigned char mbedtls_base64_table_lookup(const unsigned char * const table,
const size_t table_size, const size_t table_index)
{
size_t i;
unsigned char result = 0;
for( i = 0; i < table_size; ++i )
{
mbedtls_base64_cond_assign(&result, &table[i], mbedtls_base64_eq(i, table_index));
}
return result;
}
/*
* Encode a buffer into base64 format
*/
@ -136,10 +174,17 @@ int mbedtls_base64_encode( unsigned char *dst, size_t dlen, size_t *olen,
C2 = *src++;
C3 = *src++;
*p++ = base64_enc_map[(C1 >> 2) & 0x3F];
*p++ = base64_enc_map[(((C1 & 3) << 4) + (C2 >> 4)) & 0x3F];
*p++ = base64_enc_map[(((C2 & 15) << 2) + (C3 >> 6)) & 0x3F];
*p++ = base64_enc_map[C3 & 0x3F];
*p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
( ( C1 >> 2 ) & 0x3F ) );
*p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
( ( ( ( C1 & 3 ) << 4 ) + ( C2 >> 4 ) ) & 0x3F ) );
*p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
( ( ( ( C2 & 15 ) << 2 ) + ( C3 >> 6 ) ) & 0x3F ) );
*p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
( C3 & 0x3F ) );
}
if( i < slen )
@ -147,11 +192,15 @@ int mbedtls_base64_encode( unsigned char *dst, size_t dlen, size_t *olen,
C1 = *src++;
C2 = ( ( i + 1 ) < slen ) ? *src++ : 0;
*p++ = base64_enc_map[(C1 >> 2) & 0x3F];
*p++ = base64_enc_map[(((C1 & 3) << 4) + (C2 >> 4)) & 0x3F];
*p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
( ( C1 >> 2 ) & 0x3F ) );
*p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
( ( ( ( C1 & 3 ) << 4 ) + ( C2 >> 4 ) ) & 0x3F ) );
if( ( i + 1 ) < slen )
*p++ = base64_enc_map[((C2 & 15) << 2) & 0x3F];
*p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
( ( ( C2 & 15 ) << 2 ) & 0x3F ) );
else *p++ = '=';
*p++ = '=';
@ -202,7 +251,8 @@ int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
if( src[i] == '=' && ++j > 2 )
return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
if( src[i] > 127 || base64_dec_map[src[i]] == 127 )
if( src[i] > 127 ||
mbedtls_base64_table_lookup( base64_dec_map, sizeof( base64_dec_map ), src[i] ) == 127 )
return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
if( base64_dec_map[src[i]] < 64 && j != 0 )
@ -235,8 +285,9 @@ int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
if( *src == '\r' || *src == '\n' || *src == ' ' )
continue;
j -= ( base64_dec_map[*src] == 64 );
x = ( x << 6 ) | ( base64_dec_map[*src] & 0x3F );
j -= ( mbedtls_base64_table_lookup(base64_dec_map, sizeof( base64_dec_map ), *src ) == 64 );
x = ( x << 6 ) |
( mbedtls_base64_table_lookup( base64_dec_map, sizeof( base64_dec_map ), *src ) & 0x3F );
if( ++n == 4 )
{