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Base64 encoding: use ranges instead of tables

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Instead of doing constant-flow table lookup, which requires 64 memory loads
for each lookup into a 64-entry table, do a range-based calculation, which
requires more CPU instructions per range but there are only 5 ranges.

I expect a significant performance gain (although smaller than for decoding
since the encoding table is half the size), but I haven't measured. Code
size is slightly smaller.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-07-28 14:31:39 +02:00
parent 1121cd29b6
commit 2c4a3686bb
1 changed files with 25 additions and 97 deletions
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122
library/base64.c
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@ -35,87 +35,8 @@
#endif /* MBEDTLS_PLATFORM_C */
#endif /* MBEDTLS_SELF_TEST */
static const unsigned char base64_enc_map[64] =
{
'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',
'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T',
'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd',
'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x',
'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7',
'8', '9', '+', '/'
};
#define BASE64_SIZE_T_MAX ( (size_t) -1 ) /* SIZE_T_MAX is not standard */
/*
* Constant flow conditional assignment to unsigned char
*/
static void mbedtls_base64_cond_assign_uchar( unsigned char * dest, const unsigned char * const src,
unsigned char 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 0xFF or 0 */
unsigned char mask = ( condition | -condition );
mask >>= 7;
mask = -mask;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
*dest = ( ( *src ) & mask ) | ( ( *dest ) & ~mask );
}
/*
* Constant flow check for equality
*/
static unsigned char mbedtls_base64_eq( size_t in_a, size_t in_b )
{
size_t difference = in_a ^ in_b;
/* 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
difference |= -difference;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
/* cope with the varying size of size_t per platform */
difference >>= ( sizeof( difference ) * 8 - 1 );
return (unsigned char) ( 1 ^ difference );
}
/*
* Constant flow 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_uchar( &result, &table[i], mbedtls_base64_eq( i, table_index ) );
}
return result;
}
/* Return 0xff if low <= c <= high, 0 otherwise.
*
* Constant flow with respect to c.
@ -128,6 +49,24 @@ static unsigned char mask_of_range( unsigned char low, unsigned char high,
return( ~low_mask & high_mask & 0xff );
}
/* Given a value in the range 0..63, return the corresponding Base64 digit.
* The implementation assumes that letters are consecutive (e.g. ASCII
* but not EBCDIC).
*/
static unsigned char enc_char( unsigned char val )
{
unsigned char digit = 0;
/* For each range of values, if val is in that range, mask digit with
* the corresponding value. Since val can only be in a single range,
* only at most one masking will change digit. */
digit |= mask_of_range( 0, 25, val ) & ( 'A' + val );
digit |= mask_of_range( 26, 51, val ) & ( 'a' + val - 26 );
digit |= mask_of_range( 52, 61, val ) & ( '0' + val - 52 );
digit |= mask_of_range( 62, 62, val ) & '+';
digit |= mask_of_range( 63, 63, val ) & '/';
return( digit );
}
/*
* Encode a buffer into base64 format
*/
@ -168,17 +107,10 @@ int mbedtls_base64_encode( unsigned char *dst, size_t dlen, size_t *olen,
C2 = *src++;
C3 = *src++;
*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 ) );
*p++ = enc_char( ( C1 >> 2 ) & 0x3F );
*p++ = enc_char( ( ( ( C1 & 3 ) << 4 ) + ( C2 >> 4 ) ) & 0x3F );
*p++ = enc_char( ( ( ( C2 & 15 ) << 2 ) + ( C3 >> 6 ) ) & 0x3F );
*p++ = enc_char( C3 & 0x3F );
}
if( i < slen )
@ -186,15 +118,11 @@ int mbedtls_base64_encode( unsigned char *dst, size_t dlen, size_t *olen,
C1 = *src++;
C2 = ( ( i + 1 ) < slen ) ? *src++ : 0;
*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++ = enc_char( ( C1 >> 2 ) & 0x3F );
*p++ = enc_char( ( ( ( C1 & 3 ) << 4 ) + ( C2 >> 4 ) ) & 0x3F );
if( ( i + 1 ) < slen )
*p++ = mbedtls_base64_table_lookup( base64_enc_map, sizeof( base64_enc_map ),
( ( ( C2 & 15 ) << 2 ) & 0x3F ) );
*p++ = enc_char( ( ( C2 & 15 ) << 2 ) & 0x3F );
else *p++ = '=';
*p++ = '=';