From d6e3f46daaae078c94d1828ab13f6e45326e560c Mon Sep 17 00:00:00 2001 From: Gilles Peskine Date: Wed, 28 Jul 2021 14:31:39 +0200 Subject: [PATCH] Base64 encoding: use ranges instead of tables 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 --- library/base64.c | 122 ++++++++++------------------------------------- 1 file changed, 25 insertions(+), 97 deletions(-) diff --git a/library/base64.c b/library/base64.c index 960f778ca..832484f98 100644 --- a/library/base64.c +++ b/library/base64.c @@ -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++ = '=';