yuzu-emu/mbedtls
2
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity

Base64 decoding: use ranges instead of tables

Browse source 
Instead of doing constant-flow table lookup, which requires 128 memory loads
for each lookup into a 128-entry table, do a range-based calculation, which
requires more CPU instructions per range but there are only 5 ranges.

Experimentally, this is ~12x faster on my PC (based on
programs/x509/load_roots). The code is slightly smaller, too.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-07-28 13:54:02 +02:00
parent b553eaabea
commit ab04335052
1 changed files with 42 additions and 22 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

64
library/base64.c
View file

@ -46,23 +46,6 @@ static const unsigned char base64_enc_map[64] =
'8', '9', '+', '/'
};
static const unsigned char base64_dec_map[128] =
{
127, 127, 127, 127, 127, 127, 127, 127, 127, 127,
127, 127, 127, 127, 127, 127, 127, 127, 127, 127,
127, 127, 127, 127, 127, 127, 127, 127, 127, 127,
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, 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,
29, 30, 31, 32, 33, 34, 35, 36, 37, 38,
39, 40, 41, 42, 43, 44, 45, 46, 47, 48,
49, 50, 51, 127, 127, 127, 127, 127
};
#define BASE64_SIZE_T_MAX ( (size_t) -1 ) /* SIZE_T_MAX is not standard */
/*
@ -133,6 +116,18 @@ static unsigned char mbedtls_base64_table_lookup( const unsigned char * const ta
return result;
}
/* Return 0xff if low <= c <= high, 0 otherwise.
*
* Constant flow with respect to c.
*/
static unsigned char mask_of_range( unsigned char low, unsigned char high,
unsigned char c )
{
unsigned low_mask = ( c - low ) >> 8;
unsigned high_mask = ( c - high - 1 ) >> 8;
return( ~low_mask & high_mask & 0xff );
}
/*
* Encode a buffer into base64 format
*/
@ -211,6 +206,34 @@ int mbedtls_base64_encode( unsigned char *dst, size_t dlen, size_t *olen,
return( 0 );
}
/* Given a Base64 digit, return its value.
* If c is not a Base64 digit ('A'..'Z', 'a'..'z', '0'..'9', '+' or '/'),
* return -1.
*
* The implementation assumes that letters are consecutive (e.g. ASCII
* but not EBCDIC).
*
* The implementation is constant-flow (no branch or memory access depending
* on the value of c) unless the compiler inlines and optimizes a specific
* access.
*/
static signed char dec_value( unsigned char c )
{
unsigned char val = 0;
/* For each range of digits, if c is in that range, mask val with
* the corresponding value. Since c can only be in a single range,
* only at most one masking will change val. Set val to one plus
* the desired value so that it stays 0 if c is in none of the ranges. */
val |= mask_of_range( 'A', 'Z', c ) & ( c - 'A' + 0 + 1 );
val |= mask_of_range( 'a', 'z', c ) & ( c - 'a' + 26 + 1 );
val |= mask_of_range( '0', '9', c ) & ( c - '0' + 52 + 1 );
val |= mask_of_range( '+', '+', c ) & ( c - '+' + 62 + 1 );
val |= mask_of_range( '/', '/', c ) & ( c - '/' + 63 + 1 );
/* At this point, val is 0 if c is an invalid digit and v+1 if c is
* a digit with the value v. */
return( val - 1 );
}
/*
* Decode a base64-formatted buffer
*/
@ -220,7 +243,6 @@ int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
size_t i, n;
uint32_t j, x;
unsigned char *p;
unsigned char dec_map_lookup;
/* First pass: check for validity and get output length */
for( i = n = j = 0; i < slen; i++ )
@ -260,8 +282,7 @@ int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
{
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 )
if( dec_value( src[i] ) < 0 )
return( MBEDTLS_ERR_BASE64_INVALID_CHARACTER );
}
n++;
@ -298,8 +319,7 @@ int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
}
else
{
dec_map_lookup = mbedtls_base64_table_lookup( base64_dec_map, sizeof( base64_dec_map ), *src );
x = ( x << 6 ) | ( dec_map_lookup & 0x3F );
x = ( x << 6 ) | ( dec_value( *src ) & 0x3F );
}
if( ++n == 4 )