Attempt to make Base64 table access constant flow
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>
diff --git a/library/base64.c b/library/base64.c
index d39474a..0b4ed56 100644
--- a/library/base64.c
+++ b/library/base64.c
@@ -65,6 +65,44 @@
#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
*/
@@ -105,10 +143,17 @@
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 )
@@ -116,11 +161,15 @@
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++ = '=';
@@ -171,7 +220,8 @@
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 )
@@ -204,8 +254,9 @@
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 )
{
