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Make get_pkcs_padding() constant-time

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This commit is contained in:
Manuel Pégourié-Gonnard 2013-10-27 17:21:14 +01:00
parent 6888167e73
commit f8ab069d6a
2 changed files with 75 additions and 12 deletions
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25
library/cipher.c
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@ -406,7 +406,7 @@ static void add_pkcs_padding( unsigned char *output, size_t output_len,
size_t data_len )
{
size_t padding_len = output_len - data_len;
unsigned char i = 0;
unsigned char i;
for( i = 0; i < padding_len; i++ )
output[data_len + i] = (unsigned char) padding_len;
@ -415,23 +415,26 @@ static void add_pkcs_padding( unsigned char *output, size_t output_len,
static int get_pkcs_padding( unsigned char *input, size_t input_len,
size_t *data_len )
{
size_t i, padding_len = 0;
size_t i, pad_idx;
unsigned char padding_len, bad = 0;
if( NULL == input || NULL == data_len )
return POLARSSL_ERR_CIPHER_BAD_INPUT_DATA;
padding_len = input[input_len - 1];
if( padding_len > input_len || padding_len == 0 )
return POLARSSL_ERR_CIPHER_INVALID_PADDING;
for( i = input_len - padding_len; i < input_len; i++ )
if( input[i] != padding_len )
return POLARSSL_ERR_CIPHER_INVALID_PADDING;
*data_len = input_len - padding_len;
return 0;
/* Avoid logical || since it results in a branch */
bad |= padding_len > input_len;
bad |= padding_len == 0;
/* The number of bytes checked must be independent of padding_len,
* so pick input_len, which is usually 8 or 16 (one block) */
pad_idx = input_len - padding_len;
for( i = 0; i < input_len; i++ )
bad |= ( input[i] ^ padding_len ) * ( i >= pad_idx );
return POLARSSL_ERR_CIPHER_INVALID_PADDING * (bad != 0);
}
#endif /* POLARSSL_CIPHER_PADDING_PKCS7 */