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Make get_zeros_and_len_padding() constant-time

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Manuel Pégourié-Gonnard 2013-10-27 17:32:43 +01:00
parent f8ab069d6a
commit d17df51277
1 changed files with 12 additions and 10 deletions
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22
library/cipher.c
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@ -491,23 +491,25 @@ static void add_zeros_and_len_padding( unsigned char *output,
static int get_zeros_and_len_padding( unsigned char *input, size_t input_len, static int get_zeros_and_len_padding( unsigned char *input, size_t input_len,
size_t *data_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 ) if( NULL == input || NULL == data_len )
return POLARSSL_ERR_CIPHER_BAD_INPUT_DATA; return POLARSSL_ERR_CIPHER_BAD_INPUT_DATA;
padding_len = input[input_len - 1]; 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 - 1; i++ )
if( input[i] != 0x00 )
return POLARSSL_ERR_CIPHER_INVALID_PADDING;
*data_len = input_len - padding_len; *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 */
pad_idx = input_len - padding_len;
for( i = 0; i < input_len - 1; i++ )
bad |= input[i] * ( i >= pad_idx );
return POLARSSL_ERR_CIPHER_INVALID_PADDING * (bad != 0);
} }
#endif /* POLARSSL_CIPHER_PADDING_ZEROS_AND_LEN */ #endif /* POLARSSL_CIPHER_PADDING_ZEROS_AND_LEN */