Merged constant-time padding checks
diff --git a/ChangeLog b/ChangeLog
index c15c2ab..a4fd191 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,9 @@
* Added optional optimization for NIST MODP curves (POLARSSL_ECP_NIST_OPTIM)
* Support for Camellia-GCM mode and ciphersuites
+Changes
+ * Padding checks in cipher layer are now constant-time
+
Bugfix
* More stringent checks in cipher layer
* Server does not send out extensions not advertised by client
diff --git a/library/cipher.c b/library/cipher.c
index 495bd51..3ed6830 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -416,7 +416,7 @@
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;
@@ -425,23 +425,26 @@
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 */
@@ -463,20 +466,24 @@
static int get_one_and_zeros_padding( unsigned char *input, size_t input_len,
size_t *data_len )
{
- unsigned char *p = input + input_len - 1;
+ size_t i;
+ unsigned char done = 0, prev_done, bad;
if( NULL == input || NULL == data_len )
return POLARSSL_ERR_CIPHER_BAD_INPUT_DATA;
- while( *p == 0x00 && p > input )
- --p;
+ bad = 0xFF;
+ *data_len = 0;
+ for( i = input_len; i > 0; i-- )
+ {
+ prev_done = done;
+ done |= ( input[i-1] != 0 );
+ *data_len |= ( i - 1 ) * ( done != prev_done );
+ bad &= ( input[i-1] ^ 0x80 ) | ( done == prev_done );
+ }
- if( *p != 0x80 )
- return POLARSSL_ERR_CIPHER_INVALID_PADDING;
+ return POLARSSL_ERR_CIPHER_INVALID_PADDING * (bad != 0);
- *data_len = p - input;
-
- return 0;
}
#endif /* POLARSSL_CIPHER_PADDING_ONE_AND_ZEROS */
@@ -498,23 +505,25 @@
static int get_zeros_and_len_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 - 1; i++ )
- if( input[i] != 0x00 )
- 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 */
+ 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 */
@@ -534,14 +543,19 @@
static int get_zeros_padding( unsigned char *input, size_t input_len,
size_t *data_len )
{
- unsigned char *p = input + input_len - 1;
+ size_t i;
+ unsigned char done = 0, prev_done;
+
if( NULL == input || NULL == data_len )
return POLARSSL_ERR_CIPHER_BAD_INPUT_DATA;
- while( *p == 0x00 && p > input )
- --p;
-
- *data_len = *p == 0x00 ? 0 : p - input + 1;
+ *data_len = 0;
+ for( i = input_len; i > 0; i-- )
+ {
+ prev_done = done;
+ done |= ( input[i-1] != 0 );
+ *data_len |= i * ( done != prev_done );
+ }
return 0;
}
diff --git a/tests/suites/test_suite_cipher.padding.data b/tests/suites/test_suite_cipher.padding.data
index 2cad0a9..8776a28 100644
--- a/tests/suites/test_suite_cipher.padding.data
+++ b/tests/suites/test_suite_cipher.padding.data
@@ -82,10 +82,70 @@
depends_on:POLARSSL_CIPHER_PADDING_PKCS7
check_padding:POLARSSL_PADDING_PKCS7:"DABBAD0002":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
-Check PKCS padding #7 (non-uniform padding bytes)
+Check PKCS padding #7 (non-uniform padding bytes #1)
depends_on:POLARSSL_CIPHER_PADDING_PKCS7
check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00030203":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+Check PKCS padding #7 (non-uniform padding bytes #2)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00030103":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #3)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00030703":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #4)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00030b03":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #5)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00031303":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #6)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00032303":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #7)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00034203":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #8)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00038303":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #9)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00020303":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #10)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00010303":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #11)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00070303":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #12)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD000b0303":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #13)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00130303":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #14)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00230303":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #15)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00420303":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
+Check PKCS padding #7 (non-uniform padding bytes #16)
+depends_on:POLARSSL_CIPHER_PADDING_PKCS7
+check_padding:POLARSSL_PADDING_PKCS7:"DABBAD00830303":POLARSSL_ERR_CIPHER_INVALID_PADDING:0
+
Check PKCS padding #8 (overlong)
depends_on:POLARSSL_CIPHER_PADDING_PKCS7
check_padding:POLARSSL_PADDING_PKCS7:"040404":POLARSSL_ERR_CIPHER_INVALID_PADDING:0