Fix Lucky 13 cache attack on MD/SHA padding
The basis for the Lucky 13 family of attacks is for an attacker to be able to
distinguish between (long) valid TLS-CBC padding and invalid TLS-CBC padding.
Since our code sets padlen = 0 for invalid padding, the length of the input to
the HMAC function gives information about that.
Information about this length (modulo the MD/SHA block size) can be deduced
from how much MD/SHA padding (this is distinct from TLS-CBC padding) is used.
If MD/SHA padding is read from a (static) buffer, a local attacker could get
information about how much is used via a cache attack targeting that buffer.
Let's get rid of this buffer. Now the only buffer used is the internal MD/SHA
one, which is always read fully by the process() function.
diff --git a/library/md5.c b/library/md5.c
index 8238c2b..2a740cd 100644
--- a/library/md5.c
+++ b/library/md5.c
@@ -309,14 +309,6 @@
}
#endif
-static const unsigned char md5_padding[64] =
-{
- 0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
- 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
- 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
- 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
-};
-
/*
* MD5 final digest
*/
@@ -324,26 +316,48 @@
unsigned char output[16] )
{
int ret;
- uint32_t last, padn;
+ uint32_t used;
uint32_t high, low;
- unsigned char msglen[8];
+ /*
+ * Add padding: 0x80 then 0x00 until 8 bytes remain for the length
+ */
+ used = ctx->total[0] & 0x3F;
+
+ ctx->buffer[used++] = 0x80;
+
+ if( used <= 56 )
+ {
+ /* Enough room for padding + length in current block */
+ memset( ctx->buffer + used, 0, 56 - used );
+ }
+ else
+ {
+ /* We'll need an extra block */
+ memset( ctx->buffer + used, 0, 64 - used );
+
+ if( ( ret = mbedtls_internal_md5_process( ctx, ctx->buffer ) ) != 0 )
+ return( ret );
+
+ memset( ctx->buffer, 0, 56 );
+ }
+
+ /*
+ * Add message length
+ */
high = ( ctx->total[0] >> 29 )
| ( ctx->total[1] << 3 );
low = ( ctx->total[0] << 3 );
- PUT_UINT32_LE( low, msglen, 0 );
- PUT_UINT32_LE( high, msglen, 4 );
+ PUT_UINT32_LE( low, ctx->buffer, 56 );
+ PUT_UINT32_LE( high, ctx->buffer, 60 );
- last = ctx->total[0] & 0x3F;
- padn = ( last < 56 ) ? ( 56 - last ) : ( 120 - last );
+ if( ( ret = mbedtls_internal_md5_process( ctx, ctx->buffer ) ) != 0 )
+ return( ret );
- if( ( ret = mbedtls_md5_update_ret( ctx, md5_padding, padn ) ) != 0 )
- return( ret );
-
- if( ( ret = mbedtls_md5_update_ret( ctx, msglen, 8 ) ) != 0 )
- return( ret );
-
+ /*
+ * Output final state
+ */
PUT_UINT32_LE( ctx->state[0], output, 0 );
PUT_UINT32_LE( ctx->state[1], output, 4 );
PUT_UINT32_LE( ctx->state[2], output, 8 );
