rsa: Verify RSA padding programatically
Padding verification was done against static SHA/RSA pair arrays which
take up a lot of static memory, are mostly 0xff, and cannot be reused
for additional SHA/RSA pairings. The padding can be easily computed
according to PKCS#1v2.1 as:
EM = 0x00 || 0x01 || PS || 0x00 || T
where PS is (emLen - tLen - 3) octets of 0xff and T is DER encoding
of the hash.
Store DER prefix in checksum_algo and create rsa_verify_padding
function to handle verification of a message for any SHA/RSA pairing.
Signed-off-by: Andrew Duda <aduda@meraki.com>
Signed-off-by: aduda <aduda@meraki.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/rsa/rsa-verify.c b/lib/rsa/rsa-verify.c
index 5418f59..ee8988d 100644
--- a/lib/rsa/rsa-verify.c
+++ b/lib/rsa/rsa-verify.c
@@ -25,6 +25,40 @@
#define RSA_DEFAULT_PUBEXP 65537
/**
+ * rsa_verify_padding() - Verify RSA message padding is valid
+ *
+ * Verify a RSA message's padding is consistent with PKCS1.5
+ * padding as described in the RSA PKCS#1 v2.1 standard.
+ *
+ * @msg: Padded message
+ * @pad_len: Number of expected padding bytes
+ * @algo: Checksum algo structure having information on DER encoding etc.
+ * @return 0 on success, != 0 on failure
+ */
+static int rsa_verify_padding(const uint8_t *msg, const int pad_len,
+ struct checksum_algo *algo)
+{
+ int ff_len;
+ int ret;
+
+ /* first byte must be 0x00 */
+ ret = *msg++;
+ /* second byte must be 0x01 */
+ ret |= *msg++ ^ 0x01;
+ /* next ff_len bytes must be 0xff */
+ ff_len = pad_len - algo->der_len - 3;
+ ret |= *msg ^ 0xff;
+ ret |= memcmp(msg, msg+1, ff_len-1);
+ msg += ff_len;
+ /* next byte must be 0x00 */
+ ret |= *msg++;
+ /* next der_len bytes must match der_prefix */
+ ret |= memcmp(msg, algo->der_prefix, algo->der_len);
+
+ return ret;
+}
+
+/**
* rsa_verify_key() - Verify a signature against some data using RSA Key
*
* Verify a RSA PKCS1.5 signature against an expected hash using
@@ -83,11 +117,11 @@
return ret;
}
- padding = algo->rsa_padding;
pad_len = algo->key_len - algo->checksum_len;
/* Check pkcs1.5 padding bytes. */
- if (memcmp(buf, padding, pad_len)) {
+ ret = rsa_verify_padding(buf, pad_len, algo);
+ if (ret) {
debug("In RSAVerify(): Padding check failed!\n");
return -EINVAL;
}