Add comment about ensuring no other data follows the hash value in PKCS #1

Change-Id: I92419b7d2d8ded8f4868588ad3c24b70ac7f7b1b
Reviewed-on: https://boringssl-review.googlesource.com/14864
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/rsa/rsa.c

diff --git a/crypto/rsa/rsa.c b/crypto/rsa/rsa.c
index 6d56238..b26737d 100644
--- a/crypto/rsa/rsa.c
+++ b/crypto/rsa/rsa.c
@@ -525,6 +525,8 @@
     goto out;
   }
 
+  /* Check that no other information follows the hash value (FIPS 186-4 Section
+   * 5.5) and it matches the expected hash. */
   if (len != signed_msg_len || OPENSSL_memcmp(buf, signed_msg, len) != 0) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_BAD_SIGNATURE);
     goto out;