Add check that message length to _pkcs1_sec_decrypt is valid.

* pkcs1-sec-decrypt.c (_pkcs1_sec_decrypt): Check that message
length is valid, for given key size.
* testsuite/rsa-sec-decrypt-test.c (test_main): Add test cases for
calls to rsa_sec_decrypt specifying a too large message length.

(cherry picked from commit 7616541e6eff73353bf682c62e3a68e4fe696707)

1 files changed, 16 insertions, 1 deletions

diff --git a/testsuite/rsa-sec-decrypt-test.c b/testsuite/rsa-sec-decrypt-test.c
index fb0ed3a1..3419322e 100644
--- a/testsuite/rsa-sec-decrypt-test.c
+++ b/testsuite/rsa-sec-decrypt-test.c
@@ -55,6 +55,7 @@ rsa_decrypt_for_test(const struct rsa_public_key *pub,
 #endif
 
 #define PAYLOAD_SIZE 50
+#define DECRYPTED_SIZE 256
 void
 test_main(void)
 {
@@ -63,7 +64,7 @@ test_main(void)
   struct knuth_lfib_ctx random_ctx;
 
   uint8_t plaintext[PAYLOAD_SIZE];
-  uint8_t decrypted[PAYLOAD_SIZE];
+  uint8_t decrypted[DECRYPTED_SIZE];
   uint8_t verifybad[PAYLOAD_SIZE];
   unsigned n_size = 1024;
   mpz_t gibberish;
@@ -99,6 +100,20 @@ test_main(void)
                                     PAYLOAD_SIZE, decrypted, gibberish) == 1);
       ASSERT (MEMEQ (PAYLOAD_SIZE, plaintext, decrypted));
 
+      ASSERT (pub.size > 10);
+      ASSERT (pub.size <= DECRYPTED_SIZE);
+
+      /* Check that too large message length is rejected, largest
+	 valid size is pub.size - 11. */
+      ASSERT (!rsa_decrypt_for_test (&pub, &key, &random_ctx,
+				     (nettle_random_func *) knuth_lfib_random,
+				     pub.size - 10, decrypted, gibberish));
+
+      /* This case used to result in arithmetic underflow and a crash. */
+      ASSERT (!rsa_decrypt_for_test (&pub, &key, &random_ctx,
+				     (nettle_random_func *) knuth_lfib_random,
+				     pub.size, decrypted, gibberish));
+
       /* bad one */
       memcpy(decrypted, verifybad, PAYLOAD_SIZE);
       nettle_mpz_random_size(garbage, &random_ctx,