tests: fix negative private key sync test. (envoyproxy#8264)

Description:

Fix a test flake.

If crypto error option is set in the signing test, just leave the digest to zeroes instead of trying to modify it. The digest modification might just cause ASN object structure changes.

BoringSSL source appears to have a similar test where the ASN structure is parsed before modification:
https://github.com/google/boringssl/blob/a7d9ac2af4684747c4524cbeba9737b04dce3e3e/crypto/fipsmodule/ecdsa/ecdsa_test.cc#L143

Risk Level: low
Testing: `bazel test //test/extensions/transport_sockets/tls:ssl_socket_test --runs_per_test=1000`
Docs Changes: N/A
Release Notes: N/A
Fixes: envoyproxy#8255 

Signed-off-by: Ismo Puustinen <[email protected]>

test/extensions/transport_sockets/tls/test_private_key_method_provider.cc

@@ -99,8 +99,9 @@ static ssl_private_key_result_t rsaPrivateKeySign(SSL* ssl, uint8_t* out, size_t
                                                   const uint8_t* in, size_t in_len) {
   TestPrivateKeyConnection* ops = static_cast<TestPrivateKeyConnection*>(
       SSL_get_ex_data(ssl, TestPrivateKeyMethodProvider::rsaConnectionIndex()));
-  unsigned char hash[EVP_MAX_MD_SIZE];
-  unsigned int hash_len;
+  unsigned char hash[EVP_MAX_MD_SIZE] = {0};
+  unsigned int hash_len = EVP_MAX_MD_SIZE;
+  std::vector<uint8_t> in2;
 
   if (!ops) {
     return ssl_private_key_failure;
@@ -119,7 +120,17 @@ static ssl_private_key_result_t rsaPrivateKeySign(SSL* ssl, uint8_t* out, size_t
     return ssl_private_key_failure;
   }
 
-  if (!calculateDigest(md, in, in_len, hash, &hash_len)) {
+  in2.assign(in, in + in_len);
+
+  // If crypto error is set, we'll modify the incoming token by flipping
+  // the bits.
+  if (ops->test_options_.crypto_error_) {
+    for (size_t i = 0; i < in_len; i++) {
+      in2[i] = ~in2[i];
+    }
+  }
+
+  if (!calculateDigest(md, in2.data(), in_len, hash, &hash_len)) {
     return ssl_private_key_failure;
   }
 
@@ -128,14 +139,11 @@ static ssl_private_key_result_t rsaPrivateKeySign(SSL* ssl, uint8_t* out, size_t
     return ssl_private_key_failure;
   }
 
-  if (ops->test_options_.crypto_error_) {
-    // Flip the bits in the first byte of the digest so that the handshake will fail.
-    hash[0] ^= hash[0];
-  }
-
   // Perform RSA signing.
   if (SSL_is_signature_algorithm_rsa_pss(signature_algorithm)) {
-    RSA_sign_pss_mgf1(rsa, out_len, out, max_out, hash, hash_len, md, nullptr, -1);
+    if (!RSA_sign_pss_mgf1(rsa, out_len, out, max_out, hash, hash_len, md, nullptr, -1)) {
+      return ssl_private_key_failure;
+    }
   } else {
     unsigned int out_len_unsigned;
     if (!RSA_sign(EVP_MD_type(md), hash, hash_len, out, &out_len_unsigned, rsa)) {