Merge #20248: test: fix length of R check in key_signature_tests

8989577 Fix length of R check in test/key_tests.cpp:key_signature_tests (Dmitry Petukhov)

Pull request description:

  The code before the fix only checked the length of R value of the last
  signature in the loop, and only for equality (but the length can be
  less than 32)

  The fixed code checks that length of the R value is less than or equal
  to 32 on each iteration of the loop

ACKs for top commit:
  laanwj:
    ACK 8989577

Tree-SHA512: 73eb2bb4a6c1c5fc11dd16851b28b43037ac06ef8cfc3b1f957429a1fca295c9422d67ec6c73c0e4bb4919f92e22dc5d733e84840b0d01ad1a529aa20d906ebf

Author: laanwj

src/test/key_tests.cpp

@@ -172,20 +172,30 @@ BOOST_AUTO_TEST_CASE(key_signature_tests)
     }
     BOOST_CHECK(found);
 
-    // When entropy is not specified, we should always see low R signatures that are less than 70 bytes in 256 tries
+    // When entropy is not specified, we should always see low R signatures that are less than or equal to 70 bytes in 256 tries
+    // The low R signatures should always have the value of their "length of R" byte less than or equal to 32
     // We should see at least one signature that is less than 70 bytes.
-    found = true;
     bool found_small = false;
+    bool found_big = false;
+    bool bad_sign = false;
     for (int i = 0; i < 256; ++i) {
         sig.clear();
         std::string msg = "A message to be signed" + ToString(i);
         msg_hash = Hash(msg);
-        BOOST_CHECK(key.Sign(msg_hash, sig));
-        found = sig[3] == 0x20;
-        BOOST_CHECK(sig.size() <= 70);
+        if (!key.Sign(msg_hash, sig)) {
+            bad_sign = true;
+            break;
+        }
+        // sig.size() > 70 implies sig[3] > 32, because S is always low.
+        // But check both conditions anyway, just in case this implication is broken for some reason
+        if (sig[3] > 32 || sig.size() > 70) {
+            found_big = true;
+            break;
+        }
         found_small |= sig.size() < 70;
     }
-    BOOST_CHECK(found);
+    BOOST_CHECK(!bad_sign);
+    BOOST_CHECK(!found_big);
     BOOST_CHECK(found_small);
 }