Merge #14047: Add HKDF_HMAC256_L32 and method to negate a private key

8794a4b3ae4d34a4cd21a7dee9f694eef7726a4f QA: add test for HKDF HMAC_SHA256 L32 (Jonas Schnelli)
551d489416339dae8f9d896013cd060a21406e2b Add HKDF HMAC_SHA256 L=32 implementations (Jonas Schnelli)
3b64f852e400c552f031697d6a86829dc6e74bd6 QA: add test for CKey::Negate() (Jonas Schnelli)
463921bb649d644f79f9d7f0f96f10aa0d165f76 CKey: add method to negate the key (Jonas Schnelli)

Pull request description:

  This adds a limited implementation of `HKDF` (defined by rfc5869) that supports only HMAC-SHA256  and length output of 32 bytes (will be required for v2 transport protocol).

  This PR also includes a method to negate a private key which is useful to enforce public keys starting with 0x02 (or 0x03) (a requirement for the v2 transport protocol). The new `CKey::Negate()` method is pretty much a wrapper around `secp256k1_ec_privkey_negate()`.

  Including tests.

  This is a subset of #14032 and a pre-requirement for the v2 transport protocol.

ACKs for commit 8794a4:

Tree-SHA512: 5341929dfa29f5da766ec3612784baec6a3ad69972f08b5a985a8aafdae4dae36f104a2b888d1f5d1f33561456bd111f960d7e32c2cc4fd18e48358468f26c1a

1 files changed, 32 insertions, 0 deletions

diff --git a/src/test/key_tests.cpp b/src/test/key_tests.cpp
index ceed73b14d..1b95105eab 100644
--- a/src/test/key_tests.cpp
+++ b/src/test/key_tests.cpp
@@ -188,4 +188,36 @@ BOOST_AUTO_TEST_CASE(key_signature_tests)
     BOOST_CHECK(found_small);
 }
 
+BOOST_AUTO_TEST_CASE(key_key_negation)
+{
+    // create a dummy hash for signature comparison
+    unsigned char rnd[8];
+    std::string str = "Bitcoin key verification\n";
+    GetRandBytes(rnd, sizeof(rnd));
+    uint256 hash;
+    CHash256().Write((unsigned char*)str.data(), str.size()).Write(rnd, sizeof(rnd)).Finalize(hash.begin());
+
+    // import the static test key
+    CKey key = DecodeSecret(strSecret1C);
+
+    // create a signature
+    std::vector<unsigned char> vch_sig;
+    std::vector<unsigned char> vch_sig_cmp;
+    key.Sign(hash, vch_sig);
+
+    // negate the key twice
+    BOOST_CHECK(key.GetPubKey().data()[0] == 0x03);
+    key.Negate();
+    // after the first negation, the signature must be different
+    key.Sign(hash, vch_sig_cmp);
+    BOOST_CHECK(vch_sig_cmp != vch_sig);
+    BOOST_CHECK(key.GetPubKey().data()[0] == 0x02);
+    key.Negate();
+    // after the second negation, we should have the original key and thus the
+    // same signature
+    key.Sign(hash, vch_sig_cmp);
+    BOOST_CHECK(vch_sig_cmp == vch_sig);
+    BOOST_CHECK(key.GetPubKey().data()[0] == 0x03);
+}
+
 BOOST_AUTO_TEST_SUITE_END()