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

8794a4b3ae QA: add test for HKDF HMAC_SHA256 L32 (Jonas Schnelli)
551d489416 Add HKDF HMAC_SHA256 L=32 implementations (Jonas Schnelli)
3b64f852e4 QA: add test for CKey::Negate() (Jonas Schnelli)
463921bb64 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

src/crypto/hkdf_sha256_32.cpp

@@ -0,0 +1,21 @@
+// Copyright (c) 2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <crypto/hkdf_sha256_32.h>
+
+#include <assert.h>
+#include <string.h>
+
+CHKDF_HMAC_SHA256_L32::CHKDF_HMAC_SHA256_L32(const unsigned char* ikm, size_t ikmlen, const std::string& salt)
+{
+    CHMAC_SHA256((const unsigned char*)salt.c_str(), salt.size()).Write(ikm, ikmlen).Finalize(m_prk);
+}
+
+void CHKDF_HMAC_SHA256_L32::Expand32(const std::string& info, unsigned char hash[OUTPUT_SIZE])
+{
+    // expand a 32byte key (single round)
+    assert(info.size() <= 128);
+    static const unsigned char one[1] = {1};
+    CHMAC_SHA256(m_prk, 32).Write((const unsigned char*)info.data(), info.size()).Write(one, 1).Finalize(hash);
+}

src/crypto/hkdf_sha256_32.h

@@ -0,0 +1,25 @@
+// Copyright (c) 2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_CRYPTO_HKDF_SHA256_32_H
+#define BITCOIN_CRYPTO_HKDF_SHA256_32_H
+
+#include <crypto/hmac_sha256.h>
+
+#include <stdint.h>
+#include <stdlib.h>
+
+/** A rfc5869 HKDF implementation with HMAC_SHA256 and fixed key output length of 32 bytes (L=32) */
+class CHKDF_HMAC_SHA256_L32
+{
+private:
+    unsigned char m_prk[32];
+    static const size_t OUTPUT_SIZE = 32;
+
+public:
+    CHKDF_HMAC_SHA256_L32(const unsigned char* ikm, size_t ikmlen, const std::string& salt);
+    void Expand32(const std::string& info, unsigned char hash[OUTPUT_SIZE]);
+};
+
+#endif // BITCOIN_CRYPTO_HKDF_SHA256_32_H