Add HKDF HMAC_SHA256 L=32 implementations

src/Makefile.am

@@ -342,6 +342,8 @@ crypto_libbitcoin_crypto_base_a_SOURCES = \
   crypto/chacha20.h \
   crypto/chacha20.cpp \
   crypto/common.h \
+  crypto/hkdf_sha256_32.cpp \
+  crypto/hkdf_sha256_32.h \
   crypto/hmac_sha256.cpp \
   crypto/hmac_sha256.h \
   crypto/hmac_sha512.cpp \

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