The crux of ECIES. Generate a secret from two EC keys.

At least one private key must be known, but it doesn't matter which.

src/DeterministicKeys.cpp

@@ -285,4 +285,39 @@ EC_KEY* CKey::GeneratePrivateDeterministicKey(const NewcoinAddress& family, cons
 
 	return pkey;
 }
+
+static void* ecies_key_derivation(const void *input, size_t ilen, void *output, size_t *olen)
+{ // This function must not be changed as it must be what ECDH_compute_key expects
+	if (*olen < SHA512_DIGEST_LENGTH)
+		return NULL;
+	*olen = SHA512_DIGEST_LENGTH;
+	return SHA512(static_cast<const unsigned char *>(input), ilen, static_cast<unsigned char *>(output));
+}
+
+
+std::vector<unsigned char> CKey::getECIESSecret(CKey& otherKey)
+{ // Retrieve a secret generated from an EC key pair. At least one private key must be known.
+	if(!pkey || !otherKey.pkey)
+		throw std::runtime_error("missing key");
+
+	EC_KEY *pubkey, *privkey;
+	if(EC_KEY_get0_private_key(pkey))
+	{
+		privkey=pkey;
+		pubkey=otherKey.pkey;
+	}
+	else if(EC_KEY_get0_private_key(otherKey.pkey))
+	{
+		privkey=otherKey.pkey;
+		pubkey=pkey;
+	}
+	else throw std::runtime_error("no private key");
+
+	std::vector<unsigned char> ret(SHA512_DIGEST_LENGTH);
+	if (ECDH_compute_key(&(ret.front()), SHA512_DIGEST_LENGTH, EC_KEY_get0_public_key(pubkey),
+			privkey, ecies_key_derivation) != SHA512_DIGEST_LENGTH)
+		throw std::runtime_error("ecdh key failed");
+	return ret;
+}
+
 // vim:ts=4

src/key.h

@@ -273,6 +273,9 @@ public:
 			return false;
 		return true;
 	}
+
+	// Returns a 64-byte secret unique to these two keys. At least one private key must be known.
+	std::vector<unsigned char> getECIESSecret(CKey& otherKey);
 };
 
 #endif