Implement DHKeyExchange (dashpay#43)

* Implement DHKeyExchange

* Call CheckRelicErrors in DHKeyExchange

src/bls.cpp

@@ -111,6 +111,16 @@ void BLS::HashPubKeys(bn_t* output, size_t numOutputs,
     CheckRelicErrors();
 }
 
+PublicKey BLS::DHKeyExchange(const PrivateKey& privKey, const PublicKey& pubKey) {
+    AssertInitialized();
+    if (!privKey.keydata) {
+        throw std::string("keydata not initialized");
+    }
+    PublicKey ret = pubKey.Exp(*privKey.keydata);
+    CheckRelicErrors();
+    return ret;
+}
+
 void BLS::CheckRelicErrors() {
     if (!core_get()) {
         throw std::string("Library not initialized properly. Call BLS::Init()");

src/bls.hpp

@@ -63,6 +63,8 @@ class BLS {
             std::vector<uint8_t*> const &serPubKeys,
             std::vector<size_t> const &sortedIndices);
 
+    static PublicKey DHKeyExchange(const PrivateKey& privKey, const PublicKey& pubKey);
+
     static void CheckRelicErrors();
 };
 } // end namespace bls

src/privatekey.hpp

@@ -25,6 +25,7 @@
 #include "signature.hpp"
 namespace bls {
 class PrivateKey {
+ friend class BLS;
  public:
     // Private keys are represented as 32 byte field elements. Note that
     // not all 32 byte integers are valid keys, the private key must be

src/publickey.hpp

@@ -31,6 +31,7 @@ class PublicKey {
  friend class InsecureSignature;
  friend class Signature;
  friend class ExtendedPublicKey;
+ friend class BLS;
  public:
     static const size_t PUBLIC_KEY_SIZE = 48;