Address issues identified by external review:

* RIPD-1617, RIPD-1619, RIPD-1621:
  Verify serialized public keys more strictly before
  using them.

* RIPD-1618:
    * Simplify the base58 decoder logic.
    * Reduce the complexity of the base58 encoder and
      eliminate a potential out-of-bounds memory access.
    * Improve type safety by using an `enum class` to
      enforce strict type checking for token types.

* RIPD-1616:
  Avoid calling `memcpy` with a null pointer even if the
  size is specified as zero, since it results in undefined
  behavior.

Acknowledgements:
Ripple thanks Guido Vranken for responsibly disclosing these
issues.

Bug Bounties and Responsible Disclosures:
We welcome reviews of the rippled code and urge researchers
to responsibly disclose any issues that they may find. For
more on Ripple's Bug Bounty program, please visit:
https://ripple.com/bug-bounty

src/ripple/overlay/impl/PeerImp.cpp

@@ -256,7 +256,7 @@ PeerImp::json()
     Json::Value ret (Json::objectValue);
 
     ret[jss::public_key]   = toBase58 (
-        TokenType::TOKEN_NODE_PUBLIC, publicKey_);
+        TokenType::NodePublic, publicKey_);
     ret[jss::address]      = remote_address_.to_string();
 
     if (m_inbound)
@@ -629,7 +629,7 @@ void PeerImp::doAccept()
     JLOG(journal_.info()) << "Protocol: " << to_string(protocol);
     JLOG(journal_.info()) <<
         "Public Key: " << toBase58 (
-            TokenType::TOKEN_NODE_PUBLIC,
+            TokenType::NodePublic,
             publicKey_);
     if (auto member = app_.cluster().member(publicKey_))
     {
@@ -932,7 +932,7 @@ PeerImp::onMessage (std::shared_ptr <protocol::TMCluster> const& m)
             name = node.nodename();
 
         auto const publicKey = parseBase58<PublicKey>(
-            TokenType::TOKEN_NODE_PUBLIC, node.publickey());
+            TokenType::NodePublic, node.publickey());
 
         // NIKB NOTE We should drop the peer immediately if
         // they send us a public key we can't parse
@@ -1610,8 +1610,7 @@ PeerImp::onMessage (std::shared_ptr <protocol::TMValidation> const& m)
         STValidation::pointer val;
         {
             SerialIter sit (makeSlice(m->validation()));
-            val = std::make_shared <
-                STValidation> (std::ref (sit), false);
+            val = std::make_shared <STValidation> (std::ref (sit), false);
             val->setSeen (closeTime);
         }