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/protocol/impl/Seed.cpp

@@ -85,8 +85,7 @@ template <>
 boost::optional<Seed>
 parseBase58 (std::string const& s)
 {
-    auto const result = decodeBase58Token(
-         s, TokenType::TOKEN_FAMILY_SEED);
+    auto const result = decodeBase58Token(s, TokenType::FamilySeed);
     if (result.empty())
         return boost::none;
     if (result.size() != 16)
@@ -101,10 +100,10 @@ parseGenericSeed (std::string const& str)
         return boost::none;
 
     if (parseBase58<AccountID>(str) ||
-        parseBase58<PublicKey>(TokenType::TOKEN_NODE_PUBLIC, str) ||
-        parseBase58<PublicKey>(TokenType::TOKEN_ACCOUNT_PUBLIC, str) ||
-        parseBase58<SecretKey>(TokenType::TOKEN_NODE_PRIVATE, str) ||
-        parseBase58<SecretKey>(TokenType::TOKEN_ACCOUNT_SECRET, str))
+        parseBase58<PublicKey>(TokenType::NodePublic, str) ||
+        parseBase58<PublicKey>(TokenType::AccountPublic, str) ||
+        parseBase58<SecretKey>(TokenType::NodePrivate, str) ||
+        parseBase58<SecretKey>(TokenType::AccountSecret, str))
     {
         return boost::none;
     }