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/test/app/PayChan_test.cpp

@@ -1065,8 +1065,21 @@ struct PayChan_test : public beast::unit_test::suite
         jv.removeMember("PublicKey");
         env (jv, ter(temMALFORMED));
 
-        jv["PublicKey"] = pkHex;
-        env (jv);
+        {
+            auto const txn = R"*(
+        {
+
+        "channel_id":"5DB01B7FFED6B67E6B0414DED11E051D2EE2B7619CE0EAA6286D67A3A4D5BDB3",
+                "signature":
+        "304402204EF0AFB78AC23ED1C472E74F4299C0C21F1B21D07EFC0A3838A420F76D783A400220154FB11B6F54320666E4C36CA7F686C16A3A0456800BBC43746F34AF50290064",
+                "public_key":
+        "aKijDDiC2q2gXjMpM7i4BUS6cmixgsEe18e7CjsUxwihKfuoFgS5",
+                "amount": "1000000"
+            }
+        )*";
+            auto const r = env.rpc("json", "channel_verify", txn);
+            BEAST_EXPECT(r["result"]["error"] == "publicMalformed");
+        }
     }
 
     void