Use boost::variant to represent shielded addresses and keys

libzcash::PaymentAddress has been renamed to libzcash::SproutPaymentAddress,
and a new typedef boost::variant is now libzcash::PaymentAddress. Similarly
for ViewingKey and SpendingKey.

A new class InvalidEncoding is introduced as the default boost::variant
option for each address and key type; it is used during decoding instead
of boost::optional.

All address and key storage functions in the wallet have been modified to
refer specifically to the Sprout types, as they are used very precisely.
In most other cases, the more general type is leveraged as much as possible,
and we convert to the Sprout type when necessary. This will be subsequently
wrapped in, or replaced with, context-specific functions once Sapling
types are implemented.

src/zcash/Address.hpp

@@ -5,19 +5,26 @@
 #include "uint252.h"
 #include "serialize.h"
 
+#include <boost/variant.hpp>
+
 namespace libzcash {
+class InvalidEncoding {
+public:
+    friend bool operator==(const InvalidEncoding &a, const InvalidEncoding &b) { return true; }
+    friend bool operator<(const InvalidEncoding &a, const InvalidEncoding &b) { return true; }
+};
 
 const size_t SerializedPaymentAddressSize = 64;
 const size_t SerializedViewingKeySize = 64;
 const size_t SerializedSpendingKeySize = 32;
 
-class PaymentAddress {
+class SproutPaymentAddress {
 public:
     uint256 a_pk;
     uint256 pk_enc;
 
-    PaymentAddress() : a_pk(), pk_enc() { }
-    PaymentAddress(uint256 a_pk, uint256 pk_enc) : a_pk(a_pk), pk_enc(pk_enc) { }
+    SproutPaymentAddress() : a_pk(), pk_enc() { }
+    SproutPaymentAddress(uint256 a_pk, uint256 pk_enc) : a_pk(a_pk), pk_enc(pk_enc) { }
 
     ADD_SERIALIZE_METHODS;
 
@@ -30,10 +37,10 @@ public:
     //! Get the 256-bit SHA256d hash of this payment address.
     uint256 GetHash() const;
 
-    friend inline bool operator==(const PaymentAddress& a, const PaymentAddress& b) {
+    friend inline bool operator==(const SproutPaymentAddress& a, const SproutPaymentAddress& b) {
         return a.a_pk == b.a_pk && a.pk_enc == b.pk_enc;
     }
-    friend inline bool operator<(const PaymentAddress& a, const PaymentAddress& b) {
+    friend inline bool operator<(const SproutPaymentAddress& a, const SproutPaymentAddress& b) {
         return (a.a_pk < b.a_pk ||
                 (a.a_pk == b.a_pk && a.pk_enc < b.pk_enc));
     }
@@ -47,13 +54,13 @@ public:
     uint256 pk_enc() const;
 };
 
-class ViewingKey {
+class SproutViewingKey {
 public:
     uint256 a_pk;
     ReceivingKey sk_enc;
 
-    ViewingKey() : a_pk(), sk_enc() { }
-    ViewingKey(uint256 a_pk, ReceivingKey sk_enc) : a_pk(a_pk), sk_enc(sk_enc) { }
+    SproutViewingKey() : a_pk(), sk_enc() { }
+    SproutViewingKey(uint256 a_pk, ReceivingKey sk_enc) : a_pk(a_pk), sk_enc(sk_enc) { }
 
     ADD_SERIALIZE_METHODS;
 
@@ -63,29 +70,42 @@ public:
         READWRITE(sk_enc);
     }
 
-    PaymentAddress address() const;
+    SproutPaymentAddress address() const;
 
-    friend inline bool operator==(const ViewingKey& a, const ViewingKey& b) {
+    friend inline bool operator==(const SproutViewingKey& a, const SproutViewingKey& b) {
         return a.a_pk == b.a_pk && a.sk_enc == b.sk_enc;
     }
-    friend inline bool operator<(const ViewingKey& a, const ViewingKey& b) {
+    friend inline bool operator<(const SproutViewingKey& a, const SproutViewingKey& b) {
         return (a.a_pk < b.a_pk ||
                 (a.a_pk == b.a_pk && a.sk_enc < b.sk_enc));
     }
 };
 
-class SpendingKey : public uint252 {
+class SproutSpendingKey : public uint252 {
 public:
-    SpendingKey() : uint252() { }
-    SpendingKey(uint252 a_sk) : uint252(a_sk) { }
+    SproutSpendingKey() : uint252() { }
+    SproutSpendingKey(uint252 a_sk) : uint252(a_sk) { }
 
-    static SpendingKey random();
+    static SproutSpendingKey random();
 
     ReceivingKey receiving_key() const;
-    ViewingKey viewing_key() const;
-    PaymentAddress address() const;
+    SproutViewingKey viewing_key() const;
+    SproutPaymentAddress address() const;
 };
 
+typedef boost::variant<InvalidEncoding, SproutPaymentAddress> PaymentAddress;
+typedef boost::variant<InvalidEncoding, SproutViewingKey> ViewingKey;
+typedef boost::variant<InvalidEncoding, SproutSpendingKey> SpendingKey;
+
 }
 
+/** Check whether a PaymentAddress is not an InvalidEncoding. */
+bool IsValidPaymentAddress(const libzcash::PaymentAddress& zaddr);
+
+/** Check whether a ViewingKey is not an InvalidEncoding. */
+bool IsValidViewingKey(const libzcash::ViewingKey& vk);
+
+/** Check whether a SpendingKey is not an InvalidEncoding. */
+bool IsValidSpendingKey(const libzcash::SpendingKey& zkey);
+
 #endif // ZC_ADDRESS_H_