net: put CJDNS prefix byte in a constant

Author: vasild

src/netaddress.h

@@ -81,6 +81,10 @@ static const std::array<uint8_t, 6> INTERNAL_IN_IPV6_PREFIX{
     0xFD, 0x6B, 0x88, 0xC0, 0x87, 0x24 // 0xFD + sha256("bitcoin")[0:5].
 };
 
+/// All CJDNS addresses start with 0xFC. See
+/// https://github.com/cjdelisle/cjdns/blob/master/doc/Whitepaper.md#pulling-it-all-together
+static constexpr uint8_t CJDNS_PREFIX{0xFC};
+
 /// Size of IPv4 address (in bytes).
 static constexpr size_t ADDR_IPV4_SIZE = 4;
 
@@ -174,7 +178,7 @@ class CNetAddr
     [[nodiscard]] bool IsTor() const { return m_net == NET_ONION; }
     [[nodiscard]] bool IsI2P() const { return m_net == NET_I2P; }
     [[nodiscard]] bool IsCJDNS() const { return m_net == NET_CJDNS; }
-    [[nodiscard]] bool HasCJDNSPrefix() const { return m_addr[0] == 0xfc; }
+    [[nodiscard]] bool HasCJDNSPrefix() const { return m_addr[0] == CJDNS_PREFIX; }
     bool IsLocal() const;
     bool IsRoutable() const;
     bool IsInternal() const;

src/netgroup.cpp

@@ -52,8 +52,8 @@ std::vector<unsigned char> NetGroupManager::GetGroup(const CNetAddr& address) co
     } else if (address.IsCJDNS()) {
         // Treat in the same way as Tor and I2P because the address in all of
         // them is "random" bytes (derived from a public key). However in CJDNS
-        // the first byte is a constant 0xfc, so the random bytes come after it.
-        // Thus skip the constant 8 bits at the start.
+        // the first byte is a constant (see CJDNS_PREFIX), so the random bytes
+        // come after it. Thus skip the constant 8 bits at the start.
         nBits = 12;
     } else if (address.IsHeNet()) {
         // for he.net, use /36 groups