crypter: constify encrypt/decrypt

This makes CCrypter easier to pass aroundf for tests

src/wallet/crypter.cpp

@@ -47,7 +47,7 @@ bool CCrypter::SetKey(const CKeyingMaterial& chNewKey, const std::vector<unsigne
     return true;
 }
 
-bool CCrypter::Encrypt(const CKeyingMaterial& vchPlaintext, std::vector<unsigned char> &vchCiphertext)
+bool CCrypter::Encrypt(const CKeyingMaterial& vchPlaintext, std::vector<unsigned char> &vchCiphertext) const
 {
     if (!fKeySet)
         return false;
@@ -74,7 +74,7 @@ bool CCrypter::Encrypt(const CKeyingMaterial& vchPlaintext, std::vector<unsigned
     return true;
 }
 
-bool CCrypter::Decrypt(const std::vector<unsigned char>& vchCiphertext, CKeyingMaterial& vchPlaintext)
+bool CCrypter::Decrypt(const std::vector<unsigned char>& vchCiphertext, CKeyingMaterial& vchPlaintext) const
 {
     if (!fKeySet)
         return false;

src/wallet/crypter.h

@@ -77,8 +77,8 @@ private:
 
 public:
     bool SetKeyFromPassphrase(const SecureString &strKeyData, const std::vector<unsigned char>& chSalt, const unsigned int nRounds, const unsigned int nDerivationMethod);
-    bool Encrypt(const CKeyingMaterial& vchPlaintext, std::vector<unsigned char> &vchCiphertext);
-    bool Decrypt(const std::vector<unsigned char>& vchCiphertext, CKeyingMaterial& vchPlaintext);
+    bool Encrypt(const CKeyingMaterial& vchPlaintext, std::vector<unsigned char> &vchCiphertext) const;
+    bool Decrypt(const std::vector<unsigned char>& vchCiphertext, CKeyingMaterial& vchPlaintext) const;
     bool SetKey(const CKeyingMaterial& chNewKey, const std::vector<unsigned char>& chNewIV);
 
     void CleanKey()