crypto: improve exception types

include/opendht/crypto.h

@@ -44,7 +44,10 @@ namespace crypto {
 
 class OPENDHT_PUBLIC CryptoException : public std::runtime_error {
 public:
-        CryptoException(const std::string& str) : std::runtime_error(str) {};
+    explicit CryptoException(const std::string& str) : std::runtime_error(str) {};
+    explicit CryptoException(const char* str) : std::runtime_error(str) {};
+    CryptoException(const CryptoException& e) noexcept = default;
+    CryptoException& operator=(const CryptoException&) noexcept = default;
 };
 
 /**
@@ -52,7 +55,10 @@ class OPENDHT_PUBLIC CryptoException : public std::runtime_error {
  */
 class OPENDHT_PUBLIC DecryptError : public CryptoException {
 public:
-        DecryptError(const std::string& str = "") : CryptoException(str) {};
+    explicit DecryptError(const std::string& str) : CryptoException(str) {};
+    explicit DecryptError(const char* str) : CryptoException(str) {};
+    DecryptError(const DecryptError& e) noexcept = default;
+    DecryptError& operator=(const DecryptError&) noexcept = default;
 };
 
 struct PrivateKey;