Add C++ functions for .narinfo processing / signing

This is currently only used by the Hydra queue runner rework, but like
eff5021eaa it presumably will be useful
for the C++ rewrite of nix-push and
download-from-binary-cache. (@shlevy)

src/libstore/crypto.hh

@@ -0,0 +1,40 @@
+#pragma once
+
+#include "types.hh"
+
+#include <map>
+
+namespace nix {
+
+struct Key
+{
+    std::string name;
+    std::string key;
+
+    /* Construct Key from a string in the format
+       ‘<name>:<key-in-base64>’. */
+    Key(const std::string & s);
+
+};
+
+struct SecretKey : Key
+{
+    SecretKey(const std::string & s);
+
+    /* Return a detached signature of the given string. */
+    std::string signDetached(const std::string & s) const;
+};
+
+struct PublicKey : Key
+{
+    PublicKey(const std::string & data);
+};
+
+typedef std::map<std::string, PublicKey> PublicKeys;
+
+/* Return true iff ‘sig’ is a correct signature over ‘data’ using one
+   of the given public keys. */
+bool verifyDetached(const std::string & data, const std::string & sig,
+    const PublicKeys & publicKeys);
+
+}