Major revamp: new key paradigm, groundwork for GPG support

The active key is now stored in .git/git-crypt/key instead of being
stored outside the repo.  This will facilitate GPG support, where the
user may never interact directly with a key file.  It's also more
convenient, because it means you don't have to keep the key file
around in a fixed location (which can't be moved without breaking
git-crypt).

'git-crypt init' now takes no arguments and is used only when initializing
git-crypt for the very first time.  It generates a brand-new key, so
there's no longer a separate keygen step.

To export the key (for conveyance to another system or to a collaborator),
run 'git-crypt export-key FILENAME'.

To decrypt an existing repo using an exported key, run 'git-crypt unlock
KEYFILE'.  After running unlock, you can delete the key file you passed
to unlock.

Key files now use a new format that supports key versioning (which will
facilitate secure revocation in the future).

I've made these changes as backwards-compatible as possible.  Repos
already configured with git-crypt will continue to work without changes.
However, 'git-crypt unlock' expects a new format key.  You can use
the 'git-crypt migrate-key KEYFILE' command to migrate old keys to the
new format.

Note that old repos won't be able to use the new commands, like
export-key, or the future GPG support.  To migrate an old repo, migrate
its key file and then unlock the repo using the unlock command, as
described above.

While making these changes, I cleaned up the code significantly, adding
better error handling and improving robustness.

Next up: GPG support.

key.hpp

@@ -0,0 +1,84 @@
+/*
+ * Copyright 2014 Andrew Ayer
+ *
+ * This file is part of git-crypt.
+ *
+ * git-crypt is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * git-crypt is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with git-crypt.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Additional permission under GNU GPL version 3 section 7:
+ *
+ * If you modify the Program, or any covered work, by linking or
+ * combining it with the OpenSSL project's OpenSSL library (or a
+ * modified version of that library), containing parts covered by the
+ * terms of the OpenSSL or SSLeay licenses, the licensors of the Program
+ * grant you additional permission to convey the resulting work.
+ * Corresponding Source for a non-source form of such a combination
+ * shall include the source code for the parts of OpenSSL used as well
+ * as that of the covered work.
+ */
+
+#ifndef _KEY_H
+#define _KEY_H
+
+#include <map>
+#include <functional>
+#include <stdint.h>
+#include <iosfwd>
+
+enum {
+	HMAC_KEY_LEN = 64,
+	AES_KEY_LEN = 32
+};
+
+struct Key_file {
+public:
+	struct Entry {
+		unsigned char		aes_key[AES_KEY_LEN];
+		unsigned char		hmac_key[HMAC_KEY_LEN];
+
+		void			load (std::istream&);
+		void			store (std::ostream&) const;
+		void			generate ();
+	};
+
+	struct Malformed { }; // exception class
+	struct Incompatible { }; // exception class
+
+	const Entry*			get_latest () const;
+
+	const Entry*			get (uint32_t version) const;
+	void				add (uint32_t version, const Entry&);
+
+	void				load_legacy (std::istream&);
+	void				load (std::istream&);
+	void				store (std::ostream&) const;
+
+	bool				load (const char* filename);
+	bool				store (const char* filename) const;
+
+	void				generate ();
+
+	bool				is_empty () const { return entries.empty(); }
+	bool				is_filled () const { return !is_empty(); }
+
+	uint32_t			latest () const;
+
+private:
+	typedef std::map<uint32_t, Entry, std::greater<uint32_t> > Map;
+	enum { FORMAT_VERSION = 1 };
+
+	Map				entries;
+};
+
+#endif