ptth/bare_minimum_crypto/cpp/signing_key.cpp

#include "signing_key.h"

#include <filesystem>
#include <fstream>
#include <iostream>

#include "json.hpp"

#include "sodium_helpers.h"

namespace BareMinimumCrypto {
	using nlohmann::json;
	namespace fs = std::filesystem;
	
	string get_machine_id () {
		ifstream f;
		f.open ("/etc/machine-id", ifstream::binary);
		string machine_id;
		if (! f.is_open ()) {
			return machine_id;
		}
		
		f >> machine_id;
		return machine_id;
	}
	
	Bytes HumanKeyFile::to_msgpack () const {
		const auto j = json {
			// All BMC msgpack artifacts should have this string
			{"app", "4B27CL32"},
			// Breaking changes should generate a new Base32 schema.
			{"schema", "3T6XF5DZ"},
			{"salt", json::binary (salt)},
			{"time_created", time_created.x},
			{"pubkey", json::binary (pubkey)},
			{"machine_id", machine_id},
		};
		return json::to_msgpack (j);
	}
	
	optional <HumanKeyFile> HumanKeyFile::try_from_msgpack (const json & j)
	{
		if (j ["app"] != "4B27CL32") {
			return nullopt;
		}
		if (j ["schema"] != "3T6XF5DZ") {
			return nullopt;
		}
		
		return HumanKeyFile {
			j ["salt"].get_binary (),
			Instant (j ["time_created"]),
			j ["pubkey"].get_binary (),
			j ["machine_id"],
		};
	}
	
	Bytes MachineKeyFile::to_msgpack () const {
		const auto j = json {
			// All BMC msgpack artifacts should have this string
			{"app", "4B27CL32"},
			// Breaking changes should generate a new Base32 schema.
			{"schema", "2PVHIKMA"},
			{"secretkey", json::binary (secretkey)},
			{"time_created", time_created.x},
			{"machine_id", machine_id},
		};
		return json::to_msgpack (j);
	}
	
	optional <MachineKeyFile> MachineKeyFile::try_from_msgpack (const json & j)
	{
		if (j ["app"] != "4B27CL32") {
			return nullopt;
		}
		if (j ["schema"] != "2PVHIKMA") {
			return nullopt;
		}
		
		return MachineKeyFile {
			j ["secretkey"].get_binary (),
			Instant (j ["time_created"]),
			j ["machine_id"],
		};
	}
	
	Bytes MachineKeyFile::pubkey () const {
		Bytes pk;
		pk.resize (crypto_sign_PUBLICKEYBYTES);
		crypto_sign_ed25519_sk_to_pk (pk.data (), secretkey.data ());
		return pk;
	}
	
	bool save_key_file (const string & file_path, const Bytes msg)
	{
		ofstream f;
		f.open (file_path, ofstream::binary);
		if (! f.is_open ()) {
			return false;
		}
		// Best-effort. It probably fails on Windows.
		fs::permissions (file_path,
			fs::perms::owner_read,
			fs::perm_options::replace
		);
		
		f.write ((const char *)msg.data (), msg.size ());
		f.close ();
		
		return true;
	}
	
	string get_passphrase_from_user () {
		cout << "Type or paste passphrase (it will be visible in the console)" << endl;
		string passphrase;
		cin >> passphrase;
		
		return passphrase;
	}
	
	optional <json> try_load_msgpack_file (const string & file_path) {
		ifstream f;
		f.open (file_path, ifstream::binary);
		if (! f.is_open ()) {
			return nullopt;
		}
		
		f.seekg (0, ifstream::end);
		const auto len = f.tellg ();
		f.seekg (0, ifstream::beg);
		
		if (len > 1024 * 1024) {
			return nullopt;
		}
		
		Bytes bytes;
		bytes.resize (len);
		
		f.read ((char *)bytes.data (), bytes.size ());
		
		return json::from_msgpack (bytes);
	}
	
	optional <SigningKey> HumanKeyFile::unlock_key (const Bytes & salt, const string & passphrase)
	{
		Bytes seed;
		seed.resize (crypto_sign_SEEDBYTES);
		
		if (crypto_pwhash (
			seed.data (), seed.size (), 
			passphrase.data (), passphrase.size (), 
			salt.data (),
			crypto_pwhash_OPSLIMIT_INTERACTIVE, crypto_pwhash_MEMLIMIT_INTERACTIVE,
			crypto_pwhash_ALG_DEFAULT
		) != 0) {
			return nullopt;
		}
		
		// This generates a redundant key but that's fine.
		SigningKey key;
		//key.pk.resize (crypto_sign_PUBLICKEYBYTES);
		key.sk.resize (crypto_sign_SECRETKEYBYTES);
		
		Bytes pk;
		pk.resize (crypto_sign_PUBLICKEYBYTES);
		if (crypto_sign_seed_keypair (pk.data (), key.sk.data (), seed.data ()) != 0) {
			return nullopt;
		}
		
		return key;
	}
	
	// The whole process for a passphrased key is like this:
	// Passphrase + random salt -->
	// Seed -->
	// Secret key + Public key
	
	// Passphrases should be mandatory for keys that can sign other keys.
	
	optional <SigningKey> HumanKeyFile::generate (const string & file_path, const string & passphrase) 
	{
		try_sodium_init ();
		
		if (passphrase.size () < 8) {
			return nullopt;
		}
		
		Bytes salt;
		salt.resize (crypto_pwhash_SALTBYTES);
		randombytes_buf (salt.data (), salt.size ());
		
		auto key_opt = unlock_key (salt, passphrase);
		if (! key_opt) {
			return nullopt;
		}
		const auto key = std::move (*key_opt);
		
		const auto machine_id = get_machine_id ();
		
		HumanKeyFile key_on_disk {
			salt,
			Instant::now (),
			key.pubkey (),
			machine_id,
		};
		const auto msg = key_on_disk.to_msgpack ();
		
		if (! save_key_file (file_path, msg)) {
			return nullopt;
		}
		
		return key;
	}
	
	optional <SigningKey> HumanKeyFile::load (const string & file_path, const string & passphrase)
	{
		const auto j = std::move (*try_load_msgpack_file (file_path));
		const auto human_key = std::move (*HumanKeyFile::try_from_msgpack (j));
		
		const auto key = std::move (*unlock_key (human_key.salt, passphrase));
		
		return key;
	}
	
	optional <SigningKey> MachineKeyFile::generate (const string & file_path)
	{
		const SigningKey key;
		
		const auto machine_id = get_machine_id ();
		
		MachineKeyFile key_on_disk {
			key.sk,
			Instant::now (),
			machine_id,
		};
		const auto msg = key_on_disk.to_msgpack ();
		
		if (! save_key_file (file_path, msg)) {
			return nullopt;
		}
		
		return key;
	}
	
	SigningKey::SigningKey () {
		try_sodium_init ();
		
		Bytes pk;
		pk.resize (crypto_sign_PUBLICKEYBYTES);
		sk.resize (crypto_sign_SECRETKEYBYTES);
		
		crypto_sign_keypair (pk.data (), sk.data ());
	}
	
	Bytes SigningKey::pubkey () const {
		Bytes pk;
		pk.resize (crypto_sign_PUBLICKEYBYTES);
		crypto_sign_ed25519_sk_to_pk (pk.data (), sk.data ());
		return pk;
	}
	
	Bytes SigningKey::pub_to_msgpack () const {
		const json j = {
			{"key", json::binary (pubkey ())},
		};
		return json::to_msgpack (j);
	}
	
	optional <ExpiringSignature> SigningKey::sign (
		const Bytes & payload,
		TimeRange tr
	) const {
		try_sodium_init ();
		
		if (tr.duration () > about_1_year) {
			return nullopt;
		}
		
		const json j {
			{"not_before", tr.not_before},
			{"not_after", tr.not_after},
			{"payload", json::binary (payload)},
		};
		
		const auto cert = json::to_msgpack (j);
		
		Bytes sig;
		sig.resize (crypto_sign_BYTES);
		
		crypto_sign_detached (sig.data (), nullptr, cert.data (), cert.size (), sk.data ());
		
		return ExpiringSignature {
			cert,
			sig,
			pubkey (),
		};
	}
	
	optional <ExpiringSignature> SigningKey::sign_key (const Bytes & pubkey, Instant now) const 
	{
		return sign (pubkey, TimeRange::from_start_and_dur (now, about_3_months));
	}
	
	optional <ExpiringSignature> SigningKey::sign_data (const Bytes & v, Instant now) const 
	{
		return sign (v, TimeRange::from_start_and_dur (now, about_1_week));
	}
}