diff --git a/bare_minimum_crypto/cpp/.gitignore b/bare_minimum_crypto/cpp/.gitignore
index 61d165a..35aba71 100644
--- a/bare_minimum_crypto/cpp/.gitignore
+++ b/bare_minimum_crypto/cpp/.gitignore
@@ -1,3 +1,4 @@
 /bmc
+*.cert
 *.key
 *.o
diff --git a/bare_minimum_crypto/cpp/bmc_main.cpp b/bare_minimum_crypto/cpp/bmc_main.cpp
index 8aec328..dd4c5ee 100644
--- a/bare_minimum_crypto/cpp/bmc_main.cpp
+++ b/bare_minimum_crypto/cpp/bmc_main.cpp
@@ -28,12 +28,24 @@ int file (const string & file_path) {
 	
 	const auto j = std::move (*try_load_msgpack_file (file_path));
 	
+	if (! j.contains ("app")) {
+		cout << "Can't find app magic string. This file was probably not made by BMC." << endl;
+		return 1;
+	}
+	
+	const string app = j ["app"];
+	
+	if (app != "4B27CL32") {
+		cout << "Can't find app magic string. This file was probably not made by BMC." << endl;
+		return 1;
+	}
+	
 	const string schema = j ["schema"];
 	
 	cout << "Schema: " << schema << endl;
 	
 	if (schema == "3T6XF5DZ") {
-		cout << "File is a secret key for humans (It's passphrase-protected)" << endl;
+		cout << "File is a secret key for humans. (It's passphrase-protected)" << endl;
 		
 		// Read msgpack fields
 		const auto key = std::move (*HumanKeyFile::try_from_msgpack (j));
@@ -60,7 +72,7 @@ int file (const string & file_path) {
 		}
 	}
 	else if (schema == "2PVHIKMA") {
-		cout << "File is a secret key for machines (No passphrase)" << endl;
+		cout << "File is a secret key for machines. (No passphrase)" << endl;
 		
 		// Read msgpack fields
 		const auto key = std::move (*MachineKeyFile::try_from_msgpack (j));
@@ -86,8 +98,47 @@ int file (const string & file_path) {
 			cout << "* The key doesn't have the right permissions. Try `chmod 400` on it." << endl;
 		}
 	}
+	else if (schema == "MSYZGDBI") {
+		cout << "File is a signed certificate, possibly for a pubkey." << endl;
+		
+		cout << "Claims to be signed by pubkey `" << base64_encode (j ["signer_pubkey"].get_binary ()) << "`" << endl;
+		
+		const auto cert_bytes = j ["cert"].get_binary ();
+		const auto cert = json::from_msgpack (cert_bytes);
+		
+		const TimeRange tr {
+			cert ["not_before"],
+			cert ["not_after"],
+		};
+		
+		const auto now = Instant::now ();
+		
+		if (! tr.contains (now)) {
+			cout << "Cert is NOT valid." << endl;
+			
+			if (now.x <= tr.not_before) {
+				cout << "Cert is from the future. Is your clock behind? Is the signer's clock ahead?" << endl;
+			}
+			else if (now.x >= tr.not_after) {
+				cout << "Cert expired " << now.x - tr.not_after << " seconds ago. Don't trust it." << endl;
+			}
+			return 1;
+		}
+		
+		cout << "Cert is valid for another " << tr.not_after - now.x << " seconds." << endl;
+		
+		const auto payload = cert ["payload"].get_binary ();
+		
+		if (payload.size () == crypto_sign_PUBLICKEYBYTES) {
+			cout << "Payload might be the pubkey `" << base64_encode (payload) << "`" << endl;
+		}
+		else {
+			cout << "Payload doesn't have the right size to be a pubkey." << endl;
+		}
+	}
 	else {
 		cout << "Unknown schema. Maybe this file is from a newer version of BMC?" << endl;
+		return 1;
 	}
 	
 	return 0;
@@ -118,7 +169,7 @@ int test () {
 	cerr << "Sender key " << base64_encode (sender_key.pubkey ()) << endl;
 	
 	// The root signs the sender key
-	const ExpiringSignature sender_cert = std::move (*root_key.sign_key (sender_key, now));
+	const auto sender_cert = std::move (*root_key.sign_key (sender_key.pubkey (), now));
 	
 	const auto sender = std::move (*Sender::create (sender_key, sender_cert));
 	
@@ -221,8 +272,17 @@ int main (int argc, char ** argv) {
 			return 1;
 		}
 		const auto sig = std::move (*sig_opt);
+		const auto sig_bytes = sig.to_msgpack ();
 		
+		ofstream f;
+		f.open (output_path, ofstream::binary);
 		
+		if (! f.is_open ()) {
+			cerr << "Error, could not open output file" << endl;
+			return 1;
+		}
+		
+		f.write ((const char *)sig_bytes.data (), sig_bytes.size ());
 	}
 	else if (result.count ("test")) {
 		if (test () != 0) {
diff --git a/bare_minimum_crypto/cpp/expiring_signature.cpp b/bare_minimum_crypto/cpp/expiring_signature.cpp
index 8853a0b..171f759 100644
--- a/bare_minimum_crypto/cpp/expiring_signature.cpp
+++ b/bare_minimum_crypto/cpp/expiring_signature.cpp
@@ -13,6 +13,18 @@ namespace BareMinimumCrypto {
 		return ! (*this == o);
 	}
 	
+	Bytes ExpiringSignature::to_msgpack () const {
+		const json j {
+			{"app", "4B27CL32"},
+			{"schema", "MSYZGDBI"},
+			{"sig", json::binary (sig)},
+			{"cert", json::binary (cert)},
+			{"signer_pubkey", json::binary (signer_pubkey)},
+		};
+		
+		return json::to_msgpack (j);
+	}
+	
 	Bytes KeyCertFile::to_msgpack () const {
 		const json cert_j {
 			{"pubkey", json::binary (pubkey)},
@@ -22,12 +34,10 @@ namespace BareMinimumCrypto {
 		
 		const auto cert = json::to_msgpack (cert_j);
 		
-		const json j {
-			{"sig", json::binary (sig)},
-			{"cert", json::binary (cert)},
-		};
-		
-		return json::to_msgpack (j);
+		return ExpiringSignature {
+			cert,
+			sig,
+		}.to_msgpack ();
 	}
 	
 	optional <KeyCertFile> KeyCertFile::try_from_msgpack (const json & msg) 
diff --git a/bare_minimum_crypto/cpp/expiring_signature.h b/bare_minimum_crypto/cpp/expiring_signature.h
index 16a7ce1..2b7fe25 100644
--- a/bare_minimum_crypto/cpp/expiring_signature.h
+++ b/bare_minimum_crypto/cpp/expiring_signature.h
@@ -18,9 +18,12 @@ namespace BareMinimumCrypto {
 		// Payload is contained in here
 		Bytes cert;
 		Bytes sig;
+		Bytes signer_pubkey;
 		
 		bool operator == (const ExpiringSignature & o) const;
 		bool operator != (const ExpiringSignature & o) const;
+		
+		Bytes to_msgpack () const;
 	};
 	
 	struct KeyCertFile {
diff --git a/bare_minimum_crypto/cpp/signing_key.cpp b/bare_minimum_crypto/cpp/signing_key.cpp
index dc01db7..1e9dbb3 100644
--- a/bare_minimum_crypto/cpp/signing_key.cpp
+++ b/bare_minimum_crypto/cpp/signing_key.cpp
@@ -26,6 +26,8 @@ namespace BareMinimumCrypto {
 	
 	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)},
@@ -38,6 +40,9 @@ namespace BareMinimumCrypto {
 	
 	optional <HumanKeyFile> HumanKeyFile::try_from_msgpack (const json & j)
 	{
+		if (j ["app"] != "4B27CL32") {
+			return nullopt;
+		}
 		if (j ["schema"] != "3T6XF5DZ") {
 			return nullopt;
 		}
@@ -52,6 +57,8 @@ namespace BareMinimumCrypto {
 	
 	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)},
@@ -63,6 +70,9 @@ namespace BareMinimumCrypto {
 	
 	optional <MachineKeyFile> MachineKeyFile::try_from_msgpack (const json & j)
 	{
+		if (j ["app"] != "4B27CL32") {
+			return nullopt;
+		}
 		if (j ["schema"] != "2PVHIKMA") {
 			return nullopt;
 		}
@@ -282,12 +292,13 @@ namespace BareMinimumCrypto {
 		return ExpiringSignature {
 			cert,
 			sig,
+			pubkey (),
 		};
 	}
 	
-	optional <ExpiringSignature> SigningKey::sign_key (const SigningKey & k, Instant now) const 
+	optional <ExpiringSignature> SigningKey::sign_key (const Bytes & pubkey, Instant now) const 
 	{
-		return sign (k.pub_to_msgpack (), TimeRange::from_start_and_dur (now, about_3_months));
+		return sign (pubkey, TimeRange::from_start_and_dur (now, about_3_months));
 	}
 	
 	optional <ExpiringSignature> SigningKey::sign_data (const Bytes & v, Instant now) const 
diff --git a/bare_minimum_crypto/cpp/signing_key.h b/bare_minimum_crypto/cpp/signing_key.h
index 4fe934e..0334cb6 100644
--- a/bare_minimum_crypto/cpp/signing_key.h
+++ b/bare_minimum_crypto/cpp/signing_key.h
@@ -31,7 +31,7 @@ namespace BareMinimumCrypto {
 			TimeRange tr
 		) const;
 		
-		optional <ExpiringSignature> sign_key (const SigningKey & k, Instant now) const;
+		optional <ExpiringSignature> sign_key (const Bytes & pubkey, Instant now) const;
 		optional <ExpiringSignature> sign_data (const Bytes & v, Instant now) const;
 	};
 	
diff --git a/bare_minimum_crypto/schemas.md b/bare_minimum_crypto/schemas.md
index 5eaa6d6..c59c452 100644
--- a/bare_minimum_crypto/schemas.md
+++ b/bare_minimum_crypto/schemas.md
@@ -1,5 +1,7 @@
 - 2PVHIKMA
 - 3T6XF5DZ
+- 4B27CL32
+- MSYZGDBI
 
 2PVHIKMA is a secret key for machines. It has no passphrase, so
 computer programs can use it automatically.
@@ -7,3 +9,11 @@ computer programs can use it automatically.
 3T6XF5DZ is a secret key for humans. It is protected by a passphrase.
 The passphrase should be saved in a password manager or written on paper,
 and not kept in the same disk as the key.
+
+4B27CL32 can be used by older versions of BMC to differentiate between
+generic Msgpack files, and files created by newer versions of BMC. This is
+in addition to the schema, since older versions won't recognize newer
+schemas.
+
+MSYZGDBI is a signed certificate. The payload is in the file and may be a
+public key.