Add GPG v2.1 support by signing message digest

firmware.patch

diff --git a/firmware/crypto.c b/firmware/crypto.c
index 84d611d..1f94ceb 100644
--- a/firmware/crypto.c
+++ b/firmware/crypto.c
@@ -90,6 +90,16 @@ int sshMessageSign(const uint8_t *message, size_t message_len, const uint8_t *pr
 	return ecdsa_sign(&nist256p1, privkey, message, message_len, signature + 1, NULL);
 }
 
+int gpgMessageSign(const uint8_t *message, size_t message_len, const uint8_t *privkey, uint8_t *signature)
+{
+	// GPG should sign a SHA256 digest of the original message.
+	if (message_len != 32) {
+		return 1;
+	}
+	signature[0] = 0; // prefix: pad with zero, so all signatures are 65 bytes
+	return ecdsa_sign_digest(&nist256p1, privkey, message, signature + 1, NULL);
+}
+
 int cryptoMessageSign(const uint8_t *message, size_t message_len, const uint8_t *privkey, uint8_t *signature)
 {
 	SHA256_CTX ctx;
diff --git a/firmware/crypto.h b/firmware/crypto.h
index b9d03fd..aefdc97 100644
--- a/firmware/crypto.h
+++ b/firmware/crypto.h
@@ -34,6 +34,8 @@ uint32_t ser_length_hash(SHA256_CTX *ctx, uint32_t len);
 
 int sshMessageSign(const uint8_t *message, size_t message_len, const uint8_t *privkey, uint8_t *signature);
 
+int gpgMessageSign(const uint8_t *message, size_t message_len, const uint8_t *privkey, uint8_t *signature);
+
 int cryptoMessageSign(const uint8_t *message, size_t message_len, const uint8_t *privkey, uint8_t *signature);
 
 int cryptoMessageVerify(const uint8_t *message, size_t message_len, const uint8_t *address_raw, const uint8_t *signature);
diff --git a/firmware/fsm.c b/firmware/fsm.c
index a9cd886..98b5b94 100644
--- a/firmware/fsm.c
+++ b/firmware/fsm.c
@@ -739,11 +739,14 @@ void fsm_msgSignIdentity(SignIdentity *msg)
 	}
 
 	bool sign_ssh = msg->identity.has_proto && (strcmp(msg->identity.proto, "ssh") == 0);
+	bool sign_gpg = msg->identity.has_proto && (strcmp(msg->identity.proto, "gpg") == 0);
 
 	int result = 0;
 	layoutProgressSwipe("Signing", 0);
 	if (sign_ssh) { // SSH does not sign visual challenge
 		result = sshMessageSign(msg->challenge_hidden.bytes, msg->challenge_hidden.size, node->private_key, resp->signature.bytes);
+	} else if (sign_gpg) { // GPG should sign a digest
+		result = gpgMessageSign(msg->challenge_hidden.bytes, msg->challenge_hidden.size, node->private_key, resp->signature.bytes);
 	} else {
 		uint8_t digest[64];
 		sha256_Raw(msg->challenge_hidden.bytes, msg->challenge_hidden.size, digest);