hsmd: regroup hsm_secret decryption logic

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

common/hsm_encryption.c

@@ -49,6 +49,27 @@ bool encrypt_hsm_secret(const struct secret *encryption_key,
 	return true;
 }
 
+bool decrypt_hsm_secret(const struct secret *encryption_key,
+			const struct encrypted_hsm_secret *cipher,
+			struct secret *output)
+{
+	crypto_secretstream_xchacha20poly1305_state crypto_state;
+
+	/* The header part */
+	if (crypto_secretstream_xchacha20poly1305_init_pull(&crypto_state, cipher->data,
+							    encryption_key->data) != 0)
+		return false;
+	/* The ciphertext part */
+	if (crypto_secretstream_xchacha20poly1305_pull(&crypto_state, output->data,
+						       NULL, 0,
+						       cipher->data + HS_HEADER_LEN,
+						       HS_CIPHERTEXT_LEN,
+						       NULL, 0) != 0)
+		return false;
+
+	return true;
+}
+
 void discard_key(struct secret *key TAKES)
 {
 	/* sodium_munlock() also zeroes the memory. */

common/hsm_encryption.h

@@ -38,6 +38,17 @@ bool encrypt_hsm_secret(const struct secret *encryption_key,
 			const struct secret *hsm_secret,
 			struct encrypted_hsm_secret *output);
 
+/** Decrypt the hsm_secret using a previously derived encryption key.
+ * @encryption_key: the key derived from the passphrase.
+ * @cipher: the encrypted hsm_secret to decrypt.
+ * @output: the resulting hsm_secret.
+ *
+ * Return false on decryption failure.
+ */
+bool decrypt_hsm_secret(const struct secret *encryption_key,
+			const struct encrypted_hsm_secret *cipher,
+			struct secret *output);
+
 /** Unlock and zeroize the encryption key memory after use.
  * @key: the encryption key. If taken, it will be tal_free'd
  */

hsmd/hsmd.c

@@ -689,25 +689,13 @@ static void load_hsm(const struct secret *encryption_key)
 	/*~ If an encryption key was passed and the `hsm_secret` is stored
 	 * encrypted, recover the seed from the cipher. */
 	if (encryption_key && st.st_size > 32) {
-		crypto_secretstream_xchacha20poly1305_state crypto_state;
-		u8 header[crypto_secretstream_xchacha20poly1305_HEADERBYTES];
-		/* The cipher size is static with xchacha20poly1305 */
-		u8 cipher[sizeof(struct secret) + crypto_secretstream_xchacha20poly1305_ABYTES];
+		struct encrypted_hsm_secret encrypted_secret;
 
-		if (!read_all(fd, &header, crypto_secretstream_xchacha20poly1305_HEADERBYTES))
+		if (!read_all(fd, encrypted_secret.data, ENCRYPTED_HSM_SECRET_LEN))
 			status_failed(STATUS_FAIL_INTERNAL_ERROR,
-			              "Reading xchacha20 header: %s", strerror(errno));
-		if (!read_all(fd, cipher, sizeof(cipher)))
-			status_failed(STATUS_FAIL_INTERNAL_ERROR,
-			              "Reading encrypted secret: %s", strerror(errno));
-		if (crypto_secretstream_xchacha20poly1305_init_pull(&crypto_state, header,
-		                                                    encryption_key->data) != 0)
-			status_failed(STATUS_FAIL_INTERNAL_ERROR,
-			              "Initializing the crypto state: %s", strerror(errno));
-		if (crypto_secretstream_xchacha20poly1305_pull(&crypto_state,
-		                                               secretstuff.hsm_secret.data,
-		                                               NULL, 0, cipher, sizeof(cipher),
-		                                               NULL, 0) != 0) {
+			              "Reading encrypted hsm_secret: %s", strerror(errno));
+		if (!decrypt_hsm_secret(encryption_key, &encrypted_secret,
+					&secretstuff.hsm_secret)) {
 			/* Exit but don't throw a backtrace when the user made a mistake in typing
 			 * its password. Instead exit and `lightningd` will be able to give
 			 * an error message. */

tools/hsmtool.c

@@ -19,7 +19,6 @@
 #include <fcntl.h>
 #include <inttypes.h>
 #include <limits.h>
-#include <sodium.h>
 #include <sys/stat.h>
 #include <unistd.h>
 #include <wally_bip39.h>
@@ -91,31 +90,20 @@ static void get_encrypted_hsm_secret(struct secret *hsm_secret,
 {
 	int fd;
 	struct secret key;
+	struct encrypted_hsm_secret encrypted_secret;
 	char *err;
-	crypto_secretstream_xchacha20poly1305_state crypto_state;
-	u8 header[crypto_secretstream_xchacha20poly1305_HEADERBYTES];
-	/* The cipher size is static with xchacha20poly1305. */
-	u8 cipher[sizeof(struct secret) + crypto_secretstream_xchacha20poly1305_ABYTES];
 
 	fd = open(hsm_secret_path, O_RDONLY);
 	if (fd < 0)
 		errx(ERROR_HSM_FILE, "Could not open hsm_secret");
 
-	if (!read_all(fd, header, crypto_secretstream_xchacha20poly1305_HEADERBYTES))
-		errx(ERROR_HSM_FILE, "Could not read cipher header");
-	if (!read_all(fd, cipher, sizeof(cipher)))
-		errx(ERROR_HSM_FILE, "Could not read cipher body");
+	if (!read_all(fd, encrypted_secret.data, ENCRYPTED_HSM_SECRET_LEN))
+		errx(ERROR_HSM_FILE, "Could not read encrypted hsm_secret");
 
 	err = hsm_secret_encryption_key(passwd, &key);
 	if (err)
 		errx(ERROR_LIBSODIUM, "%s", err);
-	if (crypto_secretstream_xchacha20poly1305_init_pull(&crypto_state, header,
-							    key.data) != 0)
-		errx(ERROR_LIBSODIUM, "Could not initialize the crypto state");
-	discard_key(&key);
-	if (crypto_secretstream_xchacha20poly1305_pull(&crypto_state, hsm_secret->data,
-	                                               NULL, 0, cipher, sizeof(cipher),
-	                                               NULL, 0) != 0)
+	if (!decrypt_hsm_secret(&key, &encrypted_secret, hsm_secret))
 		errx(ERROR_LIBSODIUM, "Could not retrieve the seed. Wrong password ?");
 
 	close(fd);