diff --git a/age/format/format.ha b/age/format/format.ha
index 8db29bc..cdfe4a3 100644
--- a/age/format/format.ha
+++ b/age/format/format.ha
@@ -99,7 +99,7 @@ fn split_args(line: str) (str, []str) = {
 	return (parts[0], parts[1..]);
 };
 
-fn decode_string(encoded: str) ([]u8 | parse_error) = {
+export fn decode_string(encoded: str) ([]u8 | parse_error) = {
 	if (strings::contains(encoded, "\n", "\r")) {
 		return fmt::fatalf("unexpected newline character");
 	};
diff --git a/age/x25519/x25519.ha b/age/x25519/x25519.ha
index f842563..7df4285 100644
--- a/age/x25519/x25519.ha
+++ b/age/x25519/x25519.ha
@@ -9,8 +9,17 @@ use crypto::random;
 use memio;
 use crypto::curve25519;
 use bytes;
+use crypto::sha256;
+use age::format;
+use crypto::hkdf;
+use crypto::chachapoly;
 
-const MEGABYTE: size = 1024 * 1024;
+export type incorrectidentity = !void;
+
+export type parsingerror = !void;
+
+def MEGABYTE: size = 1024 * 1024;
+def FILE_KEY_SIZE: size = 16;
 
 // an x25519 identity.
 // 
@@ -90,3 +99,63 @@ export fn read_identity_file(file: str) []identity = {
 	};
 	return identities;
 };
+
+export type incorrect_ciphertext_size = !void;
+export type aead_decrypt_error = !(void | incorrect_ciphertext_size);
+
+fn aead_decrypt(key: []u8, msg_size: size, ciphertext: []u8) ([]u8 | aead_decrypt_error) = {
+	let output: [32]u8 = [0...];
+	const nonce: []u8 = [0...];
+
+	if (len(ciphertext) > (msg_size + chachapoly::TAGSZ)) {
+		return incorrect_ciphertext_size;
+	};
+	const aead = chachapoly::chachapoly();
+	chachapoly::init(&aead, &memio::fixed(output), key, nonce);
+
+	return output;
+};
+
+export fn unwrap_file_key(identity: *identity,  block: format::stanza) ([16]u8 | incorrectidentity | parsingerror | aead_decrypt_error) = {
+	if (block.recipient_type != "X25519") {
+		return incorrectidentity;
+	};
+	if (len(block.args) != 1) {
+		return parsingerror;
+	};
+	const public_key = format::decode_string(block.args[0])!;
+	const our_public_key = match (bech32::decodestr(private_to_public(*identity))) {
+		case let data: (str, []u8) =>
+			let (hrp, key_bytes) = data;
+			if (ascii::strlower(hrp)! != "age" || len(key_bytes) != 32) {
+				fmt::fatal("invalid public key generated from private key");
+			};
+			defer bytes::zero(key_bytes);
+			const key: [32]u8 = [0...];
+			io::copy(&memio::fixed(key), &memio::fixed(key_bytes))!;
+			yield key;
+		case =>
+			fmt::fatal("invalid public key generated from private key");
+	};
+	if (len(public_key) != curve25519::POINTSZ) {
+		return parsingerror;
+	};
+
+	let shared_secret: []u8 = [];
+	curve25519::x25519(shared_secret, *identity, public_key);
+
+	let salt: [64]u8 = [0...]; 
+	salt[0..32] = public_key[0..32];
+	salt[32..64] = our_public_key[0..32];
+
+	let wrapping_key: [32]u8 = [0...];
+	let key_buf: [sha256::SZ + sha256::BLOCKSZ]u8 = [0...];
+	const h = hkdf::hkdf(&sha256::sha256(), wrapping_key, shared_secret, strings::toutf8("age-encryption.org/v1/X25519"), salt, key_buf);
+
+	const info = "age-encryption.org/v1/X25519";
+
+	let unwrapped: [16]u8 = [0...];
+	unwrapped[0..16] = aead_decrypt(wrapping_key, FILE_KEY_SIZE, block.body)![0..16];
+
+	return unwrapped;
+};
\ No newline at end of file