decrypt.ts

import { asymmetricAlgorithm, ivLength, pubKeyLength, symmetricAlgorithm } from "./config.ts";
async function decrypt(privateJwkString: string, payload: Uint8Array<ArrayBuffer>) {
	const privateJwk = JSON.parse(privateJwkString);
	const recipientPrivateKey = await crypto.subtle.importKey(
		"jwk",
		privateJwk,
		asymmetricAlgorithm,
		false, // not extractable
		["deriveKey"]
	);

	// 1. UNBUNDLE the blob using fixed offsets
	const receivedEphemeralPubKeyRaw = payload.buffer.slice(0, pubKeyLength);
	const receivedIv = payload.buffer.slice(pubKeyLength, pubKeyLength + ivLength);
	const receivedCiphertext = payload.buffer.slice(pubKeyLength + ivLength);

	// 2. Import the sender's ephemeral public key
	const receivedEphemeralPubKey = await crypto.subtle.importKey(
		"raw",
		receivedEphemeralPubKeyRaw,
		asymmetricAlgorithm,
		false, // not extractable
		[] // no key usages needed for the public key in derivation
	);

	// 3. Derive the exact same symmetric key using the recipient's private key and the sender's public key
	const derivedSymmetricKey = await crypto.subtle.deriveKey(
		{
			name: "ECDH",
			public: receivedEphemeralPubKey, // The received ephemeral public key
		},
		recipientPrivateKey, // The recipient's long-term private key
		symmetricAlgorithm,
		false, // not extractable
		["decrypt"] // We only need to decrypt
	);

	// 4. Decrypt the payload using the derived key and extracted IV
	const decryptedPayload = await crypto.subtle.decrypt(
		{ name: symmetricAlgorithm.name, iv: receivedIv },
		derivedSymmetricKey,
		receivedCiphertext
	);

	return new Uint8Array(decryptedPayload);
}

export default decrypt;