First commit in rust

backchannel-web/src/crypto.ts

@@ -0,0 +1,69 @@
+import { chacha20poly1305 } from "@noble/ciphers/chacha";
+import { x25519 } from "@noble/curves/ed25519";
+import { hkdf } from "@noble/hashes/hkdf";
+import { sha256 } from "@noble/hashes/sha256";
+import { randomBytes } from "@noble/hashes/utils";
+
+const HKDF_SALT = new TextEncoder().encode("backchannel-dm-salt-v1");
+const HKDF_INFO = new TextEncoder().encode("backchannel-dm-key-v1");
+
+const encoder = new TextEncoder();
+const decoder = new TextDecoder();
+
+export function bytesToBase64(input: Uint8Array): string {
+  let bin = "";
+  for (const b of input) {
+    bin += String.fromCharCode(b);
+  }
+  return btoa(bin);
+}
+
+export function base64ToBytes(input: string): Uint8Array {
+  const bin = atob(input);
+  const bytes = new Uint8Array(bin.length);
+  for (let i = 0; i < bin.length; i += 1) {
+    bytes[i] = bin.charCodeAt(i);
+  }
+  return bytes;
+}
+
+export function getOrCreateIdentityKey(): string {
+  const current = localStorage.getItem("bc.identity_key");
+  if (current) {
+    return current;
+  }
+
+  const key = bytesToBase64(randomBytes(32));
+  localStorage.setItem("bc.identity_key", key);
+  return key;
+}
+
+export function createEphemeralKeypair(): { privateKey: Uint8Array; publicKeyB64: string } {
+  const privateKey = x25519.utils.randomPrivateKey();
+  const publicKey = x25519.getPublicKey(privateKey);
+  return { privateKey, publicKeyB64: bytesToBase64(publicKey) };
+}
+
+export function deriveDmKey(privateKey: Uint8Array, theirPublicKeyB64: string): Uint8Array {
+  const theirPublicKey = base64ToBytes(theirPublicKeyB64);
+  const sharedSecret = x25519.getSharedSecret(privateKey, theirPublicKey);
+  return hkdf(sha256, sharedSecret, HKDF_SALT, HKDF_INFO, 32);
+}
+
+export function encryptDm(key: Uint8Array, plaintext: string): { nonce: string; ciphertext: string } {
+  const nonceBytes = randomBytes(12);
+  const cipher = chacha20poly1305(key, nonceBytes);
+  const ciphertext = cipher.encrypt(encoder.encode(plaintext));
+  return {
+    nonce: bytesToBase64(nonceBytes),
+    ciphertext: bytesToBase64(ciphertext),
+  };
+}
+
+export function decryptDm(key: Uint8Array, nonceB64: string, ciphertextB64: string): string {
+  const nonceBytes = base64ToBytes(nonceB64);
+  const ciphertext = base64ToBytes(ciphertextB64);
+  const cipher = chacha20poly1305(key, nonceBytes);
+  const plaintext = cipher.decrypt(ciphertext);
+  return decoder.decode(plaintext);
+}