add nip49 key encryption and decryption.

nip49.ts

@@ -0,0 +1,45 @@
+import { scrypt } from '@noble/hashes/scrypt'
+import { xchacha20poly1305 } from '@noble/ciphers/chacha'
+import { concatBytes, randomBytes } from '@noble/hashes/utils'
+import { Bech32MaxSize, encodeBytes } from './nip19'
+import { bech32 } from '@scure/base'
+
+export function encrypt(sec: Uint8Array, password: string, logn: number = 16, ksb: 0x00 | 0x01 | 0x02 = 0x02): string {
+  let salt = randomBytes(16)
+  let n = 2 ** logn
+  let key = scrypt(password, salt, { N: n, r: 8, p: 1, dkLen: 32 })
+  let nonce = randomBytes(24)
+  let aad = Uint8Array.from([ksb])
+  let xc2p1 = xchacha20poly1305(key, nonce, aad)
+  let ciphertext = xc2p1.encrypt(sec)
+  let b = concatBytes(Uint8Array.from([0x02]), Uint8Array.from([logn]), salt, nonce, aad, ciphertext)
+  return encodeBytes('ncryptsec', b)
+}
+
+export function decrypt(ncryptsec: string, password: string): Uint8Array {
+  let { prefix, words } = bech32.decode(ncryptsec, Bech32MaxSize)
+  if (prefix !== 'ncryptsec') {
+    throw new Error(`invalid prefix ${prefix}, expected 'ncryptsec'`)
+  }
+  let b = new Uint8Array(bech32.fromWords(words))
+
+  let version = b[0]
+  if (version !== 0x02) {
+    throw new Error(`invalid version ${version}, expected 0x02`)
+  }
+
+  let logn = b[1]
+  let n = 2 ** logn
+
+  let salt = b.slice(2, 2 + 16)
+  let nonce = b.slice(2 + 16, 2 + 16 + 24)
+  let ksb = b[2 + 16 + 24]
+  let aad = Uint8Array.from([ksb])
+  let ciphertext = b.slice(2 + 16 + 24 + 1)
+
+  let key = scrypt(password, salt, { N: n, r: 8, p: 1, dkLen: 32 })
+  let xc2p1 = xchacha20poly1305(key, nonce, aad)
+  let sec = xc2p1.decrypt(ciphertext)
+
+  return sec
+}