Introduce NIP-44 encryption standard

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+NIP-44
+======
+
+Encrypted Direct Message (Versioned)
+------------------------------------
+
+`optional` `author:paulmillr` `author:staab`
+
+The NIP introduces versioned encryption, allowing multiple algorithm choices to exist simultaneously.
+
+The algorithm described in NIP4 is potentially vulnerable to [padding oracle attacks](https://en.wikipedia.org/wiki/Padding_oracle_attack) and uses keys which are not indistinguishable from random.
+
+An encrypted payload MUST be encoded as a JSON object. Different versions may have different parameters. Every format has a `v` field specifying its version.
+
+Currently defined encryption algorithms:
+
+- `0x00` - Reserved
+- `0x01` - XChaCha with same key `sha256(ecdh)` per conversation
+
+# Version 0
+
+Version 0 is not defined, however implementations depending on this NIP MAY choose to support the payload described in NIP 04 in the same places a NIP 44 payload would otherwise be expected. This is intended to allow a smooth transition while clients and signing software adopt the new standard.
+
+# Version 1
+
+Params:
+
+1. `nonce`: base64-encoded xchacha nonce
+2. `ciphertext`: base64-encoded xchacha ciphertext, created from (key, nonce) against `plaintext`.
+
+Example:
+
+```
+{
+  "ciphertext": "FvQi1H4atMwU+FzUR/0CJ7kowjs+",
+  "nonce": "3dBKd83Pg2Q4Tu2A2e8N++c+ZW2IBc2f",
+  "v": 1
+}
+```
+
+**Note**: By default in the [libsecp256k1](https://github.com/bitcoin-core/secp256k1) ECDH implementation, the secret is the SHA256 hash of the shared point (both X and Y coordinates). We are using this exact implementation. In NIP4, unhashed shared point was used.
+
+## Code Samples
+
+### Javascript
+
+```javascript
+import {xchacha20} from "@noble/ciphers/chacha"
+import {secp256k1} from "@noble/curves/secp256k1"
+import {sha256} from "@noble/hashes/sha256"
+import {randomBytes} from "@noble/hashes/utils"
+import {base64} from "@scure/base"
+
+export const utf8Decoder = new TextDecoder()
+
+export const utf8Encoder = new TextEncoder()
+
+export const getSharedSecret = (privkey: string, pubkey: string): Uint8Array =>
+  sha256(secp256k1.getSharedSecret(privkey, "02" + pubkey).subarray(1, 33))
+
+export function encrypt(privkey: string, pubkey: string, text: string, v = 1) {
+  if (v !== 1) {
+    throw new Error("NIP44: unknown encryption version")
+  }
+
+  const key = getSharedSecret(privkey, pubkey)
+  const nonce = randomBytes(24)
+  const plaintext = utf8Encoder.encode(text)
+  const ciphertext = xchacha20(key, nonce, plaintext)
+
+  return JSON.stringify({
+    ciphertext: base64.encode(ciphertext),
+    nonce: base64.encode(nonce),
+    v,
+  })
+}
+
+export function decrypt(privkey: string, pubkey: string, payload: string) {
+  try {
+    payload = JSON.parse(payload) as {
+      ciphertext: string
+      nonce: string
+      v: number
+    }
+  } catch (e) {
+    throw new Error("NIP44: failed to parse payload")
+  }
+
+  if (data.v !== 1) {
+    throw new Error("NIP44: unknown encryption version")
+  }
+
+  const key = getSharedSecret(privkey, pubkey)
+  const nonce = base64.decode(data.nonce)
+  const ciphertext = base64.decode(data.ciphertext)
+  const plaintext = xchacha20(key, nonce, ciphertext)
+
+  return utf8Decoder.decode(plaintext)
+}
+```
+
+### Kotlin
+
+```kotlin
+// implementation 'fr.acinq.secp256k1:secp256k1-kmp-jni-android:0.10.1'
+// implementation "com.goterl:lazysodium-android:5.1.0@aar"
+// implementation "net.java.dev.jna:jna:5.12.1@aar"
+
+fun getSharedSecretNIP44(privKey: ByteArray, pubKey: ByteArray): ByteArray =
+  MessageDigest.getInstance("SHA-256").digest(
+    Secp256k1.get().pubKeyTweakMul(
+      Hex.decode("02") + pubKey,
+      privKey
+    ).copyOfRange(1, 33)
+  )
+
+fun encryptNIP44(msg: String, privKey: ByteArray, pubKey: ByteArray): EncryptedInfo {
+  val nonce = ByteArray(24).apply {
+    SecureRandom.getInstanceStrong().nextBytes(this)
+  }
+
+  val cipher = streamXChaCha20Xor(
+    message = msg.toByteArray(),
+    nonce = nonce,
+    key = getSharedSecretNIP44(privKey, pubKey)
+  )
+
+  return EncryptedInfo(
+    ciphertext = Base64.getEncoder().encodeToString(cipher),
+    nonce = Base64.getEncoder().encodeToString(nonce),
+    v = Nip24Version.XChaCha20.code
+  )
+}
+
+fun decryptNIP44(encInfo: EncryptedInfo, privKey: ByteArray, pubKey: ByteArray): String? {
+  require(encInfo.v == Nip24Version.XChaCha20.code) { "NIP44: unknown encryption version" }
+
+  return streamXChaCha20Xor(
+    message = Base64.getDecoder().decode(encInfo.ciphertext),
+    nonce = Base64.getDecoder().decode(encInfo.nonce),
+    key = getSharedSecretNIP44(privKey, pubKey)
+  )?.decodeToString()
+}
+
+// This method is not exposed in AndroidSodium yet, but it will be in the next version.
+fun streamXChaCha20Xor(message: ByteArray, nonce: ByteArray, key: ByteArray): ByteArray? {
+  return with (SodiumAndroid()) {
+    val resultCipher = ByteArray(message.size)
+
+    val isSuccessful = crypto_stream_chacha20_xor_ic(
+      resultCipher,
+      message,
+      message.size.toLong(),
+      nonce.drop(16).toByteArray(), // chacha nonce is just the last 8 bytes.
+      0,
+      ByteArray(32).apply {
+      crypto_core_hchacha20(this, nonce, key, null)
+      }
+    ) == 0
+
+    if (isSuccessful) resultCipher else null
+  }
+}
+
+data class EncryptedInfo(val ciphertext: String, val nonce: String, val v: Int)
+
+enum class Nip24Version(val code: Int) {
+  Reserved(0),
+  XChaCha20(1)
+}