move gotd fork into repo. (#111)

- update to latest telegram layer
- remove some references to fields in tg.Entities that don't exist in
the schema
- originally added here:
https://github.com/beeper/td/commit/820929062a2ba0104397bc01235ab58a9cff780e
  - referenced here
-
https://github.com/mautrix/telegramgo/commit/124f0967ed195b5a380c9bd02e170ada9710dde3
-
https://github.com/mautrix/telegramgo/commit/4205047aab2e0639217148b5d125bfaab668bd8e

pkg/gotd/crypto/temp_keys.go

@@ -0,0 +1,36 @@
+package crypto
+
+import (
+	"crypto/sha1" // #nosec
+	"math/big"
+)
+
+// sha1BigInt returns SHA1(a + b).
+func sha1BigInt(a, b *big.Int) []byte {
+	var buf []byte
+	buf = append(buf, a.Bytes()...)
+	buf = append(buf, b.Bytes()...)
+
+	h := sha1.Sum(buf) // #nosec
+	return h[:]
+}
+
+// TempAESKeys returns tmp_aes_key and tmp_aes_iv based on new_nonce and
+// server_nonce as defined in "Creating an Authorization Key".
+func TempAESKeys(newNonce, serverNonce *big.Int) (key, iv []byte) {
+	// See https://core.telegram.org/mtproto/auth_key#presenting-proof-of-work-server-authentication
+	// 5. Server responds in one of two ways: [...]
+
+	// tmp_aes_key := SHA1(new_nonce + server_nonce) + substr (SHA1(server_nonce + new_nonce), 0, 12);
+	// SHA1(new_nonce + server_nonce)
+	key = append(key, sha1BigInt(newNonce, serverNonce)...)
+	// substr (SHA1(server_nonce + new_nonce), 0, 12);
+	key = append(key, sha1BigInt(serverNonce, newNonce)[:12]...)
+
+	// tmp_aes_iv := substr (SHA1(server_nonce + new_nonce), 12, 8) + SHA1(new_nonce + new_nonce) + substr (new_nonce, 0, 4);
+	iv = append(iv, sha1BigInt(serverNonce, newNonce)[12:12+8]...)
+	iv = append(iv, sha1BigInt(newNonce, newNonce)...)
+	iv = append(iv, newNonce.Bytes()[:4]...)
+
+	return key, iv
+}