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/srp/srp.go

@@ -0,0 +1,63 @@
+// Package srp contains implementation of Secure Remote Password protocol.
+package srp
+
+import (
+	"crypto/sha256"
+	"io"
+	"math/big"
+
+	"github.com/go-faster/xor"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/crypto"
+)
+
+// SRP is client implementation of Secure Remote Password protocol.
+//
+// See https://core.telegram.org/api/srp.
+type SRP struct {
+	random io.Reader
+}
+
+// NewSRP creates new SRP instance.
+func NewSRP(random io.Reader) SRP {
+	return SRP{random: random}
+}
+
+// Input is hashing algorithm parameters from server.
+//
+// Copy of tg.PasswordKdfAlgoSHA256SHA256PBKDF2HMACSHA512iter100000SHA256ModPow
+type Input struct {
+	// One of two salts used by the derivation function (see SRP 2FA login)
+	Salt1 []byte
+	// One of two salts used by the derivation function (see SRP 2FA login)
+	Salt2 []byte
+	// Base (see SRP 2FA login)
+	G int
+	// 2048-bit modulus (see SRP 2FA login)
+	P []byte
+}
+
+// Answer is result of SRP algorithm.
+type Answer struct {
+	// A parameter (see SRP)
+	A []byte
+	// M1 parameter (see SRP)
+	M1 []byte
+}
+
+func xor32(a, b [sha256.Size]byte) (res [sha256.Size]byte) {
+	xor.Bytes(res[:], a[:], b[:])
+	return res
+}
+
+func (s SRP) bigFromBytes(b []byte) *big.Int {
+	return new(big.Int).SetBytes(b)
+}
+
+func (s SRP) bigExp(x, y, m *big.Int) *big.Int {
+	return new(big.Int).Exp(x, y, m)
+}
+
+func checkInput(g int, p *big.Int) error {
+	return crypto.CheckDH(g, p)
+}