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/exchange/generator.go

@@ -0,0 +1,81 @@
+package exchange
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"io"
+	"math/big"
+
+	"github.com/go-faster/errors"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/crypto"
+)
+
+// ServerRNG is server-side random number generator.
+type ServerRNG interface {
+	PQ() (pq *big.Int, err error)
+	GA(g int, dhPrime *big.Int) (a, ga *big.Int, err error)
+	DhPrime() (p *big.Int, err error)
+}
+
+var _ ServerRNG = TestServerRNG{}
+
+// TestServerRNG implements testing-only ServerRNG.
+type TestServerRNG struct {
+	rand io.Reader
+}
+
+func (s TestServerRNG) bigFromHex(hexString string) (p *big.Int, err error) {
+	data, err := hex.DecodeString(hexString)
+	if err != nil {
+		return nil, errors.Wrap(err, "decode hex string")
+	}
+
+	return big.NewInt(0).SetBytes(data), nil
+}
+
+// PQ always returns testing pq value.
+//
+// nolint:unparam
+func (s TestServerRNG) PQ() (pq *big.Int, err error) {
+	return big.NewInt(0x17ED48941A08F981), nil
+}
+
+// GA returns testing a and g_a params.
+func (s TestServerRNG) GA(g int, dhPrime *big.Int) (a, ga *big.Int, err error) {
+	if err := crypto.CheckGP(g, dhPrime); err != nil {
+		return nil, nil, err
+	}
+
+	gBig := big.NewInt(int64(g))
+	one := big.NewInt(1)
+	dhPrimeMinusOne := big.NewInt(0).Sub(dhPrime, one)
+
+	safetyRangeMin := big.NewInt(0).Exp(big.NewInt(2), big.NewInt(crypto.RSAKeyBits-64), nil)
+	safetyRangeMax := big.NewInt(0).Sub(dhPrime, safetyRangeMin)
+
+	randMax := big.NewInt(0).SetBit(big.NewInt(0), crypto.RSAKeyBits, 1)
+	for {
+		a, err = rand.Int(s.rand, randMax)
+		if err != nil {
+			return
+		}
+
+		ga = big.NewInt(0).Exp(gBig, a, dhPrime)
+		if crypto.InRange(ga, one, dhPrimeMinusOne) && crypto.InRange(ga, safetyRangeMin, safetyRangeMax) {
+			return
+		}
+	}
+}
+
+// DhPrime always returns testing dh_prime.
+func (s TestServerRNG) DhPrime() (p *big.Int, err error) {
+	return s.bigFromHex("C71CAEB9C6B1C9048E6C522F70F13F73980D40238E3E21C14934D037563D930F" +
+		"48198A0AA7C14058229493D22530F4DBFA336F6E0AC925139543AED44CCE7C37" +
+		"20FD51F69458705AC68CD4FE6B6B13ABDC9746512969328454F18FAF8C595F64" +
+		"2477FE96BB2A941D5BCD1D4AC8CC49880708FA9B378E3C4F3A9060BEE67CF9A4" +
+		"A4A695811051907E162753B56B0F6B410DBA74D8A84B2A14B3144E0EF1284754" +
+		"FD17ED950D5965B4B9DD46582DB1178D169C6BC465B0D6FF9CA3928FEF5B9AE4" +
+		"E418FC15E83EBEA0F87FA9FF5EED70050DED2849F47BF959D956850CE929851F" +
+		"0D8115F635B105EE2E4E15D04B2454BF6F4FADF034B10403119CD8E3B92FCC5B")
+}