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/dh.go

@@ -0,0 +1,47 @@
+package crypto
+
+import (
+	"math/big"
+
+	"github.com/go-faster/errors"
+)
+
+// CheckDHParams checks that g_a, g_b and g params meet key exchange conditions.
+//
+// https://core.telegram.org/mtproto/auth_key#dh-key-exchange-complete
+func CheckDHParams(dhPrime, g, gA, gB *big.Int) error {
+	one := big.NewInt(1)
+	dhPrimeMinusOne := big.NewInt(0).Sub(dhPrime, one)
+	if !InRange(g, one, dhPrimeMinusOne) {
+		return errors.New("kex: bad g, g must be 1 < g < dh_prime - 1")
+	}
+	if !InRange(gA, one, dhPrimeMinusOne) {
+		return errors.New("kex: bad g_a, g_a must be 1 < g_a < dh_prime - 1")
+	}
+	if !InRange(gB, one, dhPrimeMinusOne) {
+		return errors.New("kex: bad g_b, g_b must be 1 < g_b < dh_prime - 1")
+	}
+
+	// IMPORTANT: Apart from the conditions on the Diffie-Hellman prime
+	// dh_prime and generator g, both sides are to check that g, g_a and
+	// g_b are greater than 1 and less than dh_prime - 1. We recommend
+	// checking that g_a and g_b are between 2^{2048-64} and
+	// dh_prime - 2^{2048-64} as well.
+
+	// 2^{2048-64}
+	safetyRangeMin := big.NewInt(0).Exp(big.NewInt(2), big.NewInt(RSAKeyBits-64), nil)
+	safetyRangeMax := big.NewInt(0).Sub(dhPrime, safetyRangeMin)
+	if !InRange(gA, safetyRangeMin, safetyRangeMax) {
+		return errors.New("kex: bad g_a, g_a must be 2^{2048-64} < g_a < dh_prime - 2^{2048-64}")
+	}
+	if !InRange(gB, safetyRangeMin, safetyRangeMax) {
+		return errors.New("kex: bad g_b, g_b must be 2^{2048-64} < g_b < dh_prime - 2^{2048-64}")
+	}
+
+	return nil
+}
+
+// InRange checks whether x is in (min, max) range, i.e. min < x < max.
+func InRange(x, min, max *big.Int) bool {
+	return x.Cmp(min) > 0 && x.Cmp(max) < 0
+}