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/data_with_hash.go

@@ -0,0 +1,55 @@
+package crypto
+
+import (
+	"bytes"
+	"crypto/sha1" // #nosec
+	"io"
+)
+
+// GuessDataWithHash guesses data from data_with_hash.
+func GuessDataWithHash(dataWithHash []byte) []byte {
+	// data_with_hash := SHA1(data) + data + (0-15 random bytes);
+	// such that length be divisible by 16;
+	if len(dataWithHash) <= sha1.Size {
+		// Data length too small.
+		return nil
+	}
+
+	v := dataWithHash[:sha1.Size]
+	for i := 0; i < 16; i++ {
+		if len(dataWithHash)-i < sha1.Size {
+			// End of slice reached.
+			return nil
+		}
+		data := dataWithHash[sha1.Size : len(dataWithHash)-i]
+		h := sha1.Sum(data) // #nosec
+		if bytes.Equal(h[:], v) {
+			// Found.
+			return data
+		}
+	}
+	return nil
+}
+
+func paddedLen16(l int) int {
+	n := 16 * (l / 16)
+	if n < l {
+		n += 16
+	}
+	return n
+}
+
+// DataWithHash prepends data with SHA1(data) and 0..15 random bytes so result
+// length is divisible by 16.
+//
+// Use GuessDataWithHash(result) to obtain data.
+func DataWithHash(data []byte, randomSource io.Reader) ([]byte, error) {
+	dataWithHash := make([]byte, paddedLen16(len(data)+sha1.Size))
+	h := sha1.Sum(data) // #nosec
+	copy(dataWithHash, h[:])
+	copy(dataWithHash[sha1.Size:], data)
+	if _, err := io.ReadFull(randomSource, dataWithHash[sha1.Size+len(data):]); err != nil {
+		return nil, err
+	}
+	return dataWithHash, nil
+}