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/rsa_hashed.go

@@ -0,0 +1,62 @@
+package crypto
+
+import (
+	"bytes"
+	"crypto/rsa"
+	"crypto/sha1" // #nosec G505
+	"io"
+
+	"github.com/go-faster/errors"
+)
+
+// RSAEncryptHashed encrypts given data with RSA, prefixing with a hash.
+func RSAEncryptHashed(data []byte, key *rsa.PublicKey, randomSource io.Reader) ([]byte, error) {
+	// Preparing `data_with_hash`.
+	// data_with_hash := SHA1(data) + data + (any random bytes);
+	// such that the length equals 255 bytes;
+	var dataWithHash [rsaWithHashLen]byte
+	if len(data) > rsaDataLen {
+		return nil, errors.Errorf("data length %d is too big", len(data))
+	}
+
+	// Filling data_with_hash with random bytes.
+	if _, err := io.ReadFull(randomSource, dataWithHash[:]); err != nil {
+		return nil, err
+	}
+
+	h := sha1.Sum(data) // #nosec
+
+	// Replacing first 20 bytes with sha1(data).
+	copy(dataWithHash[:sha1.Size], h[:])
+	// Replacing other bytes with data itself.
+	copy(dataWithHash[sha1.Size:], data)
+
+	// Encrypting "dataWithHash" with RSA.
+	res := rsaEncrypt(dataWithHash[:], key)
+
+	return res, nil
+}
+
+// RSADecryptHashed decrypts given data with RSA.
+func RSADecryptHashed(data []byte, key *rsa.PrivateKey) ([]byte, error) {
+	var dataWithHash [rsaWithHashLen]byte
+	if !rsaDecrypt(data, key, dataWithHash[:]) {
+		return nil, errors.New("invalid data_with_hash")
+	}
+
+	hash := dataWithHash[:sha1.Size]
+	paddedData := dataWithHash[sha1.Size:]
+
+	// Guessing such data that sha1(data) == hash.
+	for i := 0; i <= len(paddedData); i++ {
+		data := paddedData[:len(paddedData)-i]
+		h := sha1.Sum(data) // #nosec
+		if bytes.Equal(h[:], hash) {
+			// Found.
+			return data, nil
+		}
+	}
+
+	// This can be caused by invalid keys or implementation bug.
+	return nil, errors.New("hash mismatch")
+}