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Encryption

MTProto v2 Rev3 enables encryption support to replace or enchanse TLS one. Unlike TLS, actor use multiple encryption schemes at one time. Actor encrypts message with US encryption and then again encrypt with Russian encryption that in result guarantee absolute encryption streight. US encryption is performed with AES-128-CBC-HMAC-SHA256 and Russian layer is Kuznechik-CBC-HMAC-Streebog.

We are not invenging the wheel and implement encryption logic exactly as TLS 1.2 CBC block chipcher is.

In Rev4 we will enable support for Axolotl Ratched like encryption directly in protocol.

Protocol uses 256 bits of block length in HMAC instead of standart one. This was caused by bug in initial implementation of HMAC, but doesn't affect security parameters.

HMAC is calculated from (seqNumber + iv + content.length + content).

Base Encrypted Package

EncryptedPackage {
  HEADER = 0xE8
  // Sequence number starting from zero for each direction
  seqNumber: long
  // First encryption level
  encryptedPackage: bytes
}

Container for encryption level. First one is AES, second one is Kuznechik. After decrypting AES package, you will get other EncryptionCBCPackage, decrypt it and you will get Plain Text Message object.

EncryptionCBCPackage {
  iv: bytes
  encryptedContent: bytes
}