Synchronous encryption algorithm "Kuznechik" (GOST R 34.12-2015, GOST R 34.13-2015)

Implementation of the block synchronous encryption algorithm "Kuznechik" in Rust lang.

Encryption modes

Struct	Title	Reduction
AlgEcb	Electronic Codebook	ЕСВ
AlgCtr	Counter	CTR
AlgOfb	Output Feedback	OFB
AlgCbc	Cipher Block Chaining	СВС
AlgCfb	Cipher Feedback	CFB
AlgMac	Message Authentication Code	MAC

Usage (AlgOfb):

The following example encrypts and decrypts a 64-byte data block data using the OFB method. Encryption by other methods is similar, in some the gamma is not required (more details in tests.rs).

extern crate kuznechik;

use self::kuznechik::{KeyStore, Kuznechik, AlgOfb};

fn main() {
    // Initialization
    let password = "Kuznechik";
    let data = Vec::from("Hello, World!");

    let gamma = vec![
        0x12, 0x34, 0x56, 0x78, 0x90, 0xab, 0xce, 0xf0, 0xa1, 0xb2, 0xc3, 0xd4, 0xe5, 0xf0, 0x01, 0x12,
        0x23, 0x34, 0x45, 0x56, 0x67, 0x78, 0x89, 0x90, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19,
    ];

    let kuz = KeyStore::with_password(password);
    let mut cipher = AlgOfb::new(&kuz).gamma(gamma.clone());

    // Encryption
    let enc_data = cipher.encrypt(data.clone());

    // Decryption
    // Setting gamma again because OFB algorithm modify it during performing
    cipher.set_gamma(gamma);
    let dec_data = cipher.decrypt(enc_data);

    // Assert
    assert_eq!(data, dec_data);
}