Kuznechik-CTR-HMAC with key ratcheting DEM.
[cm/encrypted/]'s "/dem/a" equals to "kuznechik-ctr-hmac-kr".
CEK is 64 bytes long.
Data is split on 128 KiB chunks, each of which is encrypted the following way:

    H = Streebog-512
    CK{-1} = CEK
    CKi = HKDF-Expand(H,
        prk=HKDF-Extract(H, salt="", ikm=CK{i-1}),
        info="cm/encrypted/kuznechik-ctr-hmac-kr/kr")
    Kenc = HKDF-Expand(H, prk=CKi,
        info="cm/encrypted/kuznechik-ctr-hmac-kr/enc")
    IV = HKDF-Expand(H, len=8, prk=CKi,
        info="cm/encrypted/kuznechik-ctr-hmac-kr/iv")
    Kauth || KauthTail = HKDF-Expand(H, prk=CKi,
        info="cm/encrypted/kuznechik-ctr-hmac-kr/auth")
    CIPHERTEXT = Kuznechik-CTR(key=Kenc, ctr=IV, data=chunk)
    CIPHERTEXT || HMAC(Streebog-256, key={Kauth|KauthTail}, data=CIPHERTEXT)

Chaining key (CK) advances with every chunk. 256-bit encryption key and
randomised 64-bit nonce (initialisation vector) are derived from it.

KauthTail is used only in the last chunk to explicitly signal
that it is the last one.

"/payload"'s chunk length equals to 128KiB+32 bytes.

=> https://datatracker.ietf.org/doc/html/rfc5869.html RFC 5869, HKDF
=> https://datatracker.ietf.org/doc/html/rfc2104.html RFC 2104, HMAC
=> https://datatracker.ietf.org/doc/html/rfc7801.html RFC 7801, Kuznechik, GOST R 34.12-2015 encryption algorithm
=> https://datatracker.ietf.org/doc/html/rfc6986.html RFC 6986, Streebog, GOST R 34.11-2012 hashing algorithm
