Output format is very simple:

* "GOHPENC\n" magic string
* 32-bit big-endian block length
* 128-bit random salt

                           /-------BLOCK---------\ /-------BLOCK---------\
+-------+----------+------+------------+----------+------------+----------+----
| MAGIC | BLOCKLEN | SALT | CIPHERTEXT | AUTH TAG | CIPHERTEXT | AUTH TAG | ...
+-------+----------+------+------------+----------+------------+----------+----

There is trivial key generation:
    keyOrdinary, keyLast = HKDF(SHA512, keySpecified, SALT, AD=MAGIC)

Each block is encrypted with ChaCha20-Poly1305, using keyOrdinary and
nonce containing 64-bit big-endian counter, starting at zero. Very last
block is encrypted with keyLast and it may have zero length plaintext.