Not really a Help and Support query but found no better category. The crypt page (Crypt) says this (towards the end):
#### Chunk
Each chunk will contain 64 KiB of data, except for the last one which may have less data. The data chunk is in standard NaCl SecretBox format. SecretBox uses XSalsa20 and Poly1305 to encrypt and authenticate messages.
Each chunk contains:
* 16 Bytes of Poly1305 authenticator
* 1 - 65536 bytes XSalsa20 encrypted data
64k chunk size was chosen as the best performing chunk size (the authenticator takes too much time below this and the performance drops off due to cache effects above this). Note that these chunks are buffered in memory so they can't be too big.
This uses a 32 byte (256 bit key) key derived from the user password.
#### Examples
1 byte file will encrypt to
* 32 bytes header
* 17 bytes data chunk
49 bytes total
1 MiB (1048576 bytes) file will encrypt to
* 32 bytes header
* 16 chunks of 65568 bytes
^^
1049120 bytes total (a 0.05% overhead). This is the overhead for big files.
Shouldn't it read "16 chunks of 65552 bytes"?