Please note that all text insire quotes in the EBNF here is to be taken to
mean bytes that would decode as that using either the ASCII or the UTF-8
character encoding. "\n" refers specifically to the byte 0x0a, and no
alternative newlines are acceptable.

The file has a header like:

magic   = "SSHWOT" ;
version = "0" ;
comment = " ", ? General comment about the file. Valid utf-8, no '\n'. ? ;
header  = magic, version, [comment], "\n" ;

Examples of valid headers would be "SSHWOT0\n" and "SSHWOT0 Emma G. 2018\n".

"SSHWOT0 \n" is not valid, since a space marks that there will be a comment.

After the header the entries are laid out as:

salt        = ? base64 encoded salt, 44 bytes long ? ;
hashed host = ? base64 encoded sha256(host concat salt), 44 bytes long ? ;
fingerprint = ? base64 encoded sha256-fingerprint, 44 bytes long ? ;
comment     = " ", ? Comment about the host/key. Valid utf-8, no '\n'. ? ;
entry       = salt, hashed host, fingerprint, [comment], "\n" ;

The version of base64 used uses + for 62 and / for 63, uses = for padding,
and contains no breaks.

Examples of valid entries are
"Yixx+B6zrFoubPhBddgyx0nXHmbqMW1Wzneo4JqJv0U=yPUACFC/zPt/ENoIluOuWiTXor3r7oHhac63qej637E=QUJDREVGR0hJSktMTU5PUFFSU1RVVldYWVpbXF1eX2A=\n"
and
"bd/MfFs+DMVqNQQoZGGCvpTopeS0/Jt6GS5vg7J+638=cbbdTnuIh0ZwnM+/r3sAu4iHgaN3mpkcP9kJND4vBUo=YWJjZGVmZ2hpamtsbW5vcHFyc3R1dnd4eXp7fH1+f4A= The old one\n"

Again, if there is a space following the necessary parts, there must also be
a comment or else the entry is malformed.

If port is not 22, the host is [domain]:port. This is in accordance with how
OpenSSH stores it in .ssh/known_hosts. Internationalized domain names are
punycoded and all domain names are converted into lower case. This differs
from OpenSSH, which is not IDN-aware.

Sha256 is used instead of a password hash since we want checking for whether
a host is present to be reasonably fast.