Here is the implementation:

It splits by whitespace to find 3 "parts": $type, $key, and $comment.

I have been looking around for an authoritative standard that defines this structure, but I haven't found a clear winner:

• There is the sshd manpage section on "the authorized_keys file format" which is close but incompatible:

  Public keys consist of the following space-separated fields: options, keytype, base64-encoded key, comment.

• Some blog post seems to agree:

  The OpenSSH public key format is fully documented RFC 4253. Briefly, an OpenSSH public key consists of three fields:

  • The key type
  • A chunk of PEM-encoded data
  • A comment

  But I haven't found where in RFC 4253 that is actually written. RFC 4253 seems to be concerned about each byte of the actual key, and not the type / comment fields.

• This Super User answer also only addresses the bytes of the actual key and not the type / comment fields, but adds that "The SSH protocol does not document any file formats at all".

• I thought maybe the openLDAP schema for sshPublicKey would have a clue, but it seems to be an arbitrary ascii string.