Finally, there is a line saying "Private-MAC: " plus a hex
representation of a HMAC-SHA-1 of:
string name of algorithm ("ssh-dss", "ssh-rsa")
string encryption type
string comment
string public-blob
string private-plaintext (the plaintext version of the
private part, including the final
padding)
The key to the MAC is itself a SHA-1 hash of:
data "putty-private-key-file-mac-key"
data passphrase
(An empty passphrase is used for unencrypted keys.)