* data "putty-private-key-file-mac-key"
* data passphrase
*
- * Encrypted keys should have a MAC, whereas unencrypted ones must
- * have a hash.
+ * (An empty passphrase is used for unencrypted keys.)
*
* If the key is encrypted, the encryption key is derived from the
* passphrase by means of a succession of SHA-1 hashes. Each hash