[Bug 1615305] [NEW] Output when changing comment is weird and badly documented

[Karol Babioch]

Public bug reported:

Changing a key comment within a private key might not be something you
do on a daily basis, but it is mostly a frustrating task, since the
documentation is incomplete and wrong. In particular the man page says:

       -c     Requests changing the comment in the private and public key files.  This operation is only supported for RSA1 keys and keys stored in  the  newer  OpenSSH
              format.  The program will prompt for the file containing the private keys, for the passphrase if the key has one, and for the new comment.

So, one gets the impression that this is not applicable to newer keys.
However bug #811125 dealt with this, and there was a commit (see [1])
that added support for basically all key types. In the case of RSA keys
one has to use the new key format though (-o), which can only be find
out with trial and error and is not documented at all.

Furthermore the output of an actual ssh-keygen run is also confusing:

[kbabioch at antares .ssh]$ ssh-keygen -c -C "new comment" -f id_rsa -o
Enter passphrase: 
Key now has comment '(null)'
The comment in your key file has been changed.

The output tells me that the key is now empty (null). However the
comment is correctly set, so while this works as intended, it is
confusing to the user.

[1] https://github.com/openssh/openssh-
portable/commit/4d90625b229cf6b3551d81550a9861897509a65f#diff-
8a50ef3f3b9ea11be3c3b2fc1c0555b3

** Affects: openssh (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: change comment documentation ssh

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https://bugs.launchpad.net/bugs/1615305

Title:
  Output when changing comment is weird and badly documented

Status in openssh package in Ubuntu:
  New

Bug description:
  Changing a key comment within a private key might not be something you
  do on a daily basis, but it is mostly a frustrating task, since the
  documentation is incomplete and wrong. In particular the man page
  says:

         -c     Requests changing the comment in the private and public key files.  This operation is only supported for RSA1 keys and keys stored in  the  newer  OpenSSH
                format.  The program will prompt for the file containing the private keys, for the passphrase if the key has one, and for the new comment.

  So, one gets the impression that this is not applicable to newer keys.
  However bug #811125 dealt with this, and there was a commit (see [1])
  that added support for basically all key types. In the case of RSA
  keys one has to use the new key format though (-o), which can only be
  find out with trial and error and is not documented at all.

  Furthermore the output of an actual ssh-keygen run is also confusing:

  [kbabioch at antares .ssh]$ ssh-keygen -c -C "new comment" -f id_rsa -o
  Enter passphrase: 
  Key now has comment '(null)'
  The comment in your key file has been changed.

  The output tells me that the key is now empty (null). However the
  comment is correctly set, so while this works as intended, it is
  confusing to the user.

  [1] https://github.com/openssh/openssh-
  portable/commit/4d90625b229cf6b3551d81550a9861897509a65f#diff-
  8a50ef3f3b9ea11be3c3b2fc1c0555b3

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