To start: puTTYgen does not convert public keys in one format to
public keys in another format, although the Putty developers 
are thinking about doing something about this one day.

Converting public keys from SSH.COM to OpenSSH format can even 
be done  by hand, or by the ssh_keygen utility on OpenSSH ssh 
servers (most Unixes, also available for Windows).

What is does do, is converting PRIVATE keys from one format 
to another, which makes this utility almost unique.
Given a private key, puTTYgen can generate a public key in
any supported format (OpenSSH, SSH.COM, Putty ).

As Halibut or whatever tools only live on selected systems a short 
description of how to use PuTTYgen to convert SSH2 public and private
keys from SSH.COM format ( the format used bij TCP/IP Services for
OpenVMS) to OpenSSH format (the format used by mosy *ux ssh servers).

1. Define a foreign command to point to puTTYgen.exe like this

        puTTYgen := $sys$sysdevice:[myexes]puTTYgen.exe

   do not forget the dollarsign in front of the filespecification 
   of the executable.

Examples:

a. Convert private SSH.COM key ( possibly generated by ssh-keygen
   of TCP/IP services ) to a private OpenSSH key:
        
        puTTYgen "alice.;" "-Oprivate-openssh" -o alice_ossh.;

b. Convert/create a public OpenSSH.com key from a SSH.COM
   private key ( possibly generated by ssh-keygen of TCP/IP 
   services ), to be distributed to Unix systems you want 
   to log in to:

        puTTygen blurb.; "-Opublic-openssh" -o  blurb_ossh.pub; 

c. Generate a key in Putty format and then export the keys to
   various formats :

        puTTYgen  -t rsa -b 1024 "-C" "The big knurf test" -o knurf.ppk 
        ...

   convert/export a public/private keypair in OpenSSH format :

        puTTYgen  knurf.ppk "-Oprivate-openssh" -o knurf_ossh.;
        puTTYgen  knurf.ppk "-Opublic-openssh" -o knurf_ossh.pub;

   convert/export a public/private keypair in SSH.COM format :

        puTTYgen  knurf.ppk "-Opublic" -o knurf.pub;
        puTTYgen  knurf.ppk "-Oprivate-sshcom" -o knurf.;

Jose Baars. 1-mar-2010