How do I SSH into a Linux server from a Linux bastion host

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http://serverfault.com – The situation is like this: Windows workstation -> Linux Bastion Host -> Linux server. From my windows workstation I use putty to SSH into the Linux Bastion Host which I then SSH into the Linux server. Question is, how do I do that when the authentication between the Bastion Host and Linux server is ppk authentication. I do not wish to store the ppk key to the Linux server on my Bastion Host. (HowTos)