Converting between different SSH private/public key formats.

PuTTY –> SSH

• PuTTY private key to SSH public key and private key.
  puttygen supports exporting to an OpenSSH compatible format.
  1. Open PuttyGen
  2. Click Load
  3. Load your private key
  4. Go to Conversions->Export OpenSSH and export your private key
  5. Copy your private key to ~/.ssh/id_dsa (or id_rsa).
  6. Create the RFC 4716 version of the public key using ssh-keygen:
     ssh-keygen -e -f ~/.ssh/id_dsa > ~/.ssh/id_dsa_com.pub
  7. Convert the RFC 4716 version of the public key to the OpenSSH format:
     ssh-keygen -i -f ~/.ssh/id_dsa_com.pub > ~/.ssh/id_dsa.pub
• PuTTY public key to SSH public key
  If all you have is a public key from a user in PuTTY-style format, you can convert it to standard openssh format like so:
  ssh-keygen -i -f keyfile.pub > newkeyfile.pub

See this and this for more information.