How to convert ssh keys between different formats

Issue

• We received an ssh public key that is in an odd format we don't know what to do with. Instead of the usual 1-line key, e.g. "ssh-rsa AAAAB3NzaC1yc2EA...rItP3PZn5Dtihz2LyFkufs= rsaw@sawzall", the key spans multiple lines and looks like a certificate, e.g.:

  ---- BEGIN SSH2 PUBLIC KEY ----
  Comment: "rsa-key-20100925"
  AAAAB3NzaC1yc2EAAAADAQABAAAAYQC97noqdutsU+YqIeSlZNI9/ApXZJB0b6yAz2wk36
  z98OscQa5YstNLGiuyIrOPeHAPVf0sK0+ndyZ/3+2hb072mXDOo7EmdPNkqudiVxqPHnT+
  u93GzzZvBaIc/pMp1AE=
  ---- END SSH2 PUBLIC KEY ----
  

  Adding these lines to ~/.ssh/authorized_keys does not allow the remote ssh client to connect.