SSH2 and OpenSSH

A colleague gave me his public key to be authorized for access into this server we were working on. When I opened it, it looked different from what I came to expect:

---- BEGIN SSH2 PUBLIC KEY ----
Comment: "rsa-key-20201017"
AAAAB3NzaC1yc2EAAAABJQAAAQEAoR40GT6gmARyx6N2J9L03W6pLcqg2JdWAusX
X2Q/VxuEP9YZtmwak12wR+orbOgsSz1vYX8uS6m/CCpxloGa1BhHTQQ1VjbBD3I2
wjts6Zl2eqGaMmhHAlCBY/a8SdmhbBYzqddcZIZoePApNEiugE7TT86BJVVU/DHW
W5zZF8ggINc2k2d4EWggnlotKqPAQRcK2Im++znyrflIwna+TiVMoLWmShGOpn6K
Jft/oPwbXV88X7cjkdq0sDp/pig3mqa5G+d6l0hhNLHUZbYZu97KansWe1m3q77c
fEfB34NMtv/Ni91IkRBges3o3NuVC1Ki1hJ1x9mAGfRWsA1pJQ==
---- END SSH2 PUBLIC KEY ----

As it would turn out, this is an example of a key generated in SSH2 format. This is commonly used by PuTTY, an SSH client favored by many Windows users.

For it to be recognized and accepted by our Ubuntu server, I had to convert this into OpenSSH format.

An OpenSSH format looks like this:

ssh-rsa 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

There is a simple option on ssh-keygen, the same program used to generate OpenSSH key pairs, to convert SSH2 formatted keys into OpenSSH format. It is done like so:

ssh-keygen -i -f ssh2.pub > openssh.pub

Where ssh2.pub is the SSH2 formatted key and openssh.pub is an arbitrary file name for the converted key output.