This adds a new KMS, SSHAgentKMS, which is a KMS to provide signing keys
for issuing ssh certificates signed by a key managed by a ssh-agent. It
uses the golang.org/x/crypto package to get a native Go implementation
to talk to a ssh-agent.
This was primarly written to be able to use gpg-agent to provide the
keys stored in a YubiKeys openpgp interface, but can be used for other
setups like proxying a ssh-agent over network.
That way the signing key for ssh certificates can be kept in a
"sign-only" hsm.
This code was written for my employer Intinor AB, but for simplicity
sake gifted to me to contribute upstream.
Signed-off-by: Anton Lundin <glance@acc.umu.se>
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