I was reading GitLab’s documentation (see link) on how to write to a repository from within the CI pipeline and noticed something: The described Docker executor is able to authenticate e.g. against the Git repository with only a private SSH key, being told absolutely nothing about the user’s name it is associated with.
If I’m correct, that would mean that technically, I could authenticate to an SSH server without supplying my name if I use a private key?
I know that when I don’t supply a user explicitly like ssh user@server or via .ssh/config, the active environment’s user is used automatically, that’s not what I’m asking.
The public key contains a user name/email address string, I’m aware, is the same information also encoded into the private key as well? If yes, I don’t see the need to hand that info to an SSH call. If no, how does the SSH server know which public key it’s supposed to use to challenge my private key ownership? It would have to iterate over all saved keys, which sounds rather inefficient to me and potentially unsafe (timing attacks etc.).
I hope I’m somewhat clear, for some reason I find it really hard to phrase this question.
There are instances where the user is implied, but there is always a user. As far as Git goes, the user is almost always
git.I guess this is probably the solution to my riddle. Thanks.
Yeah either that or they use a custom SSH implementation that just ignores the username because it’s not needed for the type of authentication they’re doing (like checking the keys of a specific account/project that is already known).
Technically, you always use a username, however in case of Gitlab that SSH username is always
git. When an SSH client connects to server, it offers an authentication method. Gitlab accepts that method only if it is a publickey and the fingerprint of the offered key maps to the known Gitlab user.It’s a blessing and a curse. I have two gitlab accounts on the same server - private and work. I can’t use the same key for both as the key is used to distinguish git users, and git doesn’t make it easy to select which key you want to use to pull or clone particular repo.
git config core.sshCommand 'ssh -i 'Hmm, with a similar technique one could even create git command aliases for running git with specific ssh private keys
Better than that, git config supports conditional includes, based on a repo URL or path on disk. So you can have a gitconfig per organization or whatever, which specifies an sshCommand and thus an ssh key.
Public SSH keys don’t contain any user information at all. They could have some metadata for users to easily read, but that can be deleted without repercussions.
I’m no expert, and this is probably how it does NOT work, but if you have a private key, it can generate the public key, so that could be a way to tell the server “this is me”, now try me.
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It would have to iterate over all saved keys, which sounds rather inefficient to me and potentially unsafe (timing attacks etc.)
sshd only checks for matches in the user’s
authorized_keysfile, not system wide.
