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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).

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SSH login without user name? (docs.gitlab.com)

submitted 6 months ago by [email protected] to c/[email protected]

23 comments fedilink hide all child comments

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.

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[–] [email protected] 1 points 6 months ago (1 children)

When authenticating with git over SSH, the private key should be considered secret and well protected.

That means the corresponding public key that was uploaded to the git server is enough to authenticate and no username is required. However, a password protected privare key is possible and extra layers of security can be added to the authentication mechanism.

As far as resource based attacks based on public key searching, I doubt many servers have significant enough public keys on a single host to even notice. Most repos are siloed and have specific teams/individuals assigned to them, so only a small number of public keys even gets loaded.

I dont know enough about the server side mechanics to be sure, but imo the attack surface is pretty small.

[–] [email protected] 1 points 6 months ago (1 children)

That means the corresponding public key that was uploaded to the git server is enough to authenticate and no username is required.

A username is required to authenticate with an SSH server. A public key alone is not enough.

[–] [email protected] 2 points 6 months ago (1 children)

While true, in most cases the username is simply "git" and not a specific username tied to the pub/priv keypair

[–] [email protected] 1 points 6 months ago

The post you originally replied to was misunderstanding how the username is located when authenticating with a server.

Original post:

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?

Your reply would be creating more confusion, because you implied that no username is required.

Your reply:

That means the corresponding public key that was uploaded to the git server is enough to authenticate and no username is required.

I am just clarifying if the original poster read your comment and was led to believe they wouldn't need a username. It is, in fact, required. As you expressed, it's usually "git" when connecting to a a git server, but it doesn't have to be.