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How do you know whether you're SSHing to a place you're not supposed to or not? There's no CA telling you that it's legit like there is for https. 99% of people

How do you know whether you're SSHing to a place you're not supposed to or not? There's no CA telling you that it's legit like there is for https. 99% of people don't manually verify the host key fingerprint.

yjftsjthsd-h 7y ago
SSH does support CAs, there's just no PKI in place. But you can totally set up an internal SSH CA and sign the host keys of every server in your company.
- londons_explore 7y ago
  Would anything stop you just loading let's encrypt onto it? Will regular HTTPS certificates work?
  - yjftsjthsd-h 7y ago
    No, ssh uses ssh keys to sign keys.
- fanf2 7y ago
  There is a PKI: DNSSEC with SSHFP records.
  - tptacek 7y ago
    That's principally a PKI for SSH host keys, which aren't the keys people talk about when we talk about SSH CAs.
    - yjftsjthsd-h 7y ago
      Is it really that much more common to sign user keys rather than host keys? I've been quite fond of signing hosts, personally.
      - tptacek 7y ago
        When people talk about SSH CA's, they're talking about systems where you are granted access to a fleet of servers not by having your id_rsa.pub copied into a bunch of authorized_keys files through some automated process, but rather by having the whole fleet trust a CA key, and then having the CA key sign certificates for users (typically: through an identity provider). The wins are: * It's a lot easier to provision users because the per-server configuration is static. * You can issue short-lived keys for users so that a lost laptop won't be carrying lots of dangerous SSH keys. * You can force users through your standard authentication flow and get things like MFA without requiring the SSH servers to understand MFA individually. Outside of this use case, it's not common to sign SSH keys at all. SSHFP isn't widely used; in fact, almost nobody uses it (because almost nobody uses DNSSEC in the first place). But even if you were set up to use it, SSHFP wouldn't do the things we're talking about when we talk about SSH CAs. In fact, it'd be hard to come up with a worse place to put user keys than in the DNS, which is public, even (in fact: especially) with DNSSEC.
        yjftsjthsd-h 7y ago
        Yeah, I've done it for users, but it's basically the same for host keys; you sign the host key with the CA, stick `HostCertificate /etc/ssh/ssh_host_ed25519_key-cert.pub` in your sshd config, and stop getting prompted to manually verify host keys on connection. Would that not still be called a CA?

yjftsjthsd-h 7y ago

SSH does support CAs, there's just no PKI in place. But you can totally set up an internal SSH CA and sign the host keys of every server in your company.

londons_explore 7y ago
Would anything stop you just loading let's encrypt onto it? Will regular HTTPS certificates work?
- yjftsjthsd-h 7y ago
  No, ssh uses ssh keys to sign keys.
fanf2 7y ago
There is a PKI: DNSSEC with SSHFP records.
- tptacek 7y ago
  That's principally a PKI for SSH host keys, which aren't the keys people talk about when we talk about SSH CAs.
  - yjftsjthsd-h 7y ago
    Is it really that much more common to sign user keys rather than host keys? I've been quite fond of signing hosts, personally.
    - tptacek 7y ago
      When people talk about SSH CA's, they're talking about systems where you are granted access to a fleet of servers not by having your id_rsa.pub copied into a bunch of authorized_keys files through some automated process, but rather by having the whole fleet trust a CA key, and then having the CA key sign certificates for users (typically: through an identity provider). The wins are: * It's a lot easier to provision users because the per-server configuration is static. * You can issue short-lived keys for users so that a lost laptop won't be carrying lots of dangerous SSH keys. * You can force users through your standard authentication flow and get things like MFA without requiring the SSH servers to understand MFA individually. Outside of this use case, it's not common to sign SSH keys at all. SSHFP isn't widely used; in fact, almost nobody uses it (because almost nobody uses DNSSEC in the first place). But even if you were set up to use it, SSHFP wouldn't do the things we're talking about when we talk about SSH CAs. In fact, it'd be hard to come up with a worse place to put user keys than in the DNS, which is public, even (in fact: especially) with DNSSEC.
      - yjftsjthsd-h 7y ago
        Yeah, I've done it for users, but it's basically the same for host keys; you sign the host key with the CA, stick `HostCertificate /etc/ssh/ssh_host_ed25519_key-cert.pub` in your sshd config, and stop getting prompted to manually verify host keys on connection. Would that not still be called a CA?