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SSH certificates: the better SSH experience

jpmens.net

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Re: SSH certificates: the better SSH experience

#11

Post by papyDoctor »

thomashabets2 wrote: post #2

Every couple of months someone re-discovers SSH certificates, and blogs about them. I'm guilty of it too. My blog post from 15 years ago is nowhere near as good as OP's post, but if I though me of 15 years ago lived up to my standards of today, I'd be really disappointed: https://blog.habets.se/2011/07/OpenSSH-certificates.html

Another useful feature of SSH certificates is that you can sign a user’s public key to grant them access to a remote machine for a limited time and as a specific remote user.

Re: SSH certificates: the better SSH experience

#12

Post by thomashabets2 »

kaoD wrote: post #3
thomashabets2 wrote: post #2

Every couple of months someone re-discovers SSH certificates, and blogs about them. I'm guilty of it too. My blog post from 15 years ago is nowhere near as good as OP's post, but if I though me of 15 years ago lived up to my standards of today, I'd be really disappointed: https://blog.habets.se/2011/07/OpenSSH-certificates.html

I've known SSH certs for a while but never went through the effort of migrating away from keys. I'm very frustrated about manually managing my SSH keys across my different servers and devices though. I assume you gathered a lot of thoughts over these 15 years. Should I invest in making the switch?

If your use case is such that you are frustrated about managing keys, host or user keys, then yes it does sound like SSH certs would help you. E.g. when you have many users, servers, or high enough cartesian product of the two.

In environment where they don't cause frustration they're not worth it.

Not really more to it than that, from my point of view.

Re: SSH certificates: the better SSH experience

#13

Post by akerl_ »

Tepix wrote: post #7

The author lists all the advantes of CA certificates, yet doesn't list the disadvantages. OTOH, all the many steps required to set it up make the disadvantages rather obvious. Also, I've never had a security issue due to TOFU, have you?

> Also, I've never had a security issue due to TOFU, have you?

This is a bit like suggesting you've never been in a car crash, so seat belts must not be worth considering.

Do you feel that beyond the obvious and documented work in setting them up, there are disadvantages to using SSH certificates?

Re: SSH certificates: the better SSH experience

#15

Post by jsiepkes »

jcalvinowens wrote: post #14

You can also address TOFU to some extent using SSHFP DNS records. Openssh supports checking the DNSSEC signature in the client, in theory, but it's a configure option and I'm not sure if distros build with it.

On top of that you would need something to secure DNS. Like DNSSEC or at the very least use DNS with TLS or DNS over HTTP. None of these are typically enabled by default.

Re: SSH certificates: the better SSH experience

#16

Post by otabdeveloper4 »

kaoD wrote: post #3
thomashabets2 wrote: post #2

Every couple of months someone re-discovers SSH certificates, and blogs about them. I'm guilty of it too. My blog post from 15 years ago is nowhere near as good as OP's post, but if I though me of 15 years ago lived up to my standards of today, I'd be really disappointed: https://blog.habets.se/2011/07/OpenSSH-certificates.html

I've known SSH certs for a while but never went through the effort of migrating away from keys. I'm very frustrated about manually managing my SSH keys across my different servers and devices though. I assume you gathered a lot of thoughts over these 15 years. Should I invest in making the switch?

You will have to manage your SSH CA certificates instead of your keys.

The workflows SSH CA's are extremely janky and insecure.

With some creative use of `AuthorizedKeysCommand` you can make SSH key rotation painless and secure.

With SSH certificates you have to go back to the "keys to the kingdom" antipattern and just hope for the best.

Re: SSH certificates: the better SSH experience

#17

Post by otabdeveloper4 »

akerl_ wrote: post #13
Tepix wrote: post #7

The author lists all the advantes of CA certificates, yet doesn't list the disadvantages. OTOH, all the many steps required to set it up make the disadvantages rather obvious. Also, I've never had a security issue due to TOFU, have you?

> Also, I've never had a security issue due to TOFU, have you? This is a bit like suggesting you've never been in a car crash, so seat belts must not be worth considering. Do you feel that beyond the obvious and documented work in setting them up, there are disadvantages to using SSH certificates?

Your ISP or telecom has to be compromised for TOFU to be relevant to anything. In practice that never happens.

Re: SSH certificates: the better SSH experience

#18

Post by gkoz »

adrian_b wrote: post #9
Tepix wrote: post #7

The author lists all the advantes of CA certificates, yet doesn't list the disadvantages. OTOH, all the many steps required to set it up make the disadvantages rather obvious. Also, I've never had a security issue due to TOFU, have you?

TOFU is convenient, but not necessary. Choosing to use TOFU is a distinct choice from the choice of using the keys generated by SSH, instead of using certificates. If you do not want to use TOFU, for extra security, you just have to pair the computers by copying between them the corresponding public keys through a secure channel, e.g. by using a USB memory. Using certificates does not add any simplification or any ex…

Are you pairing computers by copying certificates to visit this site?

Re: SSH certificates: the better SSH experience

#19

Post by adrian_b »

akerl_ wrote: post #13
Tepix wrote: post #7

The author lists all the advantes of CA certificates, yet doesn't list the disadvantages. OTOH, all the many steps required to set it up make the disadvantages rather obvious. Also, I've never had a security issue due to TOFU, have you?

> Also, I've never had a security issue due to TOFU, have you? This is a bit like suggesting you've never been in a car crash, so seat belts must not be worth considering. Do you feel that beyond the obvious and documented work in setting them up, there are disadvantages to using SSH certificates?

Certificates provide extra features, like revocation.

However, if you do not need the extra features provided by certificates, using SSH-generated keys is strictly equivalent with using certificates and it requires less work.

TOFU is neither necessary nor recommended, it is just a convenience feature, to be used when security may be lax.

The secure way to use SSH is to never use TOFU but to pair the user and the server by copying the public keys between the 2 computers through a secure channel, e.g. either by using a USB memory or by sending the public keys through already existing authenticated encrypted links that pass through other computers. (Such a link may be a HTTPS download link.)

When using certificates, a completely identical procedure must be used. After certificates are generated, like also after SSH keys are generated, the certificates must be copied to the client computer and the server computer through secure channels.

Re: SSH certificates: the better SSH experience

#20

Post by bobo56539 »

With the recent wave of npm hacks stealing private keys, I wanted to limit key's lifetimes.

I've set up a couple of yubikeys as SSH CAs on hosts I manage. I use them to create short lived certs (say 24h) at the start of the day. This way i only have to enter the yubikey pin once a day.

I could not find an easy way to limit maximum certificate lifetime in openssh, except for using the AuthorizedPrincipalCommand, which feels very fragile.

Does anyone else have any experience with a similar setup? How do you limit cert max lifetime?

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