FIDO2: credential management extension

[prusnak]

In order to fully support resident keys with OpenSSH we need to implement credential management extension

Let's evaluate whether this is something we want to do.

[andrewkozlik]

AFAIK, the credential management in CTAP is not documented anywhere. However, from looking at the libfido2 code, it should be pretty easy to figure out. I already have a general idea of how it works:

Looks like there are several subcommands to the GetInfo command: CRED_METADATA = 0x01 RP_BEGIN = 0x02 RP_NEXT = 0x03 RK_BEGIN = 0x04 RK_NEXT = 0x05 DELETE_CRED = 0x06

The RK_BEGIN subcommand has one parameter: 1: RP ID hash

The DELETE_CRED subcommand has one parameter: 1: Credential ID

Other subcommands don't have a parameter.

CRED_METADATA response: 1: Total number of resident credentials currently present on the authenticator. 2: Number of remaining resident credentials which can be created.

RP_BEGIN / RP_NEXT response: 3: PublicKeyCredentialRpEntity (map with keys "id" and "name") 4: rp_id_hash (bytes) 5: Number of relying parties (RP_BEGIN only)

RK_BEGIN / RK_NEXT response: 6: PublicKeyCredentialUserEntity (map with keys "name", "displayName", "id") 7: PublicKeyCredentialDescriptors (map with key "id") 8: COSE Public Key (map with "alg", "crv" etc.) 9: Number of resident keys for the requested RP (RK_BEGIN only)

[andrewkozlik]

Specification: https://fidoalliance.org/specs/fido2/fido-client-to-authenticator-protocol-v2.1-rd-20191217.html#authenticatorCredentialManagement

[zejdlikt]

Hello, is there any decision already made on this topic? If so, any eta?

Thank you

[andrewkozlik]

Another version of the CTAP proposal is out: https://fidoalliance.org/specs/fido-v2.1-rd-20201208/fido-client-to-authenticator-protocol-v2.1-rd-20201208.html#authenticatorCredentialManagement. It's still in draft form, but I think this particular part of the specification if fairly stable by now and we can think about implementing it.

@zejdlikt, you can still used OpenSSH without this feature, the only trouble is if you are using multiple machines, then you need each one to have a copy of the "secret key" file in ~/.ssh/ (when using FIDO2 I think it doesn't actually contain any secret if I recall correctly, just metadata about the credential). Personally I am under the impression that this is a shortcoming of the design decisions around the OpenSSH implementation, because the credential management extension shouldn't be required for logging in with resident credentials, but I don't see deep enough into their implementation and design rationale to give justified criticism.

Anyway, I will look into how difficult this would be to implement.

[zejdlikt]

@zejdlikt, you can still used OpenSSH without this feature, the only trouble is if you are using multiple machines, then you need each one to have a copy of the "secret key" file in ~/.ssh/ (when using FIDO2 I think it doesn't actually contain any secret if I recall correctly, just metadata about the credential). Personally I am under the impression that this is a shortcoming of the design decisions around the OpenSSH implementation, because the credential management extension shouldn't be required for logging in with resident credentials, but I don't see deep enough into their implementation and design rationale to give justified criticism.

Yes i know about the "workaround" there considering copying metadata in ~/.ssh/ - and of course it works (tested with TrezorT/Yubikey). Anyway, the idea of having just a usb key with me traveling around the world and being able to login to my servers from any other server w/o cp or scp anything additional is a nice idea. So i would say, it would be very "nice to have" (if possible).

[cs224]

Is there any update on this issue? Thanks!

[Hannsek]

No progress has been done on this.