Kerberos authentication only works with aes256-cts-hmac-sha1-96 for Windows 10 joined to RHEL IDM

[Deepak Ramanath]

I have a Windows 10 server joined to a RedHat IDM (RHEL 8.9) realm using Kerberos. When a user tries to authenticate on a Windows 10 server, the following error is shown

"We cannot sign you in with this credential because your domain isn't available"

On the IDM, looking at the `/var/log/krb5kdc.log`, I see the following...

Nov 30 23:08:17 idm.server.local krb5kdc[11775](info): AS_REQ (6 etypes {aes256-cts-hmac-sha1-96(18), aes128-cts-hmac-sha1-96(17), DEPRECATED:arcfour-hmac(23), DEPRECATED:arcfour-hmac-exp(24), UNSUPPORTED:(-135), UNSUPPORTED:des-cbc-md5(3)}) 192.168.124.55: NEEDED_PREAUTH: [e-mail] for krbtgt/[e-mail], Additional pre-authentication required
Nov 30 23:08:17 idm.server.local krb5kdc[11774](info): AS_REQ (6 etypes {aes256-cts-hmac-sha1-96(18), aes128-cts-hmac-sha1-96(17), DEPRECATED:arcfour-hmac(23), DEPRECATED:arcfour-hmac-exp(24), UNSUPPORTED:(-135), UNSUPPORTED:des-cbc-md5(3)}) 192.168.124.55: ISSUE: authtime 1701385697, etypes {rep=aes256-cts-hmac-sha1-96(18), tkt=aes256-cts-hmac-sha384-192(20), ses=aes256-cts-hmac-sha1-96(18)}, [e-mail] for krbtgt/[e-mail]
Nov 30 23:08:17 idm.server.local krb5kdc[11775](info): TGS_REQ (5 etypes {aes256-cts-hmac-sha1-96(18), aes128-cts-hmac-sha1-96(17), DEPRECATED:arcfour-hmac(23), DEPRECATED:arcfour-hmac-exp(24), UNSUPPORTED:(-135)}) 192.168.124.55: ISSUE: authtime 1701385697, etypes {rep=aes256-cts-hmac-sha1-96(18), tkt=aes256-cts-hmac-sha1-96(18), ses=aes256-cts-hmac-sha1-96(18)}, [e-mail] for host/[e-mail]

In the `/etc/crypto-policies/back-ends/krb5.config`, `libdefaults` has been set to

[libdefaults]
permitted_enctypes = aes256-cts-hmac-sha384-192 aes128-cts-hmac-sha256-128 aes256-cts-hmac-sha1-96 aes128-cts-hmac-sha1-96 camellia256-cts-cmac camellia128-cts-cmac

Interestingly, if all encryption types are removed except aes256-cts-hmac-sha1-96 from the permitted_enctypes, the authentication on Windows 10 is successful.

Any idea why only setting to aes256-cts-hmac-sha1-96 works while a list of supported methods including aes256-cts-hmac-sha1-96 does not?

[Spike White]

I don't understand why that full list of permitted_enctypes is a problem,
while your abbreviated list is not.

I do know that windows AD controllers seem to favor aes256-cts-hmac-sha1-96
and aes128-cts-hmac-sha1-96.   For most AD domains, DES was deprecated long
ago and as of last year, I think most customers are trying to deprecate RC4
as well.

Our AD DCs are W2016, 2020 and (formerly) W2012.  I have no experience with
RedHat IDM and no experience with Win10 servers (I thought Win 10 were all
desktops and integrated natively with AD).

But I do know that the krb5-libs will attempt to negotiate the encryption
types in the order they are listed in your permitted_enctypes line.  So
change your line to do aes256-cts-hmac-sha1-96 first.  something like:

permitted_enctypes = aes256-cts-hmac-sha1-96 aes128-cts-hmac-sha1-96
aes256-cts-hmac-sha384-192 aes128-cts-hmac-sha256-128 camellia256-cts-cmac
camellia128-cts-cmac

This will at least give you another data point.  to my mind, it should
proceed like this:

Attempt aes256-cts-hmac-sha384-192, fail,
Attempt aes128-cts-hmac-sha256-128, fail.
Attempt  aes256-cts-hmac-sha1-96, succeed.

Spike

On Thu, Nov 30, 2023 at 11:23 PM Deepak Ramanath 
wrote:
> I have a Windows 10 server joined to a RedHat IDM (RHEL 8.9) realm using
> Kerberos. When a user tries to authenticate on a Windows 10 server, the
> following error is shown
>
> "We cannot sign you in with this credential because your domain isn't
> available"
>
> On the IDM, looking at the `/var/log/krb5kdc.log`, I see the following...
>
> Nov 30 23:08:17 idm.server.local krb5kdc[11775](info): AS_REQ (6 etypes
> {aes256-cts-hmac-sha1-96(18), aes128-cts-hmac-sha1-96(17),
> DEPRECATED:arcfour-hmac(23), DEPRECATED:arcfour-hmac-exp(24),
> UNSUPPORTED:(-135), UNSUPPORTED:des-cbc-md5(3)}) 192.168.124.55:
> NEEDED_PREAUTH: [e-mail] for krbtgt/[e-mail],
> Additional pre-authentication required
> Nov 30 23:08:17 idm.server.local krb5kdc[11774](info): AS_REQ (6 etypes
> {aes256-cts-hmac-sha1-96(18), aes128-cts-hmac-sha1-96(17),
> DEPRECATED:arcfour-hmac(23), DEPRECATED:arcfour-hmac-exp(24),
> UNSUPPORTED:(-135), UNSUPPORTED:des-cbc-md5(3)}) 192.168.124.55: ISSUE:
> authtime 1701385697, etypes {rep=aes256-cts-hmac-sha1-96(18),
> tkt=aes256-cts-hmac-sha384-192(20), ses=aes256-cts-hmac-sha1-96(18)},
> [e-mail] for krbtgt/[e-mail]
> Nov 30 23:08:17 idm.server.local krb5kdc[11775](info): TGS_REQ (5 etypes
> {aes256-cts-hmac-sha1-96(18), aes128-cts-hmac-sha1-96(17),
> DEPRECATED:arcfour-hmac(23), DEPRECATED:arcfour-hmac-exp(24),
> UNSUPPORTED:(-135)}) 192.168.124.55: ISSUE: authtime 1701385697, etypes
> {rep=aes256-cts-hmac-sha1-96(18), tkt=aes256-cts-hmac-sha1-96(18),
> ses=aes256-cts-hmac-sha1-96(18)}, [e-mail] for
> host/[e-mail]
>
> In the `/etc/crypto-policies/back-ends/krb5.config`, `libdefaults` has
> been set to
>
> [libdefaults]
> permitted_enctypes = aes256-cts-hmac-sha384-192 aes128-cts-hmac-sha256-128
> aes256-cts-hmac-sha1-96 aes128-cts-hmac-sha1-96 camellia256-cts-cmac
> camellia128-cts-cmac
>
> Interestingly, if all encryption types are removed except
> aes256-cts-hmac-sha1-96 from the permitted_enctypes, the authentication on
> Windows 10 is successful.
>
> Any idea why only setting to aes256-cts-hmac-sha1-96 works while a list of
> supported methods including aes256-cts-hmac-sha1-96 does not?
> --
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