[Freeipa-devel] [PATCH] 0097 Add options to write lightweight CA cert or chain to file

Tue Aug 16 06:10:08 UTC 2016

On 16.8.2016 07:24, Fraser Tweedale wrote:
> On Mon, Aug 15, 2016 at 08:19:33AM +0200, Jan Cholasta wrote:
>> On 9.8.2016 16:47, Fraser Tweedale wrote:
>>> On Mon, Aug 08, 2016 at 10:49:27AM +0200, Jan Cholasta wrote:
>>>> On 8.8.2016 09:06, Fraser Tweedale wrote:
>>>>> On Mon, Aug 08, 2016 at 08:54:05AM +0200, Jan Cholasta wrote:
>>>>>> Hi,
>>>>>>
>>>>>> On 8.8.2016 06:34, Fraser Tweedale wrote:
>>>>>>> Please review the attached patch with adds --certificate-out and
>>>>>>> --certificate-chain-out options to `ca-show' command.
>>>>>>>
>>>>>>> Note that --certificate-chain-out currently writes a bogus file due
>>>>>>> to a bug in Dogtag that will be fixed in this week's build.
>>>>>>>
>>>>>>> https://fedorahosted.org/freeipa/ticket/6178
>>>>>>
>>>>>> 1) The client-side *-out options should be defined on the client side, not
>>>>>> on the server side.
>>>>>>
>>>>> Will option defined on client side be propagated to, and observable
>>>>> in the ipaserver plugin?  The ipaserver plugin needs to observe that
>>>>> *-out has been requested and executes additional command(s) on that
>>>>> basis.
>>>>
>>>> Is there a reason not to *always* return the certs?
>>>>
>>> We hit Dogtag to retrieve them.
>>
>> I don't think that's an issue in a -show command.
>>
> cert_show is invoked by other commands (cert_find*, cert_show,
> cert_request, cert_status, ca_del) but these all hit Dogtag anyway
> so I suppose that's fine.  I'll return the cert *and* the chain in
> separate attributes, unconditionally.
>
>>>
>>>>>
>>>>>>
>>>>>> 2) I don't think there should be additional information included in summary
>>>>>> (and it definitely should not be multi-line). I would rather inform the user
>>>>>> via an error message when unable to write the files.
>>>>>>
>>>>> I was just following the pattern of other commands that write certs,
>>>>> profile config, etc.  Apart from consistency with other commands I
>>>>> agree that there is no need to have it.  So I will remove it.
>>>>>
>>>>>> If you think there is an actual value in informing the user about
>>>>>> successfully writing the files, please use ipalib.messages for the job.
>>>>>>
>>>>>>
>>>>>> 3) IMO a better format for the certificate chain than PKCS#7 would be
>>>>>> concatenated PEM, as that's the most commonly used format in IPA (in
>>>>>> installers, there are no cert chains in API commands ATM).
>>>>>>
>>>>> Sure, but the main use case isn't IPA.  Other apps require PKCS #7
>>>>> or concatenated PEMs, but sometimes they must be concatenated
>>>>> forward, and othertimes backwards.  There is no one size fits all.
>>>>
>>>> True, which is exactly why I think we should at least be self-consistent and
>>>> use concatenated PEM (and multi-value DER over the wire).
>>>>
>>> Dogtag returns a PKCS7 (either DER or PEM, according to HTTP Accept
>>> header).
>>>
>>> If we want list-of-PEMs between server and client we have to convert
>>> on the server.  Do we have a good way of doing this without exec'ing
>>> `openssl pkcs7' on the server?  Is it acceptable to exec 'openssl'
>>> to do the conversion on the server?  python-nss does not have PKCS7
>>> functions and I am not keen on adding a pyasn1 PKCS7 parser just for
>>> the sake of pushing bits as list-of-PEMs.
>>
>> I'm afraid we can't avoid conversion to/from PKCS#7 one way or the other.
>> For example, if we added a call to retrieve external CA chain using certs
>> from cn=certificates,cn=ipa,cn=etc, we would have to convert the result to
>> PKCS#7 if it was our cert chain format of choice.
>>
>> What we can avoid though is executing "openssl pkcs7" to do the conversion -
>> we can use an approach similar to our DNSSEC code and use python-cffi to
>> call libcrypto's PKCS#7 conversion routines instead.
>>
> I had a look at the OpenSSL API for parsing PKCS #7; now I prefer to
> exec `openssl' to do the job :)
>
> I will transmit DER-encoded PKCS #7 object on the wire; we cannot
> used multi-valued DER attribute because order is important.   Client
> will convert to PEMs.

Well, my point was not to send PKCS#7 over the wire, so that clients 
(including 3rd party clients) do not have to convert from PKCS#7 themselves.

In fact we can use multi-valued DER - whatever you send over the wire 
from the server will be received in the exact same order by the client. 
Even if it wasn't, you can easily restore the order by matching issuer 
and subject names of the certificates.

>
> Should have new patch on list this afternoon.
>
> Thanks,
> Fraser
>
>>>
>>> FWIW, man pages and code suggest that PKCS #7 is accepted in
>>> installer, etc.
>>
>> True, but that's a relatively new feature (since 4.1) and the installer
>> internally executes "openssl pkcs7" to convert PKCS #7 to list of certs :-)
>>
>>>
>>>>> We can add an option to control the format later, but for now,
>>>>> Dogtag returns a PKCS #7 (PEM or DER) so let's go with that.  Worst
>>>>> case is an admin has to invoke `openssl pkcs7' and concat the certs
>>>>> themselves.
>>>>
>>>> AFAIK none of NSS, OpenSSL or p11-kit can use PKCS#7 cert chains directly,
>>>> so I'm afraid the worst case would happen virtually always.
>>>>
>>> If you're OK with invoking OpenSSL on the client to convert PKCS #7
>>> to list-of-PEMs (similar to what is done in
>>> ipapython.certdb.NSSDatabase) then we can have the client perform
>>> the conversion.
>>
>> See above.
>>
>>>
>>>>>
>>>>>>
>>>>>> 4) Over the wire, the certs should be DER-formatted, as that's the most
>>>>>> common wire format in other API commands.
>>>>>>
>>>>> OK.
>>>>>
>>>>>>
>>>>>> 5) What is the benefit in having the CA cert and the rest of the chain
>>>>>> separate? For end-entity certs it makes sense to separate the cert from the
>>>>>> CA chain, but for CA certs, you usually want the full chain, no?
>>>>>>
>>>>> If you want to anchor trust directly at a subca (e.g. restrict VPN
>>>>> login to certs issued by VPN sub-CA) then you often just want the
>>>>> cert.  The chain option does subsume it, at cost of more work for
>>>>> administrators with this use case.  I think it makes sense to keep
>>>>> both options.
>>>>
>>>> Does it? From what you described above, you either want just the sub-CA
>>>> cert, or the full chain including the sub-CA cert, in which case it might
>>>> make more sense to have a single --out option and a --chain flag.
>>>>
>>> How about --certificate-out which defaults to single cert, but does
>>> chain (as list-of-PEMs) when --chain flag given.
>>>
>>> Per https://fedorahosted.org/freeipa/ticket/5166 let's not add more
>>> `--out' options.
>>
>> +1
>>
>>>
>>> Thanks,
>>> Fraser
>>>
>>
>>
>> --
>> Jan Cholasta

-- 
Jan Cholasta