On 13.10.2016 17:23, Ben Lipton wrote:
Thank you, this was a really helpful clarification of your point.
Comments below. Once again, I'm sorry I missed the email for so long.
Ben
On 09/05/2016 06:52 AM, Jan Cholasta wrote:
On 27.8.2016 22:40, Ben Lipton wrote:
On 08/25/2016 04:11 PM, Rob Crittenden wrote:
Ben Lipton wrote:
On 08/23/2016 03:54 AM, Jan Cholasta wrote:
On 8.8.2016 22:23, Ben Lipton wrote:
On 07/25/2016 07:45 AM, Jan Cholasta wrote:
On 25.7.2016 13:11, Alexander Bokovoy wrote:
On Mon, 25 Jul 2016, Jan Cholasta wrote:
On 20.7.2016 16:05, Ben Lipton wrote:
Hi,
Thanks very much for the feedback! Some responses below; I hope
you'll
let me know what you think of my reasoning.
On 07/20/2016 04:20 AM, Jan Cholasta wrote:
Hi,
On 17.6.2016 00:06, Ben Lipton wrote:
On 06/14/2016 08:27 AM, Ben Lipton wrote:
Hello all,
I have written up a design proposal for making certificate
requests
easier to generate when using alternate certificate profiles:
http://www.freeipa.org/page/V4/Automatic_Certificate_Request_Generation.
The use case for this is described in
https://fedorahosted.org/freeipa/ticket/4899. I will be
working on
implementing this design over the next couple of months.
If you
have
the time and interest, please take a look and share any
comments or
concerns that you have.
Thanks!
Ben
Just a quick update to say that I've created a new document
that
covers
the proposed schema additions in a more descriptive way (with
diagrams!)
I'm very new to developing with LDAP, so some more experienced
eyes on
the proposal would be very helpful, even if you don't have
time to
absorb the full design. Please take a look at
http://www.freeipa.org/page/V4/Automatic_Certificate_Request_Generation/Schema
if you have a chance.
I finally had a chance to take a look at this, here are some
comments:
1) I don't like how transformation rules are tied to a
particular
helper and have to be duplicated for each of them. They
should be
generic and work with any helper, as helpers are just an
implementation detail and their resulting data is the same.
In fact, I think I would prefer if the CSR was generated using
python-cryptography's CertificateSigningRequestBuilder [1]
rather
than
openssl or certutil or any other command line tool.
There are lots of tools that users might want to use to manage
their
private keys, so I don't know if we can assume that whatever
library we
prefer will actually be able to access the private key to sign a
CSR,
which is why I thought it would be useful to support more than
one.
python-cryptography has the notion of backends, which allow it to
support multiple crypto implementations. Upstream it currently
supports only OpenSSL [2], but some work has been done on PKCS#11
backend [3], which provides support for HSMs and soft-tokens
(like
NSS
databases).
Alternatively, for NSS databases (and other "simple" cases), you
can
generate the private key with python-cryptography using the
default
backend, export it to a file and import the file to the target
database, so you don't actually need the PKCS#11 backend for
them.
So, the only thing that's currently lacking is HSM support, but
given
that we don't support HSMs in IPA nor in certmonger, I don't
think
it's an issue for now.
The
purpose of the mapping rule is to tie together the
transformation
rules
that produce the same data into an object that's
implementation-agnostic, so that profiles referencing those
rules
are
automatically compatible with all the helper options.
They are implementation-agnostic, as long as you consider
`openssl`
and `certutil` the only implementations :-) But I don't think
this
solution scales well to other possible implementations.
Anyway, my main grudge is that the transformation rules shouldn't
really be stored on and processed by the server. The server
should
know the *what* (mapping rules), but not the *how*
(transformation
rules). The *how* is an implementation detail and does not
change in
time, so there's no benefit in handling it on the server. It
should be
handled exclusively on the client, which I believe would also
make
the
whole thing more robust (it would not be possible for a bug on
the
server to break all the clients).
This is a good point. However, for the scope of Ben's project
can we
limit it by openssl and certutil support? Otherwise Ben
wouldn't be
able
to complete the project in time.
I'm fine with that, but I don't think it's up to me :-)
This is turning out to be a common (and, I think, reasonable)
reaction
to the proposal. It is rather complex, and I worry that it
will be
difficult to configure. On the other hand, there is some hidden
complexity to enabling a simpler config format, as well. One of
the
goals of the project as it was presented to me was to allow the
creation
of profiles that add certificate extensions *that FreeIPA
doesn't
yet
know about*. With the current proposal, one only has to add a
rule
generating text that the helper will understand.
... which will be possible only as long as the helper
understands the
extension. Which it might not, thus the current proposal works
only
for *some* extensions that FreeIPA doesn't yet support.
We can go ad infinitum here but with any helper implementation,
be it
python-cryptography or anything else, you will need to have a
support
there as well.
My point was that the current proposal is not any better than my
proposal in this regard, as neither of them allows one to use an
arbitrary extension.
The idea with unknown extensions was to allow mapping
their acceptance to a specific relationship between IPA objects
(optionally) and an input from the CSR. A simplest example would
be an
identity rule that would copy an ASN.1 encoded content from the
CSR to
the certificate.
That's on the mapping side, not on the CSR generation side, but it
would
go similarly for the CSR if you would be able to enter unknown but
otherwise correct ASN.1 stream. There is no difference at which
helper
type we are talking about because all of them support inserting
ASN.1
content.
With your suggestion,
if there's a mapping between "san_directoryname" and the
corresponding
API calls or configuration lines, we need some way for users to
augment
that mapping without changing the code. If there's no
mapping, and
it's
just done with text processing, we need enough in the config
format to
be able to generate fairly complex structures:
builder =
builder.subject_name(x509.Name(u'CN=user,O=EXAMPLE.COM'))
builder =
builder.add_extension(x509.SubjectAlternativeName([x509.RFC822Name(u'u...@example.com'),
x509.DirectoryName(x509.Name(u'CN=user,O=EXAMPLE.COM'))]),
False)
and we need to do it without it being equivalent to calling
eval() on
the config attributes. I'm not sure how to achieve this (is it
safe to
call getattr(x509, extensiontype)(value) where extensiontype and
value
are user-specified?) and it definitely would have to be tied
to a
particular library/tool.
As I pointed out above, this needs to be figured out for the
generic
case for both the current proposal and my suggestion.
I have a proof of concept[1] for using openssl-based rules to add a
subject alt name extension without using openssl's knowledge of that
extension. It's not extremely pretty, and it took some trial and
error,
but no code changes. So, I think this actually is a difference
between
the two proposals.
With the obvious catch being that it works only with OpenSSL, which
might not work for everyone, e.g. when using HSMs or SmartCards, due
to a limited PKCS#11 support in OpenSSL.
Very true. Even certutil's equivalent feature (--extGeneric) doesn't
seem like it would work very well in this context, as you are supposed
to pass in an already-encoded extension, so text-based templating
wouldn't be able to do much.
Yeah, I struggled with this myself. I ended up writing a pyasn1 script
to generate the extension I needed, wrote that to a file, and passed
it to certutil using:
--extGeneric 2.5.29.17:not-critical:/path/to/msupn.der
Next we have the easy case, extensions that we as FreeIPA developers
know are important and build support for. For these, the two
proposals
work equivalently well, but yours is simpler to configure because
the
knowledge of how to make a san_rfc822name is built into the library
instead of being stored on the server as a set of rules.
Finally, we have the case of extensions that are known to the
helper,
but not to FreeIPA. In the existing proposal, new rules can be
written
to support these extensions under a particular helper. Further,
those
rules can be used by reference in many profiles, reducing
duplication of
effort/data/errors.
As I understand it, the main objections in this thread are that
transformation rules are implementation (i.e. helper) specific data
stored in the IPA server, and that the system has several levels of
schema when it could just embed rules in the profile. But without
helper-specific rules, administrators could not take advantage of
the
additional extensions supported by the helper they are using.
There is *no* advantage in forcing the user to choose between helpers
which differ only in the set of limitations on the CSR they are able
to produce. The user should specify a) where the private key is
located and b) what profile to use, and that's it, it should just
work.
Ok, this is a good point about usability. The user creating the CSR
shouldn't have to care about helpers, and I agree that the current way
they are exposed is clunky. I do think that an administrator creating
custom rules might want to take advantage of a helper, so they
wouldn't
need to understand the ASN.1 representation of their chosen
certificate
extension. Of course, the desired extension might not be supported by
the helper either. Since I don't know what specific extensions people
will want to use this for, I don't know how to balance the better
administrator experience of adding extensions via a helper with the
limited extension support.
The original reason we arrived at the concept of "helpers" was to
support different ways of getting at private keys, but perhaps this
should not be the concern of the CSR data generator. In your opinion,
would it be sufficient to support just one key format (PKCS#12? PEM?)
and let the user deal with putting those keys into whatever
formats/databases they need? If that's ok, maybe we can stop having
*multiple* helpers, but if we want to replace helpers entirely I'm
still
not certain what to replace them with.
I'd just add an option to specify the output format, e.g PEM, NSS,
Java keystore, PKCS#12, whatever. You can probably get away with the
first two for starters. Different output format is going to mean
different options but that is probably not a big deal.
My point was that if we want to get rid of all the helpers but one, or
replace helpers with something else entirely like somehow templating
ASN1 structures directly, it will get harder to support all those
formats (or even both of the first two). For example, if we drop
certutil as a helper, how will we sign CSRs with keys stored in NSS
databases?
1. get the public part of the key from the NSS database
2. construct a CertificationRequestInfo [1] from the template and the
public key
3. sign the CertificationRequestInfo with NSS using the private key to
get a CSR
This is purely client side, will work with any crypto library (just
substitute NSS for something else) and, if done right, using very
little code.
Ok, I like this. If an encoded CertificationRequestInfo is something we
can expect to be compatible with any reasonable library (it sounds like
it should be) then the library can be used client-side to do the
key-storage-specific parts. I'm going to try writing this data ->
encoded CertificationRequestInfo -> CSR flow to make sure it works as
well as it sounds. If it does, it will also be useful for the code I'm
working on right now to connect certmonger with the current version of
the CSR autogeneration tool.
Note that this will most probably require calling C functions. You might
want to look into python-cffi.
Remember that the private key will be at rest for some period of time
while the CSR is being approved. The key needs to be protected at that
time.
rob
And
without the separation of profiles from mapping rules in the schema,
rules would need to be copy+pasted among profiles, and grouping
rules
with the same effect under different helpers would be much
uglier. We
can and should discuss whether these are the right tradeoffs, but
this
is where those decisions came from.
OTOH, I think we could use GSER encoding of the extension value:
{ rfc822Name:"u...@example.com",
directoryName:rdnSequence:"CN=user,O=EXAMPLE.COM" }
GSER is not really used widely and does not have standardized
encoding
rules beyond its own definition. If you want to allow
transformation
rules in GSER that mention existing content in IPA objects, you
would
need to deal with templating anyway. At this point it becomes
irrelevant
what you are templating, though.
True, but the goal here is not to avoid templating, but rather to
avoid implementation-specific bits on the server, and GSER is the
only
thing that is textual, implementation-neutral and, as a bonus,
standardized.
As I said elsewhere, we could use GSER as a textual output format
instead of openssl or certutil, but it still needs its own
"helper" to
build the CSR, and unlike the other options, it seems like we might
need
to implement that helper. I'm not sure it's fair to call it
implementation-neutral if no implementation exists yet :)
Right. Like I said, using GSER was just a quick idea off the top
of my
head. I would actually rather use some sort of data structure
templating rather than textual templating on top of any kind of
textual representation of said data structures. I don't know if there
is such a thing, though.
This sounds interesting, can you give an example of what this might
look
like?
It would be something like XSLT, but for ASN.1 rather than XML.
I learned that there's also an XML encoding for ASN.1, XER, but that's
still a textual representation and we'd have to insert the data
textually.
Well, yes and no. While it's true that it's still a textual
representation, what really makes a difference is that for XML, there
is a templating mechanism which understands the structure of the data
(XLST, as mentioned above).
Unforutantely, XER has the same shortcoming as GSER: to be able to
convert it to DER, you need to know the ASN.1 definition of the data
structure. If we used XER+XSLT, we would also have to provide means of
adding custom ASN.1 definitions and run them through ASN.1 compiler to
convert between XER and DER.
This is a little disappointing, but it makes sense. I don't think I
realized that we'll need to compile the ASN.1 data definitions for any
extensions we want to use in a cert. That limitation didn't come up when
we were only talking about extensions that were supported by the helper
utility. But providing the ASN.1 spec for unusual extensions an admin
wants to use in their certs is probably a reasonable expectation.
Yes, that's what I think as well. It could be a simple IPA object with
name, description, extension OID and the ASN.1 definition.
It doesn't seem to be supported by any python libraries,
either, but it does look like it's supported by the asn1 compiler
in the
IPA source distribution.I could imagine an implementation that builds
an XML representation of the CSR via python templating, then makes a
signed CSR out of it in C. I'm a little concerned about it because it
would have to implement the whole CSR structure from scratch, but is
this a prototype that you'd be interested in seeing?
I can imagine something like this might work:
1. (client) generate a key pair
2. (client) get SubjectPublicKeyInfo [2] for the public key
3. (client) encode the SubjectPublicKeyInfo as XER using asn1c and
python-cffi in API mode [3]
4. (client) call server to construct CertificationRequestInfo for
specified subject from a specified template and the SubjectPublicKeyInfo
5. (server) get the subject's LDAP entry
6. (server) create a XML document which contains the subject's LDAP
attributes and the SubjectPublicKeyInfo
7. (server) use XSLT to transform the XML document to
CertificationRequestInfo using the specified template
8. (server) return the CertificationRequestInfo to the client
9. (client) convert the CertificationRequestInfo from XER to DER using
asn1c and python-cffi in API mode
10. (client) sign the CertificationRequestInfo using the private key
to get a CSR
It would be better if the XER-DER conversion was done on the server,
but I don't think that compiling and running code on the fly on the
server is a particularly good idea. Apparently there is a ASN.1
compiler available for PyASN1 [4], maybe that could be used instead,
but we would have to write a XER codec for PyASN1 ourselves (which
shouldn't be too hard IMO).
Yeah, running programs compiled from arbitrary ASN.1 seems like a risk.
Maybe a little better because the ASN.1 is provided by an administrator,
but we'd still be depending a lot on the security of the generated code.
On the other hand, if we compile on the client, the CSR generation
feature is limited to platforms where asn1c can be installed. I wish I
could think of a way to do the compilation once when the profile is
created, but run it on the client. That seems like asking for
compatibility problems, though...
It seems you missed the most important thing in the above paragraph :-)
- that is asn1ate, the PyASN1-based compiler. The nice thing about it is
that it compiles the ASN.1 definition into a PyASN1 type object, which
means you can compile the definition and use it to (un)parse data in the
same Python program. If we used it, we could JIT-compile the ASN.1
definitions on the server, without the security risk of executing native
code and without the compatibility issues of compilation on the client.
I did a little research since my last email, and there is also another
library which allows you to compile and use ASN.1 definitions on the fly
- libtasn1 [5]. Compared to asn1ate, it seems to be pretty stable
(asn1ate is currently in alpha) and is written in C, so it makes it
possible to use the administrator-defined extensions outside of IPA
(specifically, it could be useful for certificate matching and mapping
[6] in SSSD).
On further investigation, it turns out the version of
python-cryptography in F24 includes a feature allowing arbitrary
extensions to be added by adding an UnrecognizedExtension to the
CertificateSigningRequestBuilder. This makes me feel somewhat better
both about python-cryptography as a tool for this task and about the
solution I just proposed. But I still don't have a clear idea that
answers 1) how to make templates that we can turn into encoded
extensions, and 2) how to deal with all the desired key formats.
I hope the above clarifies these a little bit.
[1] <https://tools.ietf.org/html/rfc2986#section-4.1>
[2] <https://tools.ietf.org/html/rfc5280#section-4.1.2.7>
[3] <https://cffi.readthedocs.io/en/latest/overview.html#abi-versus-api>
[4] <https://github.com/kimgr/asn1ate>
[5] <https://www.gnu.org/software/libtasn1/>
[6]
<https://www.redhat.com/archives/freeipa-devel/2016-October/msg00090.html>
--
Jan Cholasta
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