On 10/20/2016 03:52 PM, Ben Lipton wrote:
On 10/17/2016 02:16 AM, Jan Cholasta wrote: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: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 thatOn 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 hopeyou'll let me know what you think of my reasoning. On 07/20/2016 04:20 AM, Jan Cholasta wrote:There are lots of tools that users might want to use to manageHi, 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 requestseasier 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! BenJust a quick update to say that I've created a new document that coversthe proposed schema additions in a more descriptive way (withdiagrams!)I'm very new to developing with LDAP, so some more experiencedeyes on the proposal would be very helpful, even if you don't have time to absorb the full design. Please take a look athttp://www.freeipa.org/page/V4/Automatic_Certificate_Request_Generation/Schemaif 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 usingpython-cryptography's CertificateSigningRequestBuilder [1] rather than openssl or certutil or any other command line tool.their private keys, so I don't know if we can assume that whatever library weprefer will actually be able to access the private key to sign aCSR,which is why I thought it would be useful to support more thanone.python-cryptography has the notion of backends, which allow it tosupport multiple crypto implementations. Upstream it currentlysupports only OpenSSL [2], but some work has been done on PKCS#11backend [3], which provides support for HSMs and soft-tokens (like NSS databases).Alternatively, for NSS databases (and other "simple" cases), youcan 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, butgiven 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'treally 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 bedifficult to configure. On the other hand, there is some hidden complexity to enabling a simpler config format, as well. One ofthegoals of the project as it was presented to me was to allow thecreation 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 wouldbe 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 itwouldgo similarly for the CSR if you would be able to enter unknown butotherwise 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 correspondingAPI calls or configuration lines, we need some way for users toaugment 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() onthe config attributes. I'm not sure how to achieve this (is itsafe tocall getattr(x509, extensiontype)(value) where extensiontype andvalue 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.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, dueto 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 supposedto 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 scriptto 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.derNext we have the easy case, extensions that we as FreeIPA developersknow are important and build support for. For these, the two proposals work equivalently well, but yours is simpler to configure because theknowledge of how to make a san_rfc822name is built into the libraryinstead 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 thattransformation rules are implementation (i.e. helper) specific data stored in the IPA server, and that the system has several levels ofschema 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 ableto 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 CSRshouldn't have to care about helpers, and I agree that the current way they are exposed is clunky. I do think that an administrator creatingcustom rules might want to take advantage of a helper, so they wouldn't need to understand the ASN.1 representation of their chosen certificateextension. Of course, the desired extension might not be supported by the helper either. Since I don't know what specific extensions peoplewill 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 thisshould 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, orreplace 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.
For now I just went ahead and implemented it in C, for simplicity. So far it only does the data + SubjectPublicKeyInfo -> CertificationRequestInfo conversion (data in the openssl config file format), but I'm convinced that both openssl and NSS should be able to sign this to turn it into a CSR. I'm also pretty sure you were right that calling C functions is required - none of the python libraries seem to have bindings for the functions that manipulate these objects. You can see the prototype here: https://github.com/LiptonB/freeipa-prototypes/blob/master/build_requestinfo.c
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 thattime. robAndwithout 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 theonly 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" tobuild the CSR, and unlike the other options, it seems like we mightneed 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 oftextual representation of said data structures. I don't know if thereis 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'sstill 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 anyextensions we want to use in a cert. That limitation didn't come up whenwe 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 theIPA 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 isthis 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 forspecified subject from a specified template and the SubjectPublicKeyInfo5. (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.What do you see as the risks of compiling native code with asn1c and executing it that are not present when generating python code with asn1ate and loading it? I would think that, native or not, we're depending on the ASN.1 compiler to generate secure code from any ASN.1 definition the admin might give it. Even a parser like libtasn1 that interprets the structure on the fly rather than generating executable code could do something dangerous when given poorly-constructed input. I don't mean to create a false equivalence, but are the interpreted options really safer than the native code?I did a little research since my last email, andt doesn't seem to have 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).Good find. That seems quite useful for being able to interact with ASN.1 defined on the fly. I wonder how hard it would be to connect it to pyasn1 to get more flexible ASN.1 decoding within python. Still doesn't help with XER encoding/decoding, but I suppose that's a SMOP :)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>
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