Verified Correctness and Security of OpenSSL HMAC(katherineye.com)
katherineye.com
Verified Correctness and Security of OpenSSL HMAC
http://katherineye.com/post/120638230126/verified-correctness-and-security-of-openssl-hmac
5 comments
> Humans are pretty good at doing proofs
A few problems with human-style proofs:
* Cost of proof construction: Using computational tools allows us to offload large portions of the proof to decision procedures, automated proof strategies, and invariant generators. This substantially reduces the effort involved in coming up with a proof in the first place.
* Cost of validation: Humans make mistakes all the time. Peer/self reviewing a proof of that size and complexity in order to ensure that the proof does not contain errors will dominate the total proof writing time. (Edit: with computers we get proof checking for free.)
> Now obviously the state of the art is progressing and it's getting easier to formally verify properties of programs, but I wonder whether it will ever be feasible to make this part of everyday software development.
Part of the reason for doing these large verification projects is to figure out what, exactly, makes verification so difficult and time-consuming. And then developing new technology to make formal proof development cheaper. Many of these large verification efforts result in an offshoot of papers about new techniques for proving, or new design considerations for theorem provers, or new libraries that will make similar proofs in the future easier.
A few problems with human-style proofs:
* Cost of proof construction: Using computational tools allows us to offload large portions of the proof to decision procedures, automated proof strategies, and invariant generators. This substantially reduces the effort involved in coming up with a proof in the first place.
* Cost of validation: Humans make mistakes all the time. Peer/self reviewing a proof of that size and complexity in order to ensure that the proof does not contain errors will dominate the total proof writing time. (Edit: with computers we get proof checking for free.)
> Now obviously the state of the art is progressing and it's getting easier to formally verify properties of programs, but I wonder whether it will ever be feasible to make this part of everyday software development.
Part of the reason for doing these large verification projects is to figure out what, exactly, makes verification so difficult and time-consuming. And then developing new technology to make formal proof development cheaper. Many of these large verification efforts result in an offshoot of papers about new techniques for proving, or new design considerations for theorem provers, or new libraries that will make similar proofs in the future easier.
It's not really a 'bit beyond'. The proof is about the machine code that runs on the CPU! It's a huge abstraction gap from C and it is the result of years of formal proofs and PL research.
About the effort being worth it or not, I believe that security critical programs MUST have formal proofs. Vulnerabilities in this kind of software are extremely costly. The same goes for software whose failure can be of danger for humans (c.f. the Toyota debacle).
About the effort being worth it or not, I believe that security critical programs MUST have formal proofs. Vulnerabilities in this kind of software are extremely costly. The same goes for software whose failure can be of danger for humans (c.f. the Toyota debacle).
A surprisingly large number of lines of code become security critical by dint of inclusion in security critical systems designed by others, or in support systems for those critical systems.
...which is a big reason why I find it insane that people aren't more interested in microkernel (or unikernel+hypervisor, which ends up in the same place) designs.
If you can have a tiny trust-kernel that has been proof-checked, keep everything else outside of it, and the things outside of it can only communicate with (or even observe) their peers via messages sent through it, then you don't need to worry about including untrusted code in your app.
Instead, you just slap any and all untrusted code into microservices (microdaemons?) in their own security domains/sandboxes/VMs/whatever, and speak to them over the kernel's message bus (or in the VM case, a virtual network), and suddenly they can't hurt you any more.
If you can have a tiny trust-kernel that has been proof-checked, keep everything else outside of it, and the things outside of it can only communicate with (or even observe) their peers via messages sent through it, then you don't need to worry about including untrusted code in your app.
Instead, you just slap any and all untrusted code into microservices (microdaemons?) in their own security domains/sandboxes/VMs/whatever, and speak to them over the kernel's message bus (or in the VM case, a virtual network), and suddenly they can't hurt you any more.
I think the problem is that operating systems aren't that useful until they have applications and a userbase. Some projects (like Mirage[1] and a few others) try to get around this by building on top of Xen, but that creates a lot of friction.
[1]: http://openmirage.org/
[1]: http://openmirage.org/
This is true. It's also why most serious projects are paravirtualizing OS's such as NetBSD or Linux. QNX and Minix 3 leverage code from NetBSD. Most of the L4's do a L4 Linux that runs Linux in user-mode on the tiny kernel. I link to specific examples in another comment here. Even the CHERI secure processor project ported FreeBSD (CheriBSD) to it for the need to keep legacy software.
That's a serious issue that's killed a number of past projects. At least many modern ones learned the lesson and are acting accordingly.
That's a serious issue that's killed a number of past projects. At least many modern ones learned the lesson and are acting accordingly.
Do you have any examples of this type of approach being used in any projects? I'd be curious to check out how it works code-wise.
I link to quite a few microkernel-based examples in the recent post below. Skip to my reply to @Thoth with all the links.
https://www.schneier.com/blog/archives/2015/05/friday_squid_...
https://www.schneier.com/blog/archives/2015/05/friday_squid_...
I'm not sure if this matches the description, but seL4 supposedly has a pretty well-developed proof system: https://sel4.systems/
Worth keeping in mind that L4 kernels do much, much less than conventional operating systems. They're more like libraries for building useful OS's on top of.
Too be fair it looks like they're "just" using CompCert to go from C to asm. CompCert has been around for a while and is a verified compiler, which itself is quite an accomplishment.
But yes, it is remarkable how far things have come with regards to formal methods. But it is still quite tedious to do unless you're an academic working in the field.
But yes, it is remarkable how far things have come with regards to formal methods. But it is still quite tedious to do unless you're an academic working in the field.
I know the project quite well, it's led by Appel and is called Verified Software Toolchain. They not only compile their code with compcert but also use its correctness theorem to derive the validity of the Assembly code!
My main comment addresses some of this. I think the consensus on this is that most software will use other techniques with lower barrier to entry that leverage stuff built with higher assurance. For example, certain imperative (Ada) and functional (Ocaml) seem to prevent all kinds of errors with decent performance. The Middle Way is to make the toolchain and critical libraries high assurance while the developer uses the basic features of the language with a decent software process. The process aims for good requirements, conservative design, reuse of proven implementation strategies, and good coding styles. These get checked, analyzed, and turned into machine code by trustworthy components.
Such an approach knocks out the vast majority of problems. The rest get squeezed out incrementally over time as research and development continues.
Such an approach knocks out the vast majority of problems. The rest get squeezed out incrementally over time as research and development continues.
> However, it seems that there is a tremendous amount of effort that goes into proving this software and I wonder if the time investment makes sense.
I suppose the effort wasn't just to prove this software correct, but to develop a methodology that allows proving other similar software.
> I wonder whether it will ever be feasible to make this part of everyday software development
I think that's where we're headed. For one thing, verification could be much easier with a language that was designed to ease verification (not C!).
We can also expect to eventually have a lot of proved correct components that we'll glue together using some languages that makes it easy to prove that the composition is correct.
I suppose the effort wasn't just to prove this software correct, but to develop a methodology that allows proving other similar software.
> I wonder whether it will ever be feasible to make this part of everyday software development
I think that's where we're headed. For one thing, verification could be much easier with a language that was designed to ease verification (not C!).
We can also expect to eventually have a lot of proved correct components that we'll glue together using some languages that makes it easy to prove that the composition is correct.
> However, it seems that there is a tremendous amount of effort that goes into proving this software and I wonder if the time investment makes sense.
Either you reasonably need machine-checkable proofs of the software confirming to a spec, or you don't and can settle for less secure methods. Then be honest and lay out what parts of the program are verified, which are proven ~informally by hand, which has been reviewed by several people, which uses a lot of tests, and so on. Risk/reward analysis like anything else (but obviously not straightforward).
Formal verification is probably so young/not widespread as a discipline that we'll probably see peanut gallery concern trolling "this is really cool! But... <same old FUD>" like your comments for the next twenty years or so.
Either you reasonably need machine-checkable proofs of the software confirming to a spec, or you don't and can settle for less secure methods. Then be honest and lay out what parts of the program are verified, which are proven ~informally by hand, which has been reviewed by several people, which uses a lot of tests, and so on. Risk/reward analysis like anything else (but obviously not straightforward).
Formal verification is probably so young/not widespread as a discipline that we'll probably see peanut gallery concern trolling "this is really cool! But... <same old FUD>" like your comments for the next twenty years or so.
Here's the full paper: http://www.cs.princeton.edu/~appel/papers/verified-hmac.pdf
Thanks. My favorite part is your thorough breakdown of the assurance case in section one. For redoing security evaluations, I recommended [1] a while back that vendors illustrate the assurance levels of each component in their systems. Your breakdown looks closer to my recommendation than most things I've seen. Such a breakdown is both honest and shows exactly where improvements (or mitigations) need to happen.
Otherwise, verifications were as practical and good as I'd expect. My favorite section to scope out, Related Work, gave me new insights as usual. You also had a useful idea of future work [2]. All together, great work.
[1] https://www.schneier.com/blog/archives/2014/04/friday_squid_...
[2] "One important future step is to con- dense commonalities of these libraries into an ontology for crypto-related reasoning principles, reusable across multiple language levels and realised in multiple proof assistants. "
Otherwise, verifications were as practical and good as I'd expect. My favorite section to scope out, Related Work, gave me new insights as usual. You also had a useful idea of future work [2]. All together, great work.
[1] https://www.schneier.com/blog/archives/2014/04/friday_squid_...
[2] "One important future step is to con- dense commonalities of these libraries into an ontology for crypto-related reasoning principles, reusable across multiple language levels and realised in multiple proof assistants. "
Even coq have had its bugs where it was possible to prove that true is false: https://github.com/clarus/falso
True. That's why formal verification is only one of many techniques in high assurance (A1/EAL7) evaluations. The methods are all a check against each other with the minds of the designers and evaluators being the strongest check. That might seem counter-intuitive given we're doing formal verification due to people's inability to write code. Yet, people equipped properly can see mistakes in good specs or designs much better than they can do tedious work (eg low-level code) without error.
So, in high assurance, we use every tool at our disposal to counter problems and then some for redundancy. Works out fine. That said, I have a nice paper for you if you want to see how screwed up formal verification can get:
http://www.cypherpunks.to/~peter/04_verif_techniques.pdf
Things have gotten a bit better but plenty of truth left in that paper.
So, in high assurance, we use every tool at our disposal to counter problems and then some for redundancy. Works out fine. That said, I have a nice paper for you if you want to see how screwed up formal verification can get:
http://www.cypherpunks.to/~peter/04_verif_techniques.pdf
Things have gotten a bit better but plenty of truth left in that paper.
Coq's kernel isn't particularly small so bugs are to be expected. The bug exploited in "falso", however, was outside the kernel. It never threatened correctness of existing Coq proofs.
Even mathematicians have had bugs where they agreed with a proof but then it turned out to be wrong.
Writing the definition of sha 256 in Coq must have been of great fun, hehe.
"Verification of a Cryptographic Primitive: SHA-256", Andrew Appel
https://www.cs.princeton.edu/~appel/papers/verif-sha.pdf
https://www.cs.princeton.edu/~appel/papers/verif-sha.pdf
Thanks for the link. I always enjoy reading Appel's work.
Three gripes I have with many formal methods projects are (a) choose something about useless to prove, (b) reinvent the wheel unnecessarily, and (c) an assurance argument with huge gaps or on a knockoff of the actual problem. I like that this project does the opposite in each area: a useful algorithm implementation whose proof build on other's projects with end-to-end assurance. Wise. I look forward to reading the paper.
So, what to do next. I suggest working on stuff that isn't getting much attention. For security, realistic proofs are lacking of useful protocols and crypto-constructions from requirements to design to implementation. Also, much foundational software builds on libraries implementing ZIP, PNG, reg ex's, and so on. More verification of those has widespread effect. Program transformations, optimizations of CompCert, integration of covert channel analysis into such tools, more static/dynamic checking, assemblers/linkers, and so on could all use more verification work done with the nice qualities I mentioned about this one. So, any readers thinking of a project might consider the above. That said, it would be nice if this could benefit the average person without formal methods abilities, right?
I thought hard on it. Our systems stuff is usually coded in low-level, imperative languages for performance. Our high assurance work often uses functional programming (or functional style) for specs, tools, and so on. Yet, the limitations and TCB's of those in systems space are huge obstacles. Yet, old Scheme/LISP work showed how to turn a limited functional program into an imperative one step by step by fleshing out its state (among other things). We've also seen metaprogramming & MDD techniques allow us to specify something at a high level with low-level, fast code automatically generated for the target.
I think the trick is to combine all of this: subset of functional programming specifies high-level operation of program and low-level operation of target language (esp fast parts); verification of useful primitives (eg stacks, pointer arithmetic) with high-level interfaces macro-style; a coding style + methodology for going from high functional to low imperative; verified transformations for optimization, macro expansion, and code generation. Each of these exist in some form, most verified in some way. What's left is to verify all of them and their integration. Such an integrated approach might dramatically simplify verification of software by letting developers simply describe it in a high-level, functional way with a step-by-step process to deployment. Verification, depending on talent, might range from manual inspection to machine-checked proofs. Yet, doing it this way should be much easier than converting them to formal verification experts.
What do you developers or formal methods people think of this?
So, what to do next. I suggest working on stuff that isn't getting much attention. For security, realistic proofs are lacking of useful protocols and crypto-constructions from requirements to design to implementation. Also, much foundational software builds on libraries implementing ZIP, PNG, reg ex's, and so on. More verification of those has widespread effect. Program transformations, optimizations of CompCert, integration of covert channel analysis into such tools, more static/dynamic checking, assemblers/linkers, and so on could all use more verification work done with the nice qualities I mentioned about this one. So, any readers thinking of a project might consider the above. That said, it would be nice if this could benefit the average person without formal methods abilities, right?
I thought hard on it. Our systems stuff is usually coded in low-level, imperative languages for performance. Our high assurance work often uses functional programming (or functional style) for specs, tools, and so on. Yet, the limitations and TCB's of those in systems space are huge obstacles. Yet, old Scheme/LISP work showed how to turn a limited functional program into an imperative one step by step by fleshing out its state (among other things). We've also seen metaprogramming & MDD techniques allow us to specify something at a high level with low-level, fast code automatically generated for the target.
I think the trick is to combine all of this: subset of functional programming specifies high-level operation of program and low-level operation of target language (esp fast parts); verification of useful primitives (eg stacks, pointer arithmetic) with high-level interfaces macro-style; a coding style + methodology for going from high functional to low imperative; verified transformations for optimization, macro expansion, and code generation. Each of these exist in some form, most verified in some way. What's left is to verify all of them and their integration. Such an integrated approach might dramatically simplify verification of software by letting developers simply describe it in a high-level, functional way with a step-by-step process to deployment. Verification, depending on talent, might range from manual inspection to machine-checked proofs. Yet, doing it this way should be much easier than converting them to formal verification experts.
What do you developers or formal methods people think of this?
However, it seems that there is a tremendous amount of effort that goes into proving this software and I wonder if the time investment makes sense. Humans are pretty good at doing proofs, so it seems that with a comparable amount of time you could just have a human verify that the C implementation matched the spec. (Yes, I know that this result goes a bit beyond that by using compcert, etc.)
Now obviously the state of the art is progressing and it's getting easier to formally verify properties of programs, but I wonder whether it will ever be feasible to make this part of everyday software development. Anyways, these are just my half-baked ramblings since this isn't a topic I've thought about a whole lot, but I'm interested to hear other peoples perspectives.