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author 2018-08-07 09:58:45 -0700 committer 2019-03-22 00:50:53 -0600 commit f5afd0e402e6703dc9aa5ff5d8cb1da9acee305d (patch) tree 2e62c319b6f6736b7685c2dcb7fa31a7e6e1abe4 parent f54839ddad64ad23a3076dd8c9084fd34c039057 (diff) download linux-zinc.tar.gz



zinc zinc: introduce minimal cryptography library

Zinc stands for "Zinc Is Neat Crypto" or "Zinc as IN Crypto". It's also short, easy to type, and plays nicely with the recent trend of naming crypto libraries after elements. The guiding principle is "don't overdo it". It's less of a library and more of a directory tree for organizing well-curated direct implementations of cryptography primitives. Zinc is a new cryptography API that is much more minimal and lower-level than the current one. It intends to complement it and provide a basis upon which the current crypto API might build, as the provider of software implementations of cryptographic primitives. It is motivated by three primary observations in crypto API design: * Highly composable "cipher modes" and related abstractions from the 90s did not turn out to be as terrific an idea as hoped, leading to a host of API misuse problems. * Most programmers are afraid of crypto code, and so prefer to integrate it into libraries in a highly abstracted manner, so as to shield themselves from implementation details. Cryptographers, on the other hand, prefer simple direct implementations, which they're able to verify for high assurance and optimize in accordance with their expertise. * Overly abstracted and flexible cryptography APIs lead to a host of dangerous problems and performance issues. The kernel is in the business usually not of coming up with new uses of crypto, but rather implementing various constructions, which means it essentially needs a library of primitives, not a highly abstracted enterprise-ready pluggable system, with a few particular exceptions. This last observation has seen itself play out several times over and over again within the kernel: * The perennial move of actual primitives away from crypto/ and into lib/, so that users can actually call these functions directly with no overhead and without lots of allocations, function pointers, string specifier parsing, and general clunkiness. For example: sha256, chacha20, siphash, sha1, and so forth live in lib/ rather than in crypto/. Zinc intends to stop the cluttering of lib/ and introduce these direct primitives into their proper place, lib/zinc/. * An abundance of misuse bugs with the present crypto API that have been very unpleasant to clean up. * A hesitance to even use cryptography, because of the overhead and headaches involved in accessing the routines. Zinc goes in a rather different direction. Rather than providing a thoroughly designed and abstracted API, Zinc gives you simple functions, which implement some primitive, or some particular and specific construction of primitives. It is not dynamic in the least, though one could imagine implementing a complex dynamic dispatch mechanism (such as the current crypto API) on top of these basic functions. After all, dynamic dispatch is usually needed for applications with cipher agility, such as IPsec, dm-crypt, AF_ALG, and so forth, and the existing crypto API will continue to play that role. However, Zinc will provide a non- haphazard way of directly utilizing crypto routines in applications that do have neither the need nor desire for abstraction and dynamic dispatch. It also organizes the implementations in a simple, straight-forward, and direct manner, making it enjoyable and intuitive to work on. Rather than moving optimized assembly implementations into arch/, it keeps them all together in lib/zinc/, making it simple and obvious to compare and contrast what's happening. This is, notably, exactly what the lib/raid6/ tree does, and that seems to work out rather well. It's also the pattern of most successful crypto libraries. The architecture- specific glue-code is made a part of each translation unit, rather than being in a separate one, so that generic and architecture-optimized code are combined at compile-time, and incompatibility branches compiled out by the optimizer. All implementations have been extensively tested and fuzzed, and are selected for their quality, trustworthiness, and performance. Wherever possible and performant, formally verified implementations are used, such as those from HACL* [1] and Fiat-Crypto [2]. The routines also take special care to zero out secrets using memzero_explicit (and future work is planned to have gcc do this more reliably and performantly with compiler plugins). The performance of the selected implementations is state-of-the-art and unrivaled on a broad array of hardware, though of course we will continue to fine tune these to the hardware demands needed by kernel contributors. Each implementation also comes with extensive self-tests and crafted test vectors, pulled from various places such as Wycheproof [9]. Regularity of function signatures is important, so that users can easily "guess" the name of the function they want. Though, individual primitives are oftentimes not trivially interchangeable, having been designed for different things and requiring different parameters and semantics, and so the function signatures they provide will directly reflect the realities of the primitives' usages, rather than hiding it behind (inevitably leaky) abstractions. Also, in contrast to the current crypto API, Zinc functions can work on stack buffers, and can be called with different keys, without requiring allocations or locking. SIMD is used automatically when available, though some routines may benefit from either having their SIMD disabled for particular invocations, or to have the SIMD initialization calls amortized over several invocations of the function, and so Zinc utilizes function signatures enabling that in conjunction with the recently introduced simd_context_t. More generally, Zinc provides function signatures that allow just what is required by the various callers. This isn't to say that users of the functions will be permitted to pollute the function semantics with weird particular needs, but we are trying very hard not to overdo it, and that means looking carefully at what's actually necessary, and doing just that, and not much more than that. Remember: practicality and cleanliness rather than over-zealous infrastructure. Zinc provides also an opening for the best implementers in academia to contribute their time and effort to the kernel, by being sufficiently simple and inviting. In discussing this commit with some of the best and brightest over the last few years, there are many who are eager to devote rare talent and energy to this effort. To summarize, Zinc will contain implementations of cryptographic primitives that are: * Software-based and synchronous. * Expose a simple API that operate over plain chunks of data and do not need significant cumbersome scaffolding (more below). * Extremely fast, but also, in order of priority: 1) formally verified, 2) well-known code that's received a lot of eyeballs and has seen significant real-world usage, 3) simple reviewable code that is either obviously correct or hard to screw up given test-vectors that has been subjected to significant amounts of fuzzing and projects like Wycheproof. The APIs of each implementation are generally expected to take the following forms, with accepted variations for primitives that have non-standard input or output parameters: * For hash functions the classic init/update/final dance is well-known and clear to implement. Different init functions can instantiate different operating parameters of flexible hash functions (such as blake2s_init and blake2s_init_key). * Authenticated encryption functions return a simple boolean indicating whether or not decryption succeeded. They take as inputs and outputs either a pointer to a buffer and a size, or they take as inputs and outputs scatter-gather lists (for use with the network subsystem's skb_to_sgvec, for example). * Functions that are commonly called in loops inside a long-running worker thread may grow to take a simd_context_t parameter, so that the FPU can be twiddled from outside of the function (see prior commit introducing simd_get/put/relax). It is expected that most functions that fit this scenario will be ones that take scatter-gather lists. In addition to the above implementation and API considerations, inclusion criteria for Zinc will be mostly the same as for other aspects of the kernel: is there a direct user of the primitive or construction's Zinc implementation that would immediately benefit from that kind of API? Certain primitives, like MD5 for example, might be separated off into a legacy/ header subdirectory, to make it clear to new users that if they're using it, it's for a very particular purpose. Zinc is also wary of adding overly newfangled and unvetted primitives that have no immediate uptake or scrutiny: for example, implementations of a new block cipher posted on eprint just yesterday. But beyond that, we recognize that cryptographic functions have many different uses and are required in large variety of standards and circumstances, whose decisions are often made outside the scope of the kernel, and so Zinc will strive to accommodate the writing of clean and effective code and will not discriminate on the basis of fashionability. Following the merging of this, I expect for the primitives that currently exist in lib/ to work their way into lib/zinc/, after intense scrutiny of each implementation, potentially replacing them with either formally-verified implementations, or better studied and faster state-of-the-art implementations. Also following the merging of this, I expect for the old crypto API implementations to be ported over to use Zinc for their software-based implementations. As Zinc is simply library code, its config options are un-menued, with the exception of CONFIG_ZINC_SELFTEST and CONFIG_ZINC_DEBUG, which enables various selftests and debugging conditions. [1] https://github.com/project-everest/hacl-star [2] https://github.com/mit-plv/fiat-crypto [3] https://cr.yp.to/ecdh.html [4] https://cr.yp.to/chacha.html [5] https://cr.yp.to/snuffle/xsalsa-20081128.pdf [6] https://cr.yp.to/mac.html [7] https://blake2.net/ [8] https://tools.ietf.org/html/rfc8439 [9] https://github.com/google/wycheproof Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> Cc: Samuel Neves <sneves@dei.uc.pt> Cc: Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Greg KH <gregkh@linuxfoundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: kernel-hardening@lists.openwall.com Cc: linux-crypto@vger.kernel.org

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