rind 0.1.11

rind is a compendium of high quality, fast and easy to build cryptography for Rust, aimed at use with rustls and MoQ.

rind began as a fork of Graviola by Joe Birr-Pixton, and no longer tracks it. It adds what MoQ needs to drop C cryptography entirely; the plan lives in quest/. Coming from ring or aws-lc-rs? MIGRATING.md maps their API to rind's, and rind-ring offers ring's API as is.

High quality: rind incorporates assembler routines from the s2n-bignum project. These have been formally proven to correctly implement the desired mathematical operation.

Fast: rind beats or is competitive with other cryptography libraries for Rust. cargo bench compares it with aws-lc-rs, ring, and RustCrypto.

Easy and fast to build: no C compiler, assembler or other tooling needed: just the Rust compiler. Compiles in seconds.

Status

This project is very new, so exercise due caution. The overriding goal of this crate is for use with rustls via rind-rustls, but there is also a public API for general-purpose use.

Goals

• Fast and simple compilation
  • cargo build takes seconds, and requires only rustc
• Competitive performance (with ring, aws-lc-rs, and rustcrypto)
• Uses formally-verified code from other projects (where it is as fast)
• Intended to provide algorithms in wide use on web
• Intended for use as a rustls CryptoProvider, via rind-rustls.

Limitations

aarch64, x86_64, and 32-bit arm architectures only.

• aarch64 requires the neon CPU feature. Where the CPU has the crypto extension (aes, pmull, and sha2), as the Raspberry Pi 5 and most aarch64 servers and phones do, AES, GHASH, SHA1, and SHA256 run on its instructions. Without it, as on the Raspberry Pi 4, CM4, and Zero 2 W, they run aws-lc's NEON and scalar assembly instead (see Algorithms), rind::cpu::has_aes_hardware() is false, and rind-rustls and rind-dimpl prefer ChaCha20-Poly1305.
• x86_64 runs on every x86_64 CPU. Without aes and pclmulqdq, as on Core 2 and Nehalem, AES and GHASH run aws-lc's SSSE3 assembly instead (see Algorithms), rind::cpu::has_aes_hardware() is false, and rind-rustls and rind-dimpl prefer ChaCha20-Poly1305. Without avx2, as on Sandy Bridge, Ivy Bridge, and AMD's Bulldozer and Jaguar cores, ChaCha20-Poly1305 runs aws-lc's SSE4.1 assembly, ChaCha20, SHA1, and SHA256 its SSSE3 assembly, or for SHA its AVX assembly on Intel's cores, ML-KEM's arithmetic runs a Rust port of mlkem-native's portable C, and table lookups use SSE2 (see Algorithms). Without sse4.1, as on the first Core 2s and Atoms and AMD's Bobcat, ChaCha20 and Poly1305 run in separate passes. Without ssse3 either, as on Intel's CPUs before Core 2, AMD's before Bobcat and Bulldozer, and QEMU's default qemu64 model, AES and GHASH run Rust ports of aws-lc's constant-time SSE2 and integer C, and ChaCha20, SHA1, and SHA256 its integer assembly. Without adx, as on Haswell and AMD's cores before Zen, s2n-bignum's _alt routines replace those that use it, and aws-lc's Montgomery multiplication replaces its Karatsuba routines for RSA; with avx2 there, as on Haswell, RSA-2048 signing runs aws-lc's RSAZ exponentiation. With avx512ifma, as on Ice Lake, Sapphire Rapids, and AMD's Zen 4 and 5, RSA-2048, 3072, and 4096 signing runs aws-lc's dual AVX-512 RSAZ.
• 32-bit arm (tested on armv7-unknown-linux-gnueabihf, and built for armv7-linux-androideabi) runs portable Rust instead, mostly from libcrux (see Algorithms), and needs no CPU features. RSA's Montgomery multiplication runs aws-lc's assembly, with NEON where the CPU has it. Where the CPU has the ARMv8 crypto extension, as most 32-bit Android devices do, AES, GHASH, SHA1, and SHA256 run aws-lc's assembly for it instead. Without it, rind::cpu::has_aes_hardware() is false, and rind-rustls and rind-dimpl prefer ChaCha20-Poly1305, which is faster there: ChaCha20 runs aws-lc's assembly, and so does Poly1305 where the CPU has NEON. SHA-384, SHA-512, and SHA-512/256 always run aws-lc's assembly, with NEON where the CPU has it, and with NEON, X25519's shared secret runs BoringSSL's assembly.

Every operation panics on a CPU missing one of these. Call rind::cpu::check() at startup to get an error naming the missing feature instead.

Building and Testing

Compile:

cargo build --release

Run tests:

git submodule update --init thirdparty/cavp
git submodule update --init thirdparty/wycheproof
cargo test

Run performance benchmarks:

cargo bench

With Nix, nix develop (or direnv) provides CI's toolchain, and just check runs CI's gate over what the branch changed. On Linux, just armv7 runs the nightly armv7 job: it builds and lints for 32-bit ARM, then runs the tests under qemu with the ARMv8 crypto extension, with NEON alone, and with NEON denied. nix develop .#ctgrind --command admin/ctgrind runs the nightly constant-time checks under valgrind, and nix develop .#sde --command admin/ct-sde those of the AVX-512 code valgrind cannot run, under Intel SDE.

Algorithms

Each cell gives rind's time over the fastest of aws-lc-rs and ring on that path, in the benchmark where rind trails most, then whose code runs: the sources below, rind's own code, or Graviola's. A bold ratio is a gap, above the noise of about 1.02. A ✓ marks code its source formally verifies, to the extent Sources states, and a ⚡ unverified code chosen because it measured faster than a verified alternative. A cell naming no source runs the same sources as the one to its left, and an empty cell runs the same code, so it isn't timed. The rind-rustls rows time full and resumed TLS 1.3 handshakes and bulk transfer against rustls over aws-lc-rs's and ring's providers. RSA key generation, whose time swings by up to 3x from run to run, stays out of the ratios. The nightly bench workflow times every cell and keeps each benchmark's numbers in its summary. These are from run 41378010740, and the P-256, P-384, and P-521 cells from run 41399702862, except armv7's P-256, from run 41518446383; a change that moves a cell refreshes it. ML-KEM-768's cells are from run 41400124007, whose x86_64 job drew an EPYC 9V74 (Zen 4) and no AVX2 job an EPYC 7763. RSA's no ADX and no AVX2 cells are from run 41469252502, whose no ADX job drew a Xeon 6973P-C (Granite Rapids) and no AVX2 job an EPYC 7763.

x86_64 runs every feature of an AMD EPYC 7763 (Zen 3: ADX, AVX2, SHA-NI, and VAES, but no AVX-512), except where noted below. Each column to its right denies more, as bench.yml describes, and compares with aws-lc-rs alone, since ring cannot be denied a feature: no ADX denies ADX (Haswell); no AVX2 also AVX2, BMI2, SHA-NI, AVX-512, VAES, and VPCLMULQDQ (Sandy Bridge); no AVX also AVX (Westmere); no AES-NI also AES-NI, PCLMULQDQ, and SSE4.1 (Core 2); and SSE2 also SSSE3. GitHub's pool ran no ADX on an EPYC 7763 too, no AVX2 on an EPYC 9V74 (Zen 4), no AVX and SSE2 on an EPYC 9V45 (Zen 5), and no AES-NI on a Xeon Platinum 8573C (Emerald Rapids). The 1 MB transfer row is from run 41401180882, which ran x86_64 on an EPYC 9V74, whose AVX-512 runs AES-GCM's 512-bit paths rather than the 256-bit VAES ones Zen 3 runs, except its no AVX2 cell, from run 41468954426 on an EPYC 9V45. The AES-GCM row is from run 41519474246, which ran x86_64 on an EPYC 9V45, whose AVX-512 runs AES-GCM's 512-bit paths, no AVX2, no AVX, and no AES-NI on an EPYC 7763, and SSE2 on a Xeon Platinum 8573C. The SHA, HMAC, and HKDF rows are from run 41398664891, which ran every column on an EPYC 7763, except x86_64's HKDF cell, from run 41519474246 on an EPYC 9V45, and its PBKDF2 cell, from run 41464027605 on an EPYC 7763. The SHA-256 cells are from run 41485960394, which ran x86_64 on an EPYC 9V74, no AVX2 on an EPYC 7763, no AVX on an EPYC 9V45, and SSE2 on a Xeon Platinum 8573C. The x86_64 P-256 cells take key generation, signing, and ECDH from run 41467306881, which ran x86_64 and no AVX2 on an EPYC 7763 and no ADX on an EPYC 9V74, and verification from run 41399702862, which ran x86_64 on an EPYC 7763, no ADX on an EPYC 9V45, and no AVX2 on a Xeon 6975P-C (Granite Rapids). Intel CPUs without SHA-NI run aws-lc's AVX SHA-1 and SHA-256, and every Intel CPU its verified ✓ AVX SHA-384 and SHA-512.

aarch64 runs a Neoverse-N2, with the crypto extension and the SHA-512 and SHA-3 instructions; no crypto denies them, as a Raspberry Pi 4 lacks them. armv7 runs on the same core in 32-bit mode, with NEON and the crypto extension; without them it runs the code Symmetric cryptography names. The aarch64 and no crypto cells for RSA and rind-rustls handshakes are from run 41397278464, which times aarch64 RSA's current Montgomery multiplication. The SHA-256 cells and aarch64's HMAC and PBKDF2 cells are from run 41485960394, the AES-GCM and HKDF rows from run 41519474246, the other SHA and HMAC cells from run 41398664891, and armv7's RSA and handshake cells from run 41401089431.

The table describes main, and changes alongside the code it describes. The tests use Wycheproof's vectors.

Sources

rind's own code is licensed under Apache-2.0 OR ISC OR MIT-0. Copied code keeps its source's copyright and license, which LICENSE lists with the texts they require; dependencies ship their own. Every file under rind/src names its license in a SPDX-License-Identifier comment, and files Graviola wrote keep their Written for Graviola by ... header. The rind crate as a whole is therefore (Apache-2.0 OR ISC OR MIT-0) AND (Apache-2.0 OR ISC OR MIT) AND Apache-2.0 AND ISC. The adapter crates copy nothing under another license, so they stay Apache-2.0 OR ISC OR MIT-0. rind-ring runs ring's own tests, which keep ring's licenses and stay out of the published crate.

A change that copies code from a new source, or moves an algorithm to a different one, updates this table and LICENSE in the same PR.

Assorted technical details

RSA

On x86_64 and aarch64, the arithmetic is provided by s2n-bignum, under the exponentiation described below, except that x86_64 runs aws-lc's Montgomery multiplication as aws-lc's bn_mul_mont picks it (bn_mul4x_mont for moduli whose width in 64-bit words is a multiple of four, bn_sqr8x_mont for squaring a multiple of eight, each by mulx with ADX, and bn_mul_mont_nohw for other widths), other than 1024- and 2048-bit moduli on CPUs with ADX, where s2n-bignum's Karatsuba routines are as fast or faster. aarch64 runs aws-lc's bn_mul_mont, which picks its own squaring or 4x routine by the width, at every size: on a Neoverse-N2 it measured 1.15 to 2 times as fast as s2n-bignum's generic routines, and faster than its Karatsuba routines for 1024- and 2048-bit moduli. armv7 runs the same exponentiation over aws-lc's 32-bit ARM bn_mul_mont routines as aws-lc picks them (bn_mul8x_mont_neon with NEON for moduli whose width in 32-bit words is a multiple of eight, and bn_mul_mont_nohw otherwise), and the rest of the arithmetic in rind's portable Rust. On x86_64 CPUs with AVX2 but without ADX, the private operation's exponentiation modulo each 1024-bit prime of RSA-2048 runs aws-lc's RSAZ, as aws-lc does there: Montgomery products on AVX2 over 29-bit limbs, and a 5-bit fixed window whose table entry is gathered by masking all 32 entries. Otherwise x86_64 runs aws-lc's 5-bit window, as BN_mod_exp_mont_consttime does, gathering each entry by s2n-bignum's verified routine, which reads every entry. For moduli of a multiple of 512 bits other than 1024 and 2048 bits with ADX, it runs aws-lc's bn_power5 instead: five squarings and the product by an entry gathered by masking all 32, in one call. On x86_64 CPUs with AVX-512 IFMA, both exponentiations of the CRT for primes of 1024, 1536, or 2048 bits run at once by aws-lc's dual RSAZ, as aws-lc's BN_mod_exp_mont_consttime_x2 does there: almost Montgomery products on 256-bit vectors over 52-bit limbs, two moduli at a time, and a 5-bit fixed window whose table entries are gathered by masking all 32. Valgrind runs no AVX-512, so admin/ct-sde checks it under Intel SDE instead.

The RSA private operation always uses the CRT optimisation.

On aarch64 and armv7, modular exponentiation uses a 5-bit fixed exponent window, as aws-lc's does, and the term is selected by the exponent bits from the table of base powers in a side-channel-free way. The private operation is always followed by the public operation to verify the result (and the result compared in a side-channel-free way).

Only RSA signing and verification are provided. Signing keys have 2048- to 8192-bit moduli; verification also takes 1024-bit moduli and SHA-1, but only under signature::verify::legacy. Every key refuses a public exponent that is even, 1, or over 33 bits, and a private key whose components disagree is refused when it loads, as aws-lc's RSA_check_key does. RSA encryption is not provided: OAEP waits on a consumer, and PKCS#1 v1.5 encryption, which invites padding-oracle attacks, is refused.

RSA key generation is supported for five fixed key sizes between 2048-8192-bits. All generated keys have e = 0x10001 and, as in aws-lc, d reduced mod lcm(p - 1, q - 1). Generation runs in constant time in the primes, revealing only which candidates it discards.

ECC

On x86_64 and aarch64, all ECC field and scalar arithmetic are provided by s2n-bignum, except P256's on x86_64 where aws-lc's is named below, and the rest of this section describes those backends.

P256 base point multiplication on x86_64 is aws-lc's: 7-bit Booth windows over 37 rows of 64 affine points (a 152KB constant table), each row scanned whole by its SSE2 or AVX2 routine and added by its mixed addition, with mulq or with BMI2 and ADX, as aws-lc's ecp_nistz256_point_mul_base runs them. On aarch64 it is s2n-bignum's p256_scalarmulbase, over 6-bit Booth windows and a table of each window's 32 multiples (an 88KB constant table). Variable point multiplication is s2n-bignum's p256_montjscalarmul.

armv7 runs the aarch64 algorithms for P256 in portable Rust, over fiat-crypto's field and scalar arithmetic, except its Montgomery multiplication and squaring, which are rind's. LLVM compiles fiat-crypto's 32-bit multiplication to save and restore the one carry flag 171 times a call, between its interleaved carry chains; rind's carry in 64-bit accumulators (umaal) instead, in 560 instructions run to fiat-crypto's 1,375. fiat-crypto cannot generate an unsaturated P256, as it does P521: its bounds checks fail at every 32-bit limb count from 9 to 16.

P384 base point multiplication uses a comb of 20 tables of 16 affine points (a 30KB constant table) over 5-bit Booth windows, as aws-lc does. Variable point multiplication is s2n-bignum's p384_montjscalarmul.

P521's prime is a Mersenne prime, so its field arithmetic is direct rather than Montgomery's. Base point multiplication uses a comb of 27 tables of 16 affine points (a 62KB constant table) over 5-bit Booth windows, as aws-lc does. Variable point multiplication is s2n-bignum's p521_jscalarmul. s2n-bignum's P521 point addition requires points that are not at infinity, equal, or opposite, so the comb keeps to that, selecting around the cases it reaches.

ECDSA verification computes u1 G + u2 Q in one chain of doublings, as aws-lc does, adding terms from width-5 non-adjacent forms of the public scalars: u2's over the public key's odd multiples, computed once when the key is parsed, and u1's over the first row of the base point table, at width 6 for P256, as its aarch64 row holds 32 multiples. An addition of equal or opposite points gives a zero z, as infinity does, so only then is it redone by cases.

ECDSA follows RFC6979 for generation of k, but adds additional non-critical random input. We do this to avoid the theoretical fragility of RFC6979 under fault conditions. This is allowed for by RFC6979, and the HMAC-DRBG that it builds on. The code is structured such that we pass the RFC6979 test vectors.

The code which selects a term from a table of points is non-verified, and is written in AVX2/Neon intrinsics, or for P256 on x86_64 is aws-lc's SSE2 and AVX2 assembly.

On x86_64 and aarch64, X25519 directly uses the s2n-bignum implementation.

ML-KEM-768

On aarch64, and on x86_64 CPUs with AVX2, the core polynomial arithmetic -- the NTT, inverse NTT, base multiplication and reduction -- is provided by s2n-bignum (contributed by the mlkem-native developers). The surrounding FIPS-203 construction (key generation, encapsulation and decapsulation) is written in Rust. With AVX2, ciphertext compression runs mlkem-native's AVX2 assembly.

x86_64 CPUs without AVX2 run the same construction over a Rust port of mlkem-native's portable C arithmetic instead, in natural coefficient order as aarch64 keeps it, and Keccak-f four states one at a time, chosen once per process. It measured 1.3x faster than libcrux's portable ML-KEM for key generation and encapsulation there, and 2x for decapsulation.

Compression and decompression are division-free, avoiding the variable-time division behind the KyberSlash attacks.

For sampling, which is a significant factor in ML-KEM performance, we have some different options depending on CPU features:

• We have a SampleNTT() which does 8-wide Keccak-f permutation followed by rejection sampling using AVX-512 (F, BW, VBMI, VBMI2) on x86-64.
• We have a 4-wide Keccak-f using AVX2 on x86-64.
• We have a 2-wide Keccak-f using Neon+SHA3 on aarch64.
• We have a basic Keccak-f using scalar instructions on aarch64.

Symmetric cryptography

SHA1 and SHA256 have straightforward implementations using hashing intrinsics (aka "SHA-NI" on x86_64, "sha" extension on aarch64), except that aarch64 runs aws-lc's SHA256 assembly for those instructions: it keeps a register copy off each round's dependency chain, where LLVM's code for the intrinsics puts one on it. Without SHA-NI, x86_64 selects at runtime an AVX2 message schedule with scalar rounds for SHA1, as aws-lc does. For SHA256 it runs aws-lc's AVX routine on Intel, as aws-lc picks, and the same AVX2 approach elsewhere, since the AVX routine rotates with shrd, which pre-Zen AMD runs slowly. Without AVX2 either, both run aws-lc's AVX routines on Intel and its SSSE3 ones elsewhere, as aws-lc picks; they are 2.2x (SHA256) and 1.7x (SHA1) faster than the pure Rust ones on Intel. Without SSSE3 too, both run aws-lc's integer routines, as aws-lc does: 2.2x (SHA256) and 1.25x (SHA1) faster than the pure Rust ones, and 1.3-1.5x faster than HACL*'s SHA256. aarch64 without the crypto extension runs aws-lc's scalar SHA1 and SHA256 assembly. 32-bit arm runs aws-lc's SHA1 and SHA256 assembly where the CPU has the crypto extension, and the pure Rust SHA1 and SHA256 otherwise: libcrux has no SHA1, and the pure Rust SHA256 is faster than libcrux's.

SHA384, SHA512, and SHA512/256 run aws-lc's assembly everywhere. On x86_64, its AVX routine on Intel and its scalar routine on other CPUs, as aws-lc picks: the AVX routine is 1.1x faster than the scalar one on Intel, and 1.5x slower on AMD's Zen. On aarch64, its routine for the SHA-512 instructions where the CPU has them, and its scalar routine otherwise; on 32-bit arm, its NEON routine where the CPU has NEON, and its scalar routine otherwise.

x86_64 decides once per process whether the CPU has AES-NI and PCLMULQDQ, and SSSE3. Without the first two, as on Core 2 and Nehalem, AES and AES-CTR run aws-lc's vpaes-x86_64 assembly and GHASH its ghash-ssse3-x86_64, composed into AES-GCM as aws-lc composes them, taking turns over 3 KB; both modes measured level with aws-lc's. Without SSSE3 either, as on QEMU's qemu64, they run Rust ports of the C aws-lc runs there: aes_nohw.c, which bitslices eight blocks into SSE2 registers and computes the S-box as a circuit, and gcm_nohw.c, which multiplies with integer multiplies of operands masked to every fourth bit, so no carry reaches a bit it keeps. Neither branches or indexes memory by a secret. Composed the same way, AES-GCM and AES-CTR measured 0.82-0.92x aws-lc's time; libcrux's portable AES-GCM measured 3-7x slower, and its bitsliced block level with the port's for one block. With AES-NI and PCLMULQDQ, AES-CTR runs aws-lc's aes_hw_ctr32_encrypt_blocks, 1.1-2x faster than rind's by-8 intrinsics at every length, with or without AVX.

AES-GCM has a stitched by-8 AES-CTR and a by-8 GHASH, SSE-encoded on every CPU: VEX encoding measured no faster with AVX, and without it, as on Westmere and the Goldmont and Tremont Atoms, this is 1.02-1.28x faster than aws-lc's SSE aesni-x86_64 and ghash-x86_64 routines. With AVX and MOVBE but no VAES, as from Haswell through Comet Lake and on Zen 1 and 2, aws-lc's verified aesni_gcm routine, which interleaves AES-CTR and GHASH six blocks at a time, takes messages from 288 bytes to encrypt and 96 to decrypt, as aws-lc and ring run it, and the by-8 code takes their tails; on an i7-8700 (Coffee Lake) this measured 1.2-1.7x faster than the by-8 code alone from 1200 bytes to 16 KB. Every 256- and 512-bit kernel rind runs ends with vzeroupper, so SSE code after one pays no AVX-SSE transition or false dependency. There is also an optional AVX-512 by-16 AES-CTR and GHASH. It uses dynamic CPU feature detection, so AVX-512 is not required at runtime. On cores with VAES, BoringSSL's kernels, which interleave AES-CTR and GHASH, take long messages: its 512-bit kernel from 2 KB where the CPU has AVX-512 (the by-16 path is quicker below that), and its 256-bit kernel from 256 bytes otherwise. The key picks its kernel once, when it is made.

aarch64 decides once per process whether the CPU has the crypto extension. With it, AES-GCM runs s2n-bignum's kernels, which interleave a by-4 AES-CTR and GHASH over whole blocks. An AES-256 key made on a core with SHA3 runs their by-8 kernels instead, which use its three-way XOR. They run in place, outside their proofs, which assume separate input and output buffers. admin/parse-asm/inplace.py walks them at every length to 64 blocks, checking that each block is loaded before it is stored and that their branches and addresses depend only on the length.

Without it, AES runs aws-lc's vpaes-armv8, Mike Hamburg's vector permutation AES, whose S-box is NEON tbl lookups into registers, and GHASH runs aws-lc's ghash-neon-armv8, which builds each product from NEON's 8-bit polynomial multiplies; neither reads memory or branches by a secret. AES-GCM runs AES-CTR and GHASH over each eight blocks in turn, as aws-lc composes them.

32-bit arm decides once per process from AT_HWCAP2: with the crypto extension, AES and AES-GCM keys use aws-lc's aesv8-armx and ghashv8-armx assembly, composed as aws-lc does on that architecture; without it, libcrux's bitsliced AES and AES-GCM.

x86_64 CPUs without AVX2 run ChaCha20 on aws-lc's chacha-x86_64 assembly, as aws-lc picks: its integer routine up to 128 bytes, and its SSSE3 routine, four blocks at a time, above that. Both measured faster than the pure Rust block function at every length: 1.7x for one block, and 4.5x from 256 bytes. Without SSSE3, the integer routine runs at every length, as in aws-lc, and measured 1.2x faster than HACL*'s ChaCha20 at every length, and HChaCha20, for XChaCha20, rotates with SSE2 shifts rather than SSSE3's byte shuffles.

ChaCha20-Poly1305, and XChaCha20-Poly1305 after HChaCha20, run aws-lc's chacha20_poly1305 assembly, which interleaves ChaCha20 with Poly1305 in one pass: on x86_64, its AVX2 half where the CPU has AVX2 and BMI2 and its SSE4.1 half otherwise, as aws-lc picks them, and on aarch64, its NEON routine. aws-lc's x86_64 routine reads its pick from OPENSSL_ia32cap_P; rind's copy reads the same bits from the block rind passes it, so the path is decided once per process like rind's others. Without SSE4.1, ChaCha20 and Poly1305 run in separate passes, as in aws-lc.

32-bit arm runs ChaCha20 on aws-lc's chacha-armv4 assembly, switching from its integer routine to its NEON one at 192 bytes as aws-lc does, where AT_HWCAP reports NEON. Poly1305 runs aws-lc's NEON code there too, and HACL*'s verified Poly1305 without NEON.

Architecture

rind's API is its own, shaped so that misuse fails to compile rather than at run time. MIGRATING.md maps each ring and aws-lc-rs item to it.

• Each module takes an algorithm as a &'static constant, such as aead::AES_128_GCM or signature::verify::ECDSA_P256_SHA256_ASN1, and has one key type per role.
• Algorithms only old protocols should use sit in legacy submodules, such as digest::legacy::SHA1.
• Key pairs name the DER format they load or encode (from_pkcs8, from_sec1, to_spki), and every private key returns its public half from public_key().
• Encodings, signatures, and shared secrets are owned values of fixed capacity, such as signature::Pkcs8, rather than caller buffers or allocations. Keys wipe their secrets on drop.
• No call takes a random source: rind draws from the system's when it needs one, and rand::fill gives callers the same.
• Fallible calls return rind::Error, whose variant says which kind of call failed, such as KeyRejected or BadSignature.

The modules:

• aead: AES-GCM and ChaCha20-Poly1305 keys with caller nonces, an XChaCha20-Poly1305 key that picks random nonces, and QUIC header-protection keys.
• cipher: bare AES, AES-CTR, and ChaCha20, for DTLS record number protection and SRTP.
• agreement: ephemeral X25519 and ECDH on P-256, P-384, and P-521.
• kem: ML-KEM-768.
• signature: ECDSA on P-256, P-384, and P-521, RSA PKCS#1 v1.5 and PSS, and Ed25519 key pairs and verification, plus RSA keys given as their components, and SHA-1 and 1024-bit RSA for verifying old signatures.
• digest: SHA-1 (legacy only), SHA-256, SHA-384, SHA-512, and SHA-512/256.
• hmac: HMAC over those digests, keyed once and reused.
• hkdf: HKDF over those HMACs, whose output becomes an aead, QUIC header-protection, or hmac key.
• pbkdf2: PBKDF2 over those HMACs, deriving or verifying a key from a password.
• constant_time: comparing secrets such as MACs without a timing leak.
• rand: random bytes from the system.
• cpu: the startup check for required CPU features, and whether AES runs in hardware.

The public modules wrap three private layers:

• low: low level primitives. platform-specific. unsafe allowed. minimal std and alloc.
• mid: constructions, protocols and encodings. platform agnostic. no unsafe. minimal std and alloc.
• high: high level encodings and operations. platform agnostic. no unsafe.

low code should not refer to mid, nor mid to high.

low must present the same interface irrespective of platform. To this end, low::generic contains pure-rust polyfills for items we don't have assembler- or intrinsic-based implementations for a certain platform.