3 releases (breaking)

0.3.0 Sep 9, 2022
0.2.0 Sep 1, 2022
0.1.0 Aug 13, 2022

#4 in #ristretto255

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Used in double-odd

MIT license

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crrl

This library implements some primitives for purposes of cryptographic research. Its point is to provide efficient, optimized and constant-time implementations that are supposed to be representative of production-ready code, so that realistic performance benchmarks may be performed. Thus, while meant primarily for research, the code here should be fine for production use (though of course I offer no such guarantee; use at your own risks).

So far, only some primitives related to elliptic curve cryptography are implemented:

  • A generic type GF255<MQ> for finite fields of integers modulo a prime 2^255-MQ (for a value of MQ between 1 and 32767). The MQ value is provided as a type parameter, i.e. the exact field is known at compile time. This type covers the usual modulus 2^255-19 (used in Curve25519) as well as 2^255-18651 and 2^255-3957 (used in double-odd curves do255e and do255s).

  • A generic type ModInt256<M0, M1, M2, M3> for arbitrary finite fields of integers modulo a prime between 2^192 and 2^256. Montgomery representation is internally used. The modulus is provided as type parameters, allowing the compiler to apply optimizations when some parts of the modulus allow them (in particular with the modulus used for NIST curve P-256).

  • Type GFsecp256k1 implements the specific base field for curve secp256k1 (integers modulo 2^256-4294968273). The 64-bit backend has a dedicated implementation, while the 32-bit version of this type uses ModInt256.

  • Type ed25519::Point provides generic group operations in the twisted Edwards curve Curve25519. Ed25519 signatures (as per RFC 8032) are implemented. Type ed25519::Scalar implements operations on integers modulo the curve subgroup order.

  • Type ristretto255::Point provides generic group operations in the Ristretto255 group, whose prime order is exactly the size of the interesting subgroup of Curve25519.

  • Type p256::Point provides generic group operations in the NIST P-256 curve (aka "secp256r1" aka "prime256v1"). ECDSA signatures are supported. The p256::Scalar type implements the corresponding scalars (integers modulo the curve order).

  • Type secp256k1::Point provides generic group operations in the secp256k1 curve (aka "the Bitcoin curve"). ECDSA signatures are supported. The secp256k1::Scalar type implements the corresponding scalars (integers modulo the curve order). The GLV endomorphism is leveraged to speed-up point multiplication (key exchange) and signature verification.

  • Types jq255e::Point and jq255s::Point implement the double-odd curves jq255e and jq255s (along with the corresponding scalar types jq255e::Scalar and jq255s::Scalar). Key exchange and Schnorr signatures are implemented. These curves provide a prime-order group abstraction, similar to Ristretto255, but with somewhat better performance at the same security level. Moreover, the relevant signatures are both shorter (48 bytes instead of 64) and faster than the usual Ed25519 signatures.

  • Function x25519::x25519() implements the X25519 function. An optimized x25519::x2559_base() function is provided when X25519 is applied to the conventional base point.

Types GF255 and ModInt256 have a 32-bit and a 64-bit implementations each. The code is portable (it was tested on 32-bit and 64-bit x86, and 64-bit aarch64). Performance is quite decent; e.g. Ed25519 signatures are computed in about 51500 cycles, and verified in about 114000 cycles, on an Intel "Coffee Lake" CPU; this is not too far from the best assembly-optimized implementations. At the same time, use of operator overloading allows to express formulas on points and scalar with about the same syntax as their mathematical description. For instance, the core of the X25519 implementation looks like this:

        let A = x2 + z2;
        let B = x2 - z2;
        let AA = A.square();
        let BB = B.square();
        let C = x3 + z3;
        let D = x3 - z3;
        let E = AA - BB;
        let DA = D * A;
        let CB = C * B;
        x3 = (DA + CB).square();
        z3 = x1 * (DA - CB).square();
        x2 = AA * BB;
        z2 = E * (AA + E.mul_small(121665));

which is quite close to the corresponding description in RFC 7748:

       A = x_2 + z_2
       AA = A^2
       B = x_2 - z_2
       BB = B^2
       E = AA - BB
       C = x_3 + z_3
       D = x_3 - z_3
       DA = D * A
       CB = C * B
       x_3 = (DA + CB)^2
       z_3 = x_1 * (DA - CB)^2
       x_2 = AA * BB
       z_2 = E * (AA + a24 * E)

Security and Compliance

All the code is strict, both in terms of timing-based side-channels (everything is constant-time, except if explicitly stated otherwise, e.g. in a function whose name includes vartime) and in compliance to relevant standards. For instance, the Ed25519 signature support applies and enforces canonical encodings of both points and scalars.

There is no attempt at "zeroizing memory" anywhere in the code. In general, such memory cleansing is a fool's quest. Note that since most of the library use no_std rules, dynamic allocation happens only on the stack, thereby limiting the risk of leaving secret information lingering all over the RAM. The only functions that use heap allocation only store public data there.

WARNING: I reiterate what was written above: although all of the code aims at being representative of optimized production-ready code, it is still fairly recent and some bugs might still lurk, however careful I am when writing code. Any assertion of suitability to any purpose is explcitly denied. The primary purpose is to help with "trying out stuff" in cryptographic research, by offering an easy-to-use API backed by performance close enough to what can be done in actual applications.

Truncated Signatures

Support for truncated signatures is implemented for Ed25519 and ECDSA/P-256. Standard signatures can be shortened by 8 to 32 bits (i.e. the size may shrink from 64 down to 60 bytes), and the verifier rebuilds the original signature during verification (at some computational cost). This is not a ground-breaking feature, but it can be very convenient in some situations with tight constraints on bandwidth and a requirement to work with standard signature formats. See ed25519::PublicKey::verify_trunc_raw() and p256::PublicKey::verify_trunc_hash() for details.

FROST Threshold Schnorr Signatures

The FROST protocol for a distributed Schnorr signature generation scheme has been implemented, as per the v8 draft specification: draft-irtf-cfrg-frost-08. Four ciphersuites are provided, with similar APIs, in the frost::ed25519, frost::ristretto255, frost::p256 and frost::secp256k1 modules. A sample code showing how to use the API is provided in the frost-sample.rs file.

While FROST is inherently a distributed scheme, the implementation can also be used in a single signer mode by using the "group" private key directly.

Benchmarks

cargo bench runs some benchmarks, but there are a few caveats:

  • The cycle counter is used on x86. If frequency scaling ("TurboBoost") is not disabled, then you'll get wrong and meaningless results.

  • On aarch64, the cycle counter is also accessed directly, which will in general fail with some CPU exception. Access to the counter must first be enabled, which requires (on Linux) a specific kernel module. This one works for me.

  • On architectures other than i386, x86-64 and aarch64, benchmark code will simply not compile.

TODO

In general, about anything related to cryptography may show up here, if there is a use case for it.

Dependencies