Tim Ruffing 887bd1f8b6
Merge #793: Make scalar/field choice depend on C-detected __int128 availability
79f1f7a4f123765cf07be92ae894d882c5845191 Autodetect __int128 availability on the C side (Pieter Wuille)
0d7727f95e52d99c13f55c64e9d1f799ba7d7967 Add SECP256K1_FE_STORAGE_CONST_GET to 5x52 field (Pieter Wuille)

Pull request description:

  This PR does two things:
  * It removes the ability to select the 5x52 field with a 8x32 scalar, or the 10x26 field with a 4x64 scalar. It's both 128-bit wide versions, or neither.
  * The choice is made automatically by the C code, unless overridden by a USE_FORCE_WIDEMUL_INT{64,128} define (which is available through `configure` with a hidden option --with-test-override-wide-multiplication={auto,int64,int128}).

  This reduces the reliance on autoconf for this performance-critical configuration option, and also reduces the number of different combinations to test.

  This removes one theoretically useful combination: if you had x86_64 asm but no __int128 support in your compiler, it was possible to use the 64-bit field before but the 32-bit scalar. I think this doesn't matter as all compilers/systems that support (our)  x86_64 asm also support __int128. Furthermore, #767 will break this.

  As an unexpected side effect, this also means the `gen_context` static precomputation tool will now use __int128 based implementations when available (which required an addition to the 5x52 field; see first commit).

ACKs for top commit:
  real-or-random:
    ACK 79f1f7a4f123765cf07be92ae894d882c5845191 diff looks good and tests pass
  elichai:
    tACK  79f1f7a4f123765cf07be92ae894d882c5845191

Tree-SHA512: 4171732668e5c9cae5230e3a43dd6df195567e1232b89c12c5db429986b6519bb4d77334cb0bac8ce13a00a24dfffdff69b46c89b4d59bc6d297a996ea4efd3d
2020-08-12 15:27:32 +02:00
2013-04-11 12:46:39 +02:00
2017-09-24 17:53:13 -07:00
2013-05-09 15:24:32 +02:00
2019-10-28 14:59:05 +00:00
2013-05-06 13:28:46 +02:00

libsecp256k1

Build Status

Optimized C library for ECDSA signatures and secret/public key operations on curve secp256k1.

This library is intended to be the highest quality publicly available library for cryptography on the secp256k1 curve. However, the primary focus of its development has been for usage in the Bitcoin system and usage unlike Bitcoin's may be less well tested, verified, or suffer from a less well thought out interface. Correct usage requires some care and consideration that the library is fit for your application's purpose.

Features:

  • secp256k1 ECDSA signing/verification and key generation.
  • Additive and multiplicative tweaking of secret/public keys.
  • Serialization/parsing of secret keys, public keys, signatures.
  • Constant time, constant memory access signing and public key generation.
  • Derandomized ECDSA (via RFC6979 or with a caller provided function.)
  • Very efficient implementation.
  • Suitable for embedded systems.
  • Optional module for public key recovery.
  • Optional module for ECDH key exchange (experimental).

Experimental features have not received enough scrutiny to satisfy the standard of quality of this library but are made available for testing and review by the community. The APIs of these features should not be considered stable.

Implementation details

  • General
    • No runtime heap allocation.
    • Extensive testing infrastructure.
    • Structured to facilitate review and analysis.
    • Intended to be portable to any system with a C89 compiler and uint64_t support.
    • No use of floating types.
    • Expose only higher level interfaces to minimize the API surface and improve application security. ("Be difficult to use insecurely.")
  • Field operations
    • Optimized implementation of arithmetic modulo the curve's field size (2^256 - 0x1000003D1).
      • Using 5 52-bit limbs (including hand-optimized assembly for x86_64, by Diederik Huys).
      • Using 10 26-bit limbs (including hand-optimized assembly for 32-bit ARM, by Wladimir J. van der Laan).
    • Field inverses and square roots using a sliding window over blocks of 1s (by Peter Dettman).
  • Scalar operations
    • Optimized implementation without data-dependent branches of arithmetic modulo the curve's order.
      • Using 4 64-bit limbs (relying on __int128 support in the compiler).
      • Using 8 32-bit limbs.
  • Group operations
    • Point addition formula specifically simplified for the curve equation (y^2 = x^3 + 7).
    • Use addition between points in Jacobian and affine coordinates where possible.
    • Use a unified addition/doubling formula where necessary to avoid data-dependent branches.
    • Point/x comparison without a field inversion by comparison in the Jacobian coordinate space.
  • Point multiplication for verification (aP + bG).
    • Use wNAF notation for point multiplicands.
    • Use a much larger window for multiples of G, using precomputed multiples.
    • Use Shamir's trick to do the multiplication with the public key and the generator simultaneously.
    • Optionally (off by default) use secp256k1's efficiently-computable endomorphism to split the P multiplicand into 2 half-sized ones.
  • Point multiplication for signing
    • Use a precomputed table of multiples of powers of 16 multiplied with the generator, so general multiplication becomes a series of additions.
    • Intended to be completely free of timing sidechannels for secret-key operations (on reasonable hardware/toolchains)
      • Access the table with branch-free conditional moves so memory access is uniform.
      • No data-dependent branches
    • Optional runtime blinding which attempts to frustrate differential power analysis.
    • The precomputed tables add and eventually subtract points for which no known scalar (secret key) is known, preventing even an attacker with control over the secret key used to control the data internally.

Build steps

libsecp256k1 is built using autotools:

$ ./autogen.sh
$ ./configure
$ make
$ make check
$ sudo make install  # optional

Exhaustive tests

$ ./exhaustive_tests

With valgrind, you might need to increase the max stack size:

$ valgrind --max-stackframe=2500000 ./exhaustive_tests

Test coverage

This library aims to have full coverage of the reachable lines and branches.

To create a test coverage report, configure with --enable-coverage (use of GCC is necessary):

$ ./configure --enable-coverage

Run the tests:

$ make check

To create a report, gcovr is recommended, as it includes branch coverage reporting:

$ gcovr --exclude 'src/bench*' --print-summary

To create a HTML report with coloured and annotated source code:

$ gcovr --exclude 'src/bench*' --html --html-details -o coverage.html

Reporting a vulnerability

See SECURITY.md

Description
Experimental fork of libsecp256k1 with support for pedersen commitments and range proofs.
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