Hi Jonathan,

I am writing a program that has to compute the STARK public key based on the private key.

I have to private key in the form of a hex encoded string. I then decoded it into a byte slice. However, I see that the get_public_key function accepts a fixed size byte slice of &[u8; 32]. Having looked online, I couldn't find any way of converting &[u8] into &[u8; 32]. So I created a helper function which takes in the input string and attempts to convert it:

fn hex_to_32byte(inp: &str) -> Result<[u8; 32], AllErr> {
    let decoded = hex::decode(inp)?.as_slice();
    let out: [u8; 32];
    
    for i in 0..32 {
        out[i] = decoded[i];
    }

    Ok(out)
}

Is there a more efficient way?

When building on macOS in a GitHub Actions workflow with cargo build, an error occurs:

  cargo:warning=lib/starkware-crypto/src/starkware/crypto/ffi/utils.cc:1:10: fatal error: 'endian.h' file not found
  cargo:warning=#include <endian.h>
  cargo:warning=         ^~~~~~~~~~
  exit status: 0
  running: "c++" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-m64" "-arch" "x86_64" "-static" "-I" "lib/starkware-crypto/src" "-Wall" "-Wextra" "-std=c++17" "-Werror" "-Wall" "-Wextra" "-fno-strict-aliasing" "-fPIC" "-o" "/Users/runner/repos/starknet-rs/target/debug/build/starkware-crypto-sys-7e7a1853b703c604/out/lib/starkware-crypto/src/starkware/crypto/pedersen_hash.o" "-c" "lib/starkware-crypto/src/starkware/crypto/pedersen_hash.cc"
  cargo:warning=1 error generated.
  exit status: 1
  exit status: 0
  exit status: 0

  --- stderr

After inspecting the upstream code, it's found that the library converts between little endian and host endian if they're different. This forces library users to be aware of host endianess.

I'm gonna simply fork upstream and remove the conversion, and instead specifically document that all function inputs must be little-endian.

Alternative solutions considered:

• Explicitly document that function inputs must conform to host endianess

  Not good as it forces users to be aware of host endianess, which is mostly irrelevant today.

• Use target_endian to convert to big endian first if the host is big-endian

  No need to fork upstream, but has unnecessary performance costs on big-endian systems.

OS: Debian GNU/Linux 12 bookworm (testing, amd64) GCC version: 11.3.0

error: failed to run custom build command for `starkware-crypto-sys v0.1.3`

Caused by:
  process didn't exit successfully: `[...]/target/debug/build/starkware-crypto-sys-3a366d1558176dff/build-script-build` (exit status: 1)
  --- stdout
  TARGET = Some("x86_64-unknown-linux-gnu")
  HOST = Some("x86_64-unknown-linux-gnu")
  CXX_x86_64-unknown-linux-gnu = None
  CXX_x86_64_unknown_linux_gnu = None
  HOST_CXX = None
  CXX = None
  CXXFLAGS_x86_64-unknown-linux-gnu = None
  CXXFLAGS_x86_64_unknown_linux_gnu = None
  HOST_CXXFLAGS = None
  CXXFLAGS = None
  CRATE_CC_NO_DEFAULTS = None
  DEBUG = Some("true")
  running: "c++" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-m64" "-static" "-I" "lib/starkware-crypto/src" "-Wall" "-Wextra" "-std=c++17" "-Werror" "-Wall" "-Wextra" "-fno-strict-aliasing" "-fPIC" "-o" "[...]/target/debug/build/starkware-crypto-sys-e7dc842a2c877560/out/lib/starkware-crypto/src/starkware/crypto/ecdsa.o" "-c" "lib/starkware-crypto/src/starkware/crypto/ecdsa.cc"
  running: "c++" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-m64" "-static" "-I" "lib/starkware-crypto/src" "-Wall" "-Wextra" "-std=c++17" "-Werror" "-Wall" "-Wextra" "-fno-strict-aliasing" "-fPIC" "-o" "[...]/target/debug/build/starkware-crypto-sys-e7dc842a2c877560/out/lib/starkware-crypto/src/starkware/algebra/prime_field_element.o" "-c" "lib/starkware-crypto/src/starkware/algebra/prime_field_element.cc"
  exit status: 0
  running: "c++" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-m64" "-static" "-I" "lib/starkware-crypto/src" "-Wall" "-Wextra" "-std=c++17" "-Werror" "-Wall" "-Wextra" "-fno-strict-aliasing" "-fPIC" "-o" "[...]/target/debug/build/starkware-crypto-sys-e7dc842a2c877560/out/lib/starkware-crypto/src/starkware/crypto/elliptic_curve_constants.o" "-c" "lib/starkware-crypto/src/starkware/crypto/elliptic_curve_constants.cc"
  exit status: 0
  running: "c++" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-m64" "-static" "-I" "lib/starkware-crypto/src" "-Wall" "-Wextra" "-std=c++17" "-Werror" "-Wall" "-Wextra" "-fno-strict-aliasing" "-fPIC" "-o" "[...]/target/debug/build/starkware-crypto-sys-e7dc842a2c877560/out/lib/starkware-crypto/src/starkware/crypto/ffi/ecdsa.o" "-c" "lib/starkware-crypto/src/starkware/crypto/ffi/ecdsa.cc"
  exit status: 0
  running: "c++" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-m64" "-static" "-I" "lib/starkware-crypto/src" "-Wall" "-Wextra" "-std=c++17" "-Werror" "-Wall" "-Wextra" "-fno-strict-aliasing" "-fPIC" "-o" "[...]/target/debug/build/starkware-crypto-sys-e7dc842a2c877560/out/lib/starkware-crypto/src/starkware/crypto/ffi/pedersen_hash.o" "-c" "lib/starkware-crypto/src/starkware/crypto/ffi/pedersen_hash.cc"
  exit status: 0
  running: "c++" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-m64" "-static" "-I" "lib/starkware-crypto/src" "-Wall" "-Wextra" "-std=c++17" "-Werror" "-Wall" "-Wextra" "-fno-strict-aliasing" "-fPIC" "-o" "[...]/target/debug/build/starkware-crypto-sys-e7dc842a2c877560/out/lib/starkware-crypto/src/starkware/crypto/ffi/utils.o" "-c" "lib/starkware-crypto/src/starkware/crypto/ffi/utils.cc"
  cargo:warning=In file included from lib/starkware-crypto/src/starkware/crypto/pedersen_hash.h:4,
  cargo:warning=                 from lib/starkware-crypto/src/starkware/crypto/ffi/utils.h:6,
  cargo:warning=                 from lib/starkware-crypto/src/starkware/crypto/ffi/utils.cc:4:
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp: In instantiation of ‘constexpr gsl::span<T> gsl::span<T>::subspan(gsl::span<T>::index_type, gsl::span<T>::index_type) const [with T = gsl::byte; gsl::span<T>::index_type = long unsigned int]’:
  cargo:warning=lib/starkware-crypto/src/starkware/crypto/ffi/utils.cc:30:53:   required from here
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp:1726:15: error: comparison of unsigned expression in ‘>= 0’ is always true [-Werror=type-limits]
  cargo:warning= 1726 |             0 <= offset && offset <= this->size() &&
  cargo:warning=      |             ~~^~~~~~~~~
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp:579:50: note: in definition of macro ‘Expects’
  cargo:warning=  579 | # define Expects( x )  ::gsl::fail_fast_assert( (x) )
  cargo:warning=      |                                                  ^
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp:1727:15: error: comparison of unsigned expression in ‘>= 0’ is always true [-Werror=type-limits]
  cargo:warning= 1727 |             0 <= count  && count + offset <= this->size() );
  cargo:warning=      |             ~~^~~~~~~~
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp:579:50: note: in definition of macro ‘Expects’
  cargo:warning=  579 | # define Expects( x )  ::gsl::fail_fast_assert( (x) )
  cargo:warning=      |                                                  ^
  cargo:warning=cc1plus: all warnings being treated as errors
  exit status: 1
  running: "c++" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-m64" "-static" "-I" "lib/starkware-crypto/src" "-Wall" "-Wextra" "-std=c++17" "-Werror" "-Wall" "-Wextra" "-fno-strict-aliasing" "-fPIC" "-o" "[...]/target/debug/build/starkware-crypto-sys-e7dc842a2c877560/out/lib/starkware-crypto/src/starkware/crypto/pedersen_hash.o" "-c" "lib/starkware-crypto/src/starkware/crypto/pedersen_hash.cc"
  cargo:warning=In file included from lib/starkware-crypto/src/starkware/crypto/pedersen_hash.h:4,
  cargo:warning=                 from lib/starkware-crypto/src/starkware/crypto/pedersen_hash.cc:1:
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp: In instantiation of ‘constexpr gsl::span<T> gsl::span<T>::subspan(gsl::span<T>::index_type) const [with T = const starkware::EcPoint<starkware::PrimeFieldElement>; gsl::span<T>::index_type = long unsigned int]’:
  cargo:warning=lib/starkware-crypto/src/starkware/crypto/pedersen_hash.cc:42:67:   required from here
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp:1719:20: error: comparison of unsigned expression in ‘>= 0’ is always true [-Werror=type-limits]
  cargo:warning= 1719 |         Expects( 0 <= offset && offset <= this->size() );
  cargo:warning=      |                  ~~^~~~~~~~~
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp:579:50: note: in definition of macro ‘Expects’
  cargo:warning=  579 | # define Expects( x )  ::gsl::fail_fast_assert( (x) )
  cargo:warning=      |                                                  ^
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp: In instantiation of ‘constexpr gsl::span<T> gsl::span<T>::subspan(gsl::span<T>::index_type, gsl::span<T>::index_type) const [with T = const starkware::EcPoint<starkware::PrimeFieldElement>; gsl::span<T>::index_type = long unsigned int]’:
  cargo:warning=lib/starkware-crypto/src/starkware/crypto/pedersen_hash.cc:45:57:   required from here
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp:1726:15: error: comparison of unsigned expression in ‘>= 0’ is always true [-Werror=type-limits]
  cargo:warning= 1726 |             0 <= offset && offset <= this->size() &&
  cargo:warning=      |             ~~^~~~~~~~~
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp:579:50: note: in definition of macro ‘Expects’
  cargo:warning=  579 | # define Expects( x )  ::gsl::fail_fast_assert( (x) )
  cargo:warning=      |                                                  ^
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp:1727:15: error: comparison of unsigned expression in ‘>= 0’ is always true [-Werror=type-limits]
  cargo:warning= 1727 |             0 <= count  && count + offset <= this->size() );
  cargo:warning=      |             ~~^~~~~~~~
  cargo:warning=lib/starkware-crypto/src/third_party/gsl/gsl-lite.hpp:579:50: note: in definition of macro ‘Expects’
  cargo:warning=  579 | # define Expects( x )  ::gsl::fail_fast_assert( (x) )
  cargo:warning=      |                                                  ^
  cargo:warning=cc1plus: all warnings being treated as errors
  exit status: 1
  exit status: 0

  --- stderr


  error occurred: Command "c++" "-O3" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-m64" "-static" "-I" "lib/starkware-crypto/src" "-Wall" "-Wextra" "-std=c++17" "-Werror" "-Wall" "-Wextra" "-fno-strict-aliasing" "-fPIC" "-o" "[...]/openlimits-crypto-com/target/debug/build/starkware-crypto-sys-e7dc842a2c877560/out/lib/starkware-crypto/src/starkware/crypto/ffi/utils.o" "-c" "lib/starkware-crypto/src/starkware/crypto/ffi/utils.cc" with args "c++" did not execute successfully (status code exit status: 1).

After the macOS build issue was resolved, it's discovered that the library also fails to build on Windows, for both x86_64-pc-windows-gnu and x86_64-pc-windows-msvc targets.

Target triples to fix:

• [ ] x86_64-pc-windows-msvc
• [x] x86_64-pc-windows-gnu