crypto

CGO Binding Guide

Software Requirement

• CMAKE (Version at least 3.10.2)
  • Install on MacOS:

    brew install cmake
  

  • Install on Linux:

    wget http://www.cmake.org/files/v3.11/cmake-3.11.0-Linux-x86_64.tar.gz
    tar -xvf cmake-3.11.0-Linux-x86_64.tar.gz
    cd cmake-3.11.0-Linux-x86_64
    sudo  cp -r bin /usr/
    sudo  cp -r share /usr/
    sudo  cp -r doc /usr/share/
    sudo  cp -r man /usr/share/
  

CGO Development Guide

1. Declare the use of CGO and files binding

 //#include "lib/ecckey.h"
 //#include "lib/ecckey.c"
 import “C”

• Just use import “C” to start using CGO. It comes with Go. No need to install any other library.
• “//” is not a regular comment sign in CGO declaration. It starts a directive and will not be ignored by compiler.
• import “C” should be immediately preceded by //#include XXX or //#cgo.
• //#include XXX indicates the use of c header files and c source code files
• //#cgo is used to indicate the use of flags like CFLAGS, CPPFLAGS, CXXFLAGS, FFLAGS and LDFLAGS etc. to tweak the behavior of C, C++ compiler. The //#cgo directive can also include a list of build constraints that limit the os and architecture. It can also indicate the C library, so that you do not have to include the source code files.

//#cgo darwin LDFLAGS: -L${SRCDIR}/lib -lsect283k1_macos
//#cgo linux LDFLAGS: -L${SRCDIR}/lib -lsect283k1_ubuntu

This indicates the library sect283k1_macos will be used in Mac platform,
sect283k1_ubuntu in Linux platform. Both of them are located in directory ${SRCDIR}/lib. -L specifies the location of the library.

package crypto

//#include "lib/ecckey.h"
//#include "lib/ecdsa.h"
//#include "lib/sect283k1.h"
//#cgo darwin LDFLAGS: -L${SRCDIR}/lib -lsect283k1_macos
//#cgo linux LDFLAGS: -L${SRCDIR}/lib -lsect283k1_ubuntu
import "C"
import (
    "bytes"
    "encoding/binary"
    "errors"

    "github.com/iotexproject/iotex-core/pkg/enc"
    "github.com/iotexproject/go-pkgs/crypto"
)

2. Go references C

• Go calls C functions: Just call C.function_name_in_C(…)
• Go uses any type in C: C.type_in_C, like C.uint32_t, C.ec_point_aff_twist
• Pointer pass in CGO is the same as in go
• Go can access any fields of C struct in the same way in Go struct

  // C header file
  
  typedef struct
  {
      uint32_t d[9];         
      ec283_point_lambda_aff Q;
  }ec283_key_pair;
  
  void keypair_generation(ec283_key_pair *key);
  
  // Go file
  func (c *ec283) NewKeyPair() (keypair.PublicKey, keypair.PrivateKey, error) {
      var kp C.ec283_key_pair
      C.keypair_generation(&kp)
      pubKey := kp.Q
      privKey := kp.d
      ...
  }
  

• Primitive type conversion: Just cast the type from C type to Go or from Go to C

  // privkey has go type elements, privkeySer has C type elements
  for i := 0; i < privkeySize; i++ {
      privkeySer[i] = (C.uint32_t)(privkey[i])
  }
  
  // sigSer has C type elements, sig has go type elements
  for i, x := range sigSer {
      sig[i] = (uint32)(x)
  }
  

3. Go passes an array to C

Just pass the first element of the slice as a pointer, and cast the pointer to C corresponding
type. The slice in Go is a struct including header and elements, so &slice won’t give you the
first element.

// C file
// d and sig are arrays

uint32_t BLS_sign(uint32_t *d, const uint8_t *msg, uint64_t mlen, uint32_t *sig);

// Go file
// Sign signs a message given a private key

func (b *bls) Sign(privkey []uint32, msg []byte) (bool, []byte, error) {
    var sigSer [sigSize]C.uint32_t
    msgString := string(msg[:])

    if ok := C.BLS_sign((*C.uint32_t)(unsafe.Pointer(&privkey[0])), (*C.uint8_t)(&msg[0]), (C.uint64_t)(len(msgString)), &sigSer[0]); ok == 1 {
        sig, err := b.signatureSerialization(sigSer)
        if err != nil {
            return false, []byte{}, err
        }
        return true, sig, nil
    }
    return false, []byte{}, ErrSignError
}

4. Use struct defined in C

• Construct a corresponding struct in Go that mimics the same struct in C.
• (Recommended) Write function to serialize and deserialize the struct, so the information can be exposed outside independently.

  // C header file and C struct definition
  typedef struct
  {
      uint32_t x[9];     
      uint32_t l[9];     
  }ec283_point_lambda_aff;
  
  // Go file
  
  func (*ec283) publicKeySerialization(pubKey C.ec283_point_lambda_aff) (keypair.PublicKey, error) {
      var xl [18]uint32
      for i := 0; i < 9; i++ {
          xl[i] = (uint32)(pubKey.x[i])
          xl[i+9] = (uint32)(pubKey.l[i])
      }
      buf := new(bytes.Buffer)
      err := binary.Write(buf, enc.MachineEndian, xl)
      if err != nil {
          return keypair.ZeroPublicKey, err
      }
      return keypair.BytesToPublicKey(buf.Bytes())
  }
  
  func (*ec283) publicKeyDeserialization(pubKey keypair.PublicKey) (C.ec283_point_lambda_aff, error) {
      var xl [18]uint32
      var pub C.ec283_point_lambda_aff
      rbuf := bytes.NewReader(pubKey[:])
      err := binary.Read(rbuf, enc.MachineEndian, &xl)
      if err != nil {
          return pub, err
      }
      for i := 0; i < 9; i++ {
          pub.x[i] = (C.uint32_t)(xl[i])
          pub.l[i] = (C.uint32_t)(xl[i+9])
      }
      return pub, nil
  }