path: root/crypto/secp256k1/secp256.go

// Copyright 2015 Jeffrey Wilcke, Felix Lange, Gustav Simonsson. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be found in
// the LICENSE file.

// Package secp256k1 wraps the bitcoin secp256k1 C library.
package secp256k1

/*
#cgo CFLAGS: -I./libsecp256k1
#cgo CFLAGS: -I./libsecp256k1/src/
#define USE_NUM_NONE
#define USE_FIELD_10X26
#define USE_FIELD_INV_BUILTIN
#define USE_SCALAR_8X32
#define USE_SCALAR_INV_BUILTIN
#define NDEBUG
#include "./libsecp256k1/src/secp256k1.c"
#include "./libsecp256k1/src/modules/recovery/main_impl.h"
#include "ext.h"

typedef void (*callbackFunc) (const char* msg, void* data);
extern void secp256k1GoPanicIllegal(const char* msg, void* data);
extern void secp256k1GoPanicError(const char* msg, void* data);
*/
import "C"

import (
    "errors"
    "math/big"
    "unsafe"
)

var context *C.secp256k1_context

func init() {
    // around 20 ms on a modern CPU.
    context = C.secp256k1_context_create_sign_verify()
    C.secp256k1_context_set_illegal_callback(context, C.callbackFunc(C.secp256k1GoPanicIllegal), nil)
    C.secp256k1_context_set_error_callback(context, C.callbackFunc(C.secp256k1GoPanicError), nil)
}

var (
    ErrInvalidMsgLen       = errors.New("invalid message length, need 32 bytes")
    ErrInvalidSignatureLen = errors.New("invalid signature length")
    ErrInvalidRecoveryID   = errors.New("invalid signature recovery id")
    ErrInvalidKey          = errors.New("invalid private key")
    ErrInvalidPubkey       = errors.New("invalid public key")
    ErrSignFailed          = errors.New("signing failed")
    ErrRecoverFailed       = errors.New("recovery failed")
)

// Sign creates a recoverable ECDSA signature.
// The produced signature is in the 65-byte [R || S || V] format where V is 0 or 1.
//
// The caller is responsible for ensuring that msg cannot be chosen
// directly by an attacker. It is usually preferable to use a cryptographic
// hash function on any input before handing it to this function.
func Sign(msg []byte, seckey []byte) ([]byte, error) {
    if len(msg) != 32 {
        return nil, ErrInvalidMsgLen
    }
    if len(seckey) != 32 {
        return nil, ErrInvalidKey
    }
    seckeydata := (*C.uchar)(unsafe.Pointer(&seckey[0]))
    if C.secp256k1_ec_seckey_verify(context, seckeydata) != 1 {
        return nil, ErrInvalidKey
    }

    var (
        msgdata   = (*C.uchar)(unsafe.Pointer(&msg[0]))
        noncefunc = C.secp256k1_nonce_function_rfc6979
        sigstruct C.secp256k1_ecdsa_recoverable_signature
    )
    if C.secp256k1_ecdsa_sign_recoverable(context, &sigstruct, msgdata, seckeydata, noncefunc, nil) == 0 {
        return nil, ErrSignFailed
    }

    var (
        sig     = make([]byte, 65)
        sigdata = (*C.uchar)(unsafe.Pointer(&sig[0]))
        recid   C.int
    )
    C.secp256k1_ecdsa_recoverable_signature_serialize_compact(context, sigdata, &recid, &sigstruct)
    sig[64] = byte(recid) // add back recid to get 65 bytes sig
    return sig, nil
}

// RecoverPubkey returns the public key of the signer.
// msg must be the 32-byte hash of the message to be signed.
// sig must be a 65-byte compact ECDSA signature containing the
// recovery id as the last element.
func RecoverPubkey(msg []byte, sig []byte) ([]byte, error) {
    if len(msg) != 32 {
        return nil, ErrInvalidMsgLen
    }
    if err := checkSignature(sig); err != nil {
        return nil, err
    }

    var (
        pubkey  = make([]byte, 65)
        sigdata = (*C.uchar)(unsafe.Pointer(&sig[0]))
        msgdata = (*C.uchar)(unsafe.Pointer(&msg[0]))
    )
    if C.secp256k1_ext_ecdsa_recover(context, (*C.uchar)(unsafe.Pointer(&pubkey[0])), sigdata, msgdata) == 0 {
        return nil, ErrRecoverFailed
    }
    return pubkey, nil
}

// VerifySignature checks that the given pubkey created signature over message.
// The signature should be in [R || S] format.
func VerifySignature(pubkey, msg, signature []byte) bool {
    if len(msg) != 32 || len(signature) != 64 || len(pubkey) == 0 {
        return false
    }
    sigdata := (*C.uchar)(unsafe.Pointer(&signature[0]))
    msgdata := (*C.uchar)(unsafe.Pointer(&msg[0]))
    keydata := (*C.uchar)(unsafe.Pointer(&pubkey[0]))
    return C.secp256k1_ext_ecdsa_verify(context, sigdata, msgdata, keydata, C.size_t(len(pubkey))) != 0
}

// DecompressPubkey parses a public key in the 33-byte compressed format.
// It returns non-nil coordinates if the public key is valid.
func DecompressPubkey(pubkey []byte) (x, y *big.Int) {
    if len(pubkey) != 33 {
        return nil, nil
    }
    var (
        pubkeydata = (*C.uchar)(unsafe.Pointer(&pubkey[0]))
        pubkeylen  = C.size_t(len(pubkey))
        out        = make([]byte, 65)
        outdata    = (*C.uchar)(unsafe.Pointer(&out[0]))
        outlen     = C.size_t(len(out))
    )
    if C.secp256k1_ext_reencode_pubkey(context, outdata, outlen, pubkeydata, pubkeylen) == 0 {
        return nil, nil
    }
    return new(big.Int).SetBytes(out[1:33]), new(big.Int).SetBytes(out[33:])
}

// CompressPubkey encodes a public key to 33-byte compressed format.
func CompressPubkey(x, y *big.Int) []byte {
    var (
        pubkey     = S256().Marshal(x, y)
        pubkeydata = (*C.uchar)(unsafe.Pointer(&pubkey[0]))
        pubkeylen  = C.size_t(len(pubkey))
        out        = make([]byte, 33)
        outdata    = (*C.uchar)(unsafe.Pointer(&out[0]))
        outlen     = C.size_t(len(out))
    )
    if C.secp256k1_ext_reencode_pubkey(context, outdata, outlen, pubkeydata, pubkeylen) == 0 {
        panic("libsecp256k1 error")
    }
    return out
}

func checkSignature(sig []byte) error {
    if len(sig) != 65 {
        return ErrInvalidSignatureLen
    }
    if sig[64] >= 4 {
        return ErrInvalidRecoveryID
    }
    return nil
}