package codec import ( "crypto/rand" "errors" "math/big" ) // see https://www.zhihu.com/question/29383090/answer/70435297 // see https://www.ietf.org/rfc/rfc3526.txt // 2048-bit MODP Group var ( // ErrInvalidPriKey indicates the invalid private key. ErrInvalidPriKey = errors.New("invalid private key") // ErrInvalidPubKey indicates the invalid public key. ErrInvalidPubKey = errors.New("invalid public key") // ErrPubKeyOutOfBound indicates the public key is out of bound. ErrPubKeyOutOfBound = errors.New("public key out of bound") p, _ = new(big.Int).SetString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g, _ = new(big.Int).SetString("2", 16) zero = big.NewInt(0) ) // DhKey defines the Diffie Hellman key. type DhKey struct { PriKey *big.Int PubKey *big.Int } // ComputeKey returns a key from public key and private key. func ComputeKey(pubKey, priKey *big.Int) (*big.Int, error) { if pubKey == nil { return nil, ErrInvalidPubKey } if pubKey.Sign() <= 0 && p.Cmp(pubKey) <= 0 { return nil, ErrPubKeyOutOfBound } if priKey == nil { return nil, ErrInvalidPriKey } return new(big.Int).Exp(pubKey, priKey, p), nil } // GenerateKey returns a Diffie Hellman key. func GenerateKey() (*DhKey, error) { var err error var x *big.Int for { x, err = rand.Int(rand.Reader, p) if err != nil { return nil, err } if zero.Cmp(x) < 0 { break } } key := new(DhKey) key.PriKey = x key.PubKey = new(big.Int).Exp(g, x, p) return key, nil } // NewPublicKey returns a public key from the given bytes. func NewPublicKey(bs []byte) *big.Int { return new(big.Int).SetBytes(bs) } // Bytes returns public key bytes. func (k *DhKey) Bytes() []byte { if k.PubKey == nil { return nil } byteLen := (p.BitLen() + 7) >> 3 ret := make([]byte, byteLen) copyWithLeftPad(ret, k.PubKey.Bytes()) return ret } func copyWithLeftPad(dst, src []byte) { padBytes := len(dst) - len(src) for i := 0; i < padBytes; i++ { dst[i] = 0 } copy(dst[padBytes:], src) }