// Package diffiehellman povides Diffie-Hellman-Merkle key exchange.
package diffiehellman
import (
	"crypto/rand"
	"log"
	"math/big"
)
// PrivateKey returns a number in range(1, p).
func PrivateKey(p *big.Int) *big.Int {
	ret := new(big.Int)
	for {
		var err error
		ret, err = rand.Int(rand.Reader, p)
		if err != nil {
			log.Fatalf("error generating random number")
		}
		if ret.Cmp(big.NewInt(1)) > 0 && ret.Cmp(p) < 0 {
			break
		}
	}
	return ret
}
// PublicKey return a public key.
func PublicKey(private, p *big.Int, g int64) *big.Int {
	// key = g**private mod p
	key := new(big.Int)
	key.Exp(big.NewInt(g), private, p)
	return key
}
// NewPair returns a key pair (public+private)
func NewPair(p *big.Int, g int64) (private, public *big.Int) {
	private = PrivateKey(p)
	public = PublicKey(private, p, g)
	return private, public
}
// SecretKey returns the secret key.
func SecretKey(private1, public2, p *big.Int) *big.Int {
	// s = public2 ** private1 mod p == public1 ** private2 mod p
	s := new(big.Int)
	s.Exp(public2, private1, p)
	return s
}