elgamal_div.go

package main

import (
    "crypto/rand"
    "math/big"
    "io"
    "fmt"
    "os"

)

func get_g_p(size string)(g, p *big.Int) {

    if (size=="512") {
        p,_ = new(big.Int).SetString("FCA682CE8E12CABA26EFCCF7110E526DB078B05EDECBCD1EB4A208F3AE1617AE01F35B91A47E6DF63413C5E12ED0899BCD132ACD50D99151BDC43EE737592E17",16)

        g,_ = new(big.Int).SetString("678471B27A9CF44EE91A49C5147DB1A9AAF244F05A434D6486931D2D14271B9E35030B71FD73DA179069B32E2935630E1C2062354D0DA20A6C416E50BE794CA4",16)
    
    }
    if (size=="768") {
        p,_ = new(big.Int).SetString("E9E642599D355F37C97FFD3567120B8E25C9CD43E927B3A9670FBEC5D890141922D2C3B3AD2480093799869D1E846AAB49FAB0AD26D2CE6A22219D470BCE7D777D4A21FBE9C270B57F607002F3CEF8393694CF45EE3688C11A8C56AB127A3DAF",16)

        g,_ = new(big.Int).SetString("30470AD5A005FB14CE2D9DCD87E38BC7D1B1C5FACBAECBE95F190AA7A31D23C4DBBCBE06174544401A5B2C020965D8C2BD2171D3668445771F74BA084D2029D83C1C158547F3A9F1A2715BE23D51AE4D3E5A1F6A7064F316933A346D3F529252",16)
    
    }
    if (size=="1024") {
        p,_ = new(big.Int).SetString("FD7F53811D75122952DF4A9C2EECE4E7F611B7523CEF4400C31E3F80B6512669455D402251FB593D8D58FABFC5F5BA30F6CB9B556CD7813B801D346FF26660B76B9950A5A49F9FE8047B1022C24FBBA9D7FEB7C61BF83B57E7C6A8A6150F04FB83F6D3C51EC3023554135A169132F675F3AE2B61D72AEFF22203199DD14801C7",16)

        g,_ = new(big.Int).SetString("F7E1A085D69B3DDECBBCAB5C36B857B97994AFBBFA3AEA82F9574C0B3D0782675159578EBAD4594FE67107108180B449167123E84C281613B7CF09328CC8A6E13C167A8B547C8D28E0A3AE1E2BB3A675916EA37F0BFA213562F1FB627A01243BCCA4F1BEA8519089A883DFE15AE59F06928B665E807B552564014C3BFECF492A",16)
    
    }
    return 
}

type PublicKey struct {
    G, P, Y *big.Int
}

type PrivateKey struct {
    PublicKey
    X *big.Int
}


func Encrypt(random io.Reader, pub *PublicKey, msg string) (a, b *big.Int, err error) {


    k, _ := rand.Int(random, pub.P)

    m,_ := new(big.Int).SetString(msg, 10)

    a = new(big.Int).Exp(pub.G, k, pub.P)
    s := new(big.Int).Exp(pub.Y, k, pub.P)
    b = s.Mul(s, m)
    b.Mod(b, pub.P)

    return
}

func Decrypt(priv *PrivateKey, a, b *big.Int) (msg []byte, err error) {
    s := new(big.Int).Exp(a, priv.X, priv.P)
    s.ModInverse(s, priv.P)
    s.Mul(s, b)
    s.Mod(s, priv.P)
    em := s.Bytes()

    
    return em, nil
}

func valint(a []byte)(i int) {
    if (len(a)==1) { i=int(a[0])}
    if (len(a)==2) { i=(int(a[0])<<8)+ int(a[1])}
    if (len(a)==3) { i=(int(a[0])<<16)+ int(a[1])<<8+int(a[2])}
    if (len(a)==4) { i=(int(a[0])<<24)+ int(a[1])<<16+int(a[2])<<8+int(a[3])}
    return
}

func main() {


    x:="40"
    plen:="512"
    val1:="3"
    val2:="4"

    argCount := len(os.Args[1:])

    if (argCount>0) {val1 = string(os.Args[1])}
    if (argCount>1) {val2 = string(os.Args[2])}
    if (argCount>2) {x = string(os.Args[3])}
    if (argCount>3) { plen =string(os.Args[4]) }

    fmt.Printf("Prime number size: %s\n\n",plen)

    xval,_ := new(big.Int).SetString(x, 10)

    g,p:=get_g_p(plen)

    priv := &PrivateKey{
        PublicKey: PublicKey{
            G: g,
            P: p,
        },
        X: xval,
    }

    priv.Y = new(big.Int).Exp(priv.G, priv.X, priv.P)


    a1, b1, _ := Encrypt(rand.Reader, &priv.PublicKey, val1)
    a2, b2, _ := Encrypt(rand.Reader, &priv.PublicKey, val2)

    a2=a2.ModInverse(a2, priv.P)
    b2=b2.ModInverse(b2, priv.P)

    message2, _ := Decrypt(priv, a1.Mul(a1,a2), b1.Mul(b1,b2) )

    fmt.Printf("====Values (val1=%s and val2=%s)\n",val1,val2)
    fmt.Printf("====Private key (x):\nX=%d",priv.X)
    fmt.Printf("\n\n====Public key (Y,G,P):\nY=%d\nG=%d\nP=%d",priv.Y,priv.PublicKey.G,priv.PublicKey.P)


    fmt.Printf("\n\n====Cipher (a1=%s)\n\n(b1=%s): ",a1,b1)
    fmt.Printf("\n\n====Decrypted: %d",valint(message2))

}