Golang : Example for DSA(Digital Signature Algorithm) package functions

There are not many examples or tutorials out there on how to fully utilize the crypto/dsa package. In this tutorial, we will learn how to utilize the crypto/dsa functions to digitally sign our message and verify the signature.

This source code below will :

• generate a private key
• extract the public key from the generated private key
• use the private key to sign
• use the public key to verify the signature

dsaexample.go

 package main

 import (
  "crypto/rand"
  "crypto/dsa"
  "crypto/md5"
  "hash"
  "fmt"
  "os"
  "io"
  "math/big"
 )

 func main() {

 params := new(dsa.Parameters)

 // see http://golang.org/pkg/crypto/dsa/#ParameterSizes
 if err := dsa.GenerateParameters(params, rand.Reader, dsa.L1024N160); err != nil {
 fmt.Println(err)
 os.Exit(1)
 }

 privatekey := new(dsa.PrivateKey)
 privatekey.PublicKey.Parameters = *params
 dsa.GenerateKey(privatekey, rand.Reader) // this generates a public & private key pair

 var pubkey dsa.PublicKey
 pubkey = privatekey.PublicKey

 fmt.Println("Private Key :")
 fmt.Printf("%x \n", privatekey)

 fmt.Println("Public Key :")
 fmt.Printf("%x \n",pubkey)


 // Sign
 var h hash.Hash
 h = md5.New()
 r := big.NewInt(0)
 s := big.NewInt(0)

 io.WriteString(h, "This is the message to be signed and verified!")
 signhash := h.Sum(nil)

 r, s, err := dsa.Sign(rand.Reader, privatekey, signhash)
 if err != nil {
 fmt.Println(err)
 }

 signature := r.Bytes()
 signature = append(signature, s.Bytes()...)

 fmt.Printf("Signature : %x\n", signature)

 // Verify
 verifystatus := dsa.Verify(&pubkey, signhash, r, s)
 fmt.Println(verifystatus) // should be true

 // we add additional data to change the signhash
 io.WriteString(h, "This message is NOT to be signed and verified!")
 signhash = h.Sum(nil)

 verifystatus = dsa.Verify(&pubkey, signhash, r, s)
 fmt.Println(verifystatus) // should be false
 }

An example output(note : public, private keys and signature values will be different each time the code is executed) :

Private Key :

{{{de735666a2220833b0b07f88c5ff30434e4af53f5e53c0e397057902a.....}

Public Key : {{de735666a2220833b0b07f88c5ff30434e4af53f5e53c0e397057902a......}

Signature :

75483cc98f4587b9ab4e8336b873e8eddd2fb41b594db267ce4bc09285ec15d63d17f6ec82989cd3

true

false