Sign a message and verify signature with go using PKCS1. Compatible with java (SHA256withRSA)

sign_and_verify_test.go

package main

import (
	"crypto"
	"crypto/rand"
	"crypto/rsa"
	"crypto/sha256"
	"encoding/base64"
	"testing"
)

func Test_sign_a_message_and_verify_signature(t *testing.T) {

	// sign part
	privateKey, publicKey, err := generateKeyPair(512)

	if err != nil {
		t.Fatalf("could not generate keypair: %s", err.Error())
	}

	data := []byte("Eg vil ikkje vaska opp!")

	digest := sha256.Sum256(data)

	signature, signErr := rsa.SignPKCS1v15(rand.Reader, privateKey, crypto.SHA256, digest[:])

	if signErr != nil {
		t.Errorf("Could not sign message:%s", signErr.Error())
	}

	// just to check that we can survive to and from b64
	b64sig := base64.StdEncoding.EncodeToString(signature)

	decodedSignature, _ := base64.StdEncoding.DecodeString(b64sig)

	// verify part

	verifyErr := rsa.VerifyPKCS1v15(publicKey, crypto.SHA256, digest[:], decodedSignature)

	if verifyErr != nil {
		t.Errorf("Verification failed: %s", verifyErr)
	}
}

// GenerateKeyPair generates a new key pair
func generateKeyPair(bits int) (*rsa.PrivateKey, *rsa.PublicKey, error) {
	privkey, err := rsa.GenerateKey(rand.Reader, bits)
	if err != nil {
		return nil, nil, err
	}
	return privkey, &privkey.PublicKey, nil
}

Thanks this helped a lot!