how-to-override-rand-reader-behavior-golang

crypto-rand-override-test.go

package main

package endpoint

import (
	"encoding/base64"
	"io"
	"testing"

	"crypto/rand"

	"github.com/stretchr/testify/require"
)

// GenerateNonce creates a random nonce of a fixed size
func GenerateNonce() ([]byte, error) {
	nonce := make([]byte, standardGcmNonceSize)
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, err
	}
	return []byte(base64.StdEncoding.EncodeToString(nonce)), nil
}

func TestGenerateNonceSuccess(t *testing.T) {
	nonce, err := GenerateNonce()
	require.NoError(t, err)
	require.NotEmpty(t, nonce)

	// Test nonce length
	decodedNonce, err := base64.StdEncoding.DecodeString(string(nonce))
	require.NoError(t, err)
	require.Equal(t, standardGcmNonceSize, len(decodedNonce))
}

func TestGenerateNonceError(t *testing.T) {
	// Save the original rand.Reader
	originalRandReader := rand.Reader
	defer func() { rand.Reader = originalRandReader }()

	// Replace rand.Reader with a faulty reader
	rand.Reader = &faultyReader{}

	nonce, err := GenerateNonce()
	require.Error(t, err)
	require.Nil(t, nonce)
}

type faultyReader struct{}

func (f *faultyReader) Read(p []byte) (n int, err error) {
	return 0, io.ErrUnexpectedEOF
}