signutil

signutil is a program that demonstrates a use case for the signkey package.

Sample use case

Your application reads in a configuration file on local disk on startup. You do not wish the end user to modify this configuration file.

Create secret

Create a secret key on your computer. Only you should have access to this secret key.

# redirects output to file contoso.key
signutil -n user > contoso.key

The public key is embedded in the secret itself. You do not need to secure the public key.

signutil -s contoso.key > contoso.pub

Sign Configuration

Create your configuration file. To illustrate:

echo "show_trial_nag_screen = true" >> myconfig.ini
echo "force_update = true" >> myconfig.ini

Now sign your configuration file:

signutil -s contoso.key -f myconfig.ini > myconfig.ini.sig

Verify Configuration

You hardcode the content of contoso.pub into your application, then distribute the application, myconfig.ini and myconfig.ini.sig to your end user. Your application should implement something similar to the code below:

package main

import (
    "io/ioutil"
    "github.com/imacks/signkey"
)

const PUBLIC_KEY = "..."

func main() {
    // application will not work without myconfig.ini or myconfig.ini.sig
    config := ioutil.ReadAll("myconfig.ini")
    signature := ioutil.ReadAll("myconfig.ini.sig")
    // this is equivilent to: signutil -p contoso.pub -g myconfig.ini.sig -f myconfig.ini
    pubkey, _ := signkey.FromPublicKey(PUBLIC_KEY)
    if err := pubkey.Verify(config, signature); err != nil {
        panic("cannot verify configuration")
    }
    // ...
}

If your end user modify myconfig.ini or myconfig.ini.sig, the application will not be able to start.

Caveats

There are 2 ways to defeat this: modify the application to bypass the verification logic altogether, or change the value of PUBLIC_KEY in the application to something else.

Countermeasures may include code obfuscation techniques, trusted crypto processor such as TPM, and permission settings on the Operating System level.