public String getNonce() {
        return nonce;
    }
A 'nonce' is a single use, cryptographically random string,
and it's use here is to validate the authenticity of a token.
<p>
A nonce is used in the authentication flow - the hash is included in the original AuthenticationRequest
that Stroom makes to the Authentication Service. When Stroom subsequently receives an access code
it retrieves the ID token from the Authentication Service and expects to see
the hash of the nonce on the token. It can then compare the hashes.

@return The nonce string.
stroom-security/stroom-security-common-impl/src/main/java/stroom/security/common/impl/AuthenticationState.java