Creates a signer given the specified schema and key.
function createSigner(signatureScheme, signatureKey) {
  if (signatureKey) {
    // Create a SHA256-based signer that generates and validates signatures
    signatureScheme = signatureScheme || 'sha256';
    if (signatureScheme.indexOf('hmac-') === 0) {
      signatureScheme = signatureScheme.substr(5);
    }
    return {
      sign: function(values) {
        return computeSignature(values, signatureScheme, signatureKey);
      },
      validate: function(signature, values) {
        return signature === computeSignature(values, signatureScheme, signatureKey);
      }
    }
  }
  else {
    // Create a no-op signer
    return {
      sign: function() { return ''; },
      validate: function() { return true; }
    }
  }
}