code: 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; }
      }
    }
  }
docs: Creates a signer given the specified schema and key.