Example #1 for "How to make a user friendly Ethereum dApp"

EthedIn.sol

contract EthedIn {
  // Digest describing the data the user signs.
  // Needs to match what is passed to Metamask.
  bytes32 public constant delegationHash =
    keccak256("string Action", "address Address");

  // Create account for a user using the signature they provided
  //
  //   * Only approved transaction delegators are allowed to call this
  //   * _sender should be the address included in the signature as well
  //     as the address that would be submitting this transaction
  //     if it were not delegated
  function createAccountFor(
    bytes _delegationSig,
    address _sender
  ) public onlyTxDelegator {
    // Recreate the digest the user signed
    bytes32 _delegationDigest =
      keccak256(delegationHash, keccak256("Create Account", _sender));

    // Recover the signer from the digest.
    address _signer = recoverSigner(_delegationDigest, _delegationSig);
    // Check that it matches the claimed sender
    require(_sender == _signer);

    // Call the actual create account function
    createAccountForUser(_sender);
  }

  // Actually create the account
  function createAccountForUser(address _address) private { /* ... */ }
}