Some hash functions do this, e.g. bcrypt and scrypt. Theoretically, their output is no more secure than faster algorithms like SHA; but it's less practical to b

Some hash functions do this, e.g. bcrypt and scrypt. Theoretically, their output is no more secure than faster algorithms like SHA; but it's less practical to brute-force lots of guesses. Hashcash (as found in Bitcoin) is another example, albeit more egregious.