I have been reading up on various password hashing schemes, for example the SHA-based Crypt proposed by Ulrich Drepper and used in some Linux versions.
A recurring theme here is to 'strengthen' or 'stretch' the hashing function, by using many iterations of the same hash. The runtime of the hashing function can also be increased by concatenating the
password together several thousand times, to form a much longer string, and then hash the string once. However, this method seems not to be favored.
Why is that -- why does it appear that concatenating the
password together many times isn't a viable way of stretching the hash function?