you could easily define hash functions that work over A..Z if you wanted; there's nothing special about that. The misleading part is about the reduction to a u

you could easily define hash functions that work over A..Z if you wanted; there's nothing special about that. The misleading part is about the reduction to a unique fixed length string; that's not possible unless the input domain is equal to (or smaller than) the output domain (and even then it's not necessary). Any other function is guaranteed to have collisions.