hashing always produces a hash that is of a fixed short length, regardless of the length of the original plaintext. The second difference is that encryption is reversible, but hashing is not. Given the decryption key and the algorithm, ciphertext can be decryption back into the original plaintext. In contrast, it not possible to transform a hash back into the original plaintext, because hashing throws away information. For example, using MD5 to hash a 20,000-word document produces a 128-bit hash. There is no way to convert the short string of bits back into the original document.