Question

What characteristics would make an encryption absolutely unbreakable? what characteristics would make an encryption impractical to break?

Answer 1

There's no such things as an unbreakable encryption. If an encryption is devised, it will eventually be broken/decrypted. However, there are certain measures or characteristics that can be put in place to prolong decryption like using a one-time pad encryption. This means the key is used only once; using a key once is a pretty good idea since a strong algorithm will be of no use when the key becomes known. This characteristics ensures that a new key is required for decryption on successive attempt. You could also consider adding a long sequence of characters or adding an extra layer of security by upgrading to 256 or 512 bit encryption, also use salting and multiple algorithms.