In choosing which algorithm to use, there are several factors to consider.

Cryptography, the invention of new algorithms and of ways to break the codes they create, is a branch of mathematics that is constantly advancing. Attempts to crack a particular algorithm are often undertaken by serious researchers anxious to confirm whether the algorithm is really secure.

It may be possible for an expert cryptographer to find a mathematical way of breaking an algorithm, or it may be vulnerable to a "brute force" attack - just trying different keys until the correct one is found.

Just because an algorithm has been cracked in the past does not necessarily mean you must not use it - you should consider whether your data is valuable enough to tempt a hacker to devote the resources that have been shown to be needed to crack it.

Two problems with all symmetric algorithms are that the key has to be known to both sender and receiver, and so it could be intercepted when being sent from one to the other; and that if the same key is used for all your messages, any eavesdropper who finds out the key can decrypt all your messages.