natsukashi wrote:so do my substitues add or remove entropy for my 20+ symbol password? I don't get if it's the lenght that creates the entropy, or if it's simply that the substitutes somehow break the password and make it easier to crack. >_>
Entropy in this context is a term used in
communication theory to describe the complexity of a signal or piece of information. Basically, that ends up meaning how predictable it is, and consequently how compressible it is. The more predictable it is, the greater the compression possibility, and the lower the entropy. In practice, this often describes how close a string of characters comes to being completely random, because, of course, a completely random string is completely unpredictable. For cryptanalysis/cryptography,
this translates into a measure of how difficult a password is to guess. In a password, you want high entropy because it makes it more difficult to crack.
To (finally) answer your question, substitutions add entropy (more or less), and length does too, but at a much higher rate. We're basically looking at the space of possibilities. Let me try a super simple example.
Let's suppose there're 100 English words that you can spell with two letters. (According
this Scrabble link, there are about that many. Includes a bunch of weird things some people might not categorize as real "words", but oh well.) If the password cracker knows that your password is an English word that's two letters long, it will only take 100 guesses to figure it out. If you add in numbers for vowel substitutions (e.g. 1 for I, 3 for E, 0 for O, 4 for A), that increases the number of possibilities: instead of just "on, no, in, me" etc., we get "on, 0n, no, n0, in, 1n, me, m3". For each word with a vowel that can be replaced, that adds an extra possibility. So let's say that 90 of those 100 words have replaceable vowels (I'm not going to go check to see how many there actually are

). That means the password cracker now has 90 more possible words to check, for a total of 190 possibilities.
Hey, that's a pretty good improvement. But let's compare it to the number of possible three letter words. According to
this other Scrabble link, there are about 1000 three letter words. So let's compare the difference: if we stick with two letters and add vowel substitutions, we get about twice as many possibilities. If we ignore vowel substitutions, but add on another letter, we get about ten times as many possibilities. The difference between these numbers will continue to grow (literally exponentially) as the number of characters in a password increases.
So here's the
TL;DR: substitutions are better than not using substitutions; however, adding length is usually a much better strategy.