information entropy

information entropy
A measure of the uncertainty associated with a random variable ; a measure of the average information content one is missing when one does not know the value of the random variable ; usually in units such as bits.

A passphrase is similar to a password, except it can be a phrase with a series of words, punctuation, numbers, whitespace, or any string of characters you want. Good passphrases are 10-30 characters long, are not simple sentences or otherwise easily guessable (English prose has only 1-2 bits of entropy per character, and provides very bad passphrases), and contain a mix of upper and lowercase letters, numbers, and non-alphanumeric characters. — BSD General Commands Manual : ssh-keygen(1), October 2, 2010.


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