Entropic attack works by knowing how often specific letters and words normally tend to appear. For example, in English, e is the most commonly occuring letter. So if we take a message which is substitution encoded and find which symbol occurs most often we can then ‘guess’ that it must be the symbol which represents e. We can then repeat this process using a number of the most common words and letters and are likely to find that - even for suprisingly short messages - we can quickly break the code and produce a readable message.
To take the example on the secret codes page. If we look at the example of the message we see that one set of three symbols occurs twice even though it is a very short message. This makes it reasonably likely that the word may be the. When we also see how often the third symbol of this repeated word appears in the other messages we can feel confident that this symbol is e and the word is the. Using this approach and matching character-cypher mappings we know with others we don't we can quickly decypher the messages.
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