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But is it necessary to use these bytes? Yes, it is, at least for most messages that you'll see in practice. MD5 works by taking the message, and applying a fixed padding to it. This fixed padding involves, for messages which are a number of bytes (as opposed to, say, a message of 119 bits) an 0x80 byte, and for not huge messages, 0x00 bytes (in the ...


If $H(x) = x$, $x$ is a fixed point. If for a value the output of the function is the same as the input, it is called a fixed point. A length extension attack is unrelated to the concept of fixed points. There is a good question about understanding length extension attacks here.

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