4 ms·

I guess I had always assumed attacks were brute force input = output. But you seem to be implying that patterns would be looked for that would make the encrypti

I guess I had always assumed attacks were brute force input = output. But you seem to be implying that patterns would be looked for that would make the encryption/decryption vastly easier. That would probably be the piece that I wasn't grasping because if you have: a -> f(a) -> b vs. a -> g(a) -> b Then brute force really wouldn't be affected by complexity unless it was a time factor and thus either of the functions should be identical in security. But if analysis was the key to a good attack then this makes perfect sense. Thanks for the info.

tptacek 15y ago
You can brute force AES just like anything else. But AES keys are at least 2128 bits long, so you're racing the lifespan of the solar system to attempt it.
jbert 15y ago
Yes. As a simple example, if you XORd your input text with a 1-byte key, the letter frequency of english text would be unchanged. So your most common byte in your output would be likely to be the encrypted char 'e'. You could then recover the key by XOR 'e' with that most common byte. More complex systems have more complex patterns, but that's what cryptopgraphers do - try and spot weaknesses in an algorithm. As always, this is a great read: http://chargen.matasano.com/chargen/2009/7/22/if-youre-typing-the-letters-a-e-s-into-your-code-youre-doing.html http://chargen.matasano.com/chargen/2009/7/22/if-youre-typin... Search for "Garbling a block to confuse an app" and read on from there for a real-world use of output patterns (used when you can partially control the input).
- tptacek 15y ago
  Multibyte XOR keys are almost as trivial to break as single-byte XOR keys, for what it's worth. And if you know how to do that, there are "best practices" AES modes in which common implementation errors lead to the same attack.
- Groxx 15y ago
  Happen to have, or know of, a version of that that has the images intact? I'm getting broken ones on every single item in the post.

tptacek 15y ago

You can brute force AES just like anything else. But AES keys are at least 2128 bits long, so you're racing the lifespan of the solar system to attempt it.

jbert 15y ago

Yes. As a simple example, if you XORd your input text with a 1-byte key, the letter frequency of english text would be unchanged. So your most common byte in your output would be likely to be the encrypted char 'e'. You could then recover the key by XOR 'e' with that most common byte. More complex systems have more complex patterns, but that's what cryptopgraphers do - try and spot weaknesses in an algorithm. As always, this is a great read: http://chargen.matasano.com/chargen/2009/7/22/if-youre-typing-the-letters-a-e-s-into-your-code-youre-doing.html http://chargen.matasano.com/chargen/2009/7/22/if-youre-typin... Search for "Garbling a block to confuse an app" and read on from there for a real-world use of output patterns (used when you can partially control the input).

tptacek 15y ago
Multibyte XOR keys are almost as trivial to break as single-byte XOR keys, for what it's worth. And if you know how to do that, there are "best practices" AES modes in which common implementation errors lead to the same attack.
Groxx 15y ago
Happen to have, or know of, a version of that that has the images intact? I'm getting broken ones on every single item in the post.