3 ms·

I've often wondered how you know when you've actually found the solution. The massively parallel algorithm iteratively decrypts fixed-size blocks of data to fi

I've often wondered how you know when you've actually found the solution. The massively parallel algorithm iteratively decrypts fixed-size blocks of data to find keys that decrypt into ASCII numbers. This technique is often used for recovering the keys of encrypted files containing known types of data. This doesn't make sense to me. I mean, what if I didn't encrypt ASCII numbers? What if it's, say, an email message that's predominantly text in, say, UTF-8 or UTF-16? And it seems to me that as we develop richer ways to encode data, it also makes it correspondingly difficult to decrypt that data.

moron4hire 17y ago
Not sure about UTF-16, but UTF-8 is backwards compatible with ASCII. A lot of decryption requires having a general idea of what the message contains. If you know that the message is standard English text, then that gives you huge opportunities, like finding keys that make parts of the message decrypt to the word "the".
cperciva 17y ago
The field of cryptography developed in the context of nation-state actors and idiots who use crypto wrong. (Come to think of it, not much has changed.) In WW2, a lot of crypto was broken thanks to known plaintexts -- e.g., "Today's weather is..." -- or chosen plaintexts -- e.g., "Submarine sighted at...". This was made easier, of course, by the fact that military structures tend to use very consistent communications formats. You're quite right that you can slow down a brute force attack by making the input data hard to recognize -- one trivial approach is to pad it to a large size and then apply an all-or-nothing transform -- but by the time this sort of mechanism was feasible, brute force key searches stopped being useful due to increased key lengths.
- eru 17y ago
  As far as I know the allies used to spot the plaintext word "EINS" in the Enigma ciphertexts. This was helped by the omission of numbers --- or anything besides letters --- from the Enigma's alphabet. They did not even have a space key.

moron4hire 17y ago

Not sure about UTF-16, but UTF-8 is backwards compatible with ASCII. A lot of decryption requires having a general idea of what the message contains. If you know that the message is standard English text, then that gives you huge opportunities, like finding keys that make parts of the message decrypt to the word "the".

cperciva 17y ago

The field of cryptography developed in the context of nation-state actors and idiots who use crypto wrong. (Come to think of it, not much has changed.) In WW2, a lot of crypto was broken thanks to known plaintexts -- e.g., "Today's weather is..." -- or chosen plaintexts -- e.g., "Submarine sighted at...". This was made easier, of course, by the fact that military structures tend to use very consistent communications formats. You're quite right that you can slow down a brute force attack by making the input data hard to recognize -- one trivial approach is to pad it to a large size and then apply an all-or-nothing transform -- but by the time this sort of mechanism was feasible, brute force key searches stopped being useful due to increased key lengths.

eru 17y ago
As far as I know the allies used to spot the plaintext word "EINS" in the Enigma ciphertexts. This was helped by the omission of numbers --- or anything besides letters --- from the Enigma's alphabet. They did not even have a space key.