Entries Tagged "cryptanalysis"

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Spritz: A New RC4-Like Stream Cipher

Last week, Ron Rivest gave a talk at MIT about Spritz, a new stream cipher by him and Jacob Schuldt. It’s basically a redesign of RC4, given current cryptographic tools and knowledge.

RC4 is an example of what I think of as a too-good-to-be-true cipher. It looks so simple. It is so simple. In classic cryptographic terms, it’s a single rotor machine. It’s a single self-modifying rotor, but it modifies itself very slowly. Even so, it’s very hard to cryptanalyze. Even though the single rotor leaks information about its internal state with every output byte, its self-modifying structure always seems to stay ahead of analysis. But RC4 been around for over 25 years, and the best attacks are at the edge of practicality. When I talk about what sorts of secret cryptographic advances the NSA might have, a practical RC4 attack is one of the possibilities.

Spritz is Rivest and Schuldt’s redesign of RC4. It retains all of the problems that RC4 had. It’s built on a 256-element array of bytes, making it less than ideal for modern 32-bit and 64-bit CPUs. It’s not very fast. (It’s 50% slower than RC4, which was already much slower than algorithms like AES and Threefish.) It has a long key setup. But it’s a very clever design.

Here are the cores of RC4 and Spritz:

RC4:

1: i = i + 1
2: j = j + S[i]
3: SWAP(S[i];S[j])
4: z = S[S[i] + S[j]]
5: Return z

Spritz:

1: i = i + w
2: j = k + S[j + S[i]]
2a: k = i + k + S[j]
3: SWAP(S[i];S[j])
4: z = S[j + S[i + S[z + k]]]
5: Return z

S is an 8-bit permutation. In theory, it can be any size, which is nice for analysis, but in practice, it’s a 256-element array. RC4 has two pointers into the array: i and j. Spritz adds a third: k. The parameter w is basically a constant. It’s always 1 in RC4, but can be any odd number in Spritz (odd because that means it’s always relatively prime to 256). In both ciphers, i slowly walks around the array, and j—or j and k—bounce around wildly. Both have a single swap of two elements of the array. And both produce an output byte, z, a function of all the other parameters. In Spritz, the previous z is part of the calculation of the current z.

That’s the core. There are also functions for turning the key into the initial array permutation, using this as a stream cipher, using it as a hash function, and so on. It’s basically a sponge function, so it has a lot of applications.

What’s really interesting here is the way Rivest and Schuldt chose their various functions. They basically tried them all (given some constraints), and chose the ones with the best security properties. This is the sort of thing that can only be done with massive computing power.

I have always really liked RC4, and am happy to see a 21st-century redesign. I don’t know what kind of use it’ll get with its 8-bit word size, but surely there’s a niche for it somewhere.

Posted on October 27, 2014 at 9:55 AM • View Comments

The Security of al Qaeda Encryption Software

The web intelligence firm Recorded Future has posted two stories about how al Qaeda is using new encryption software in response to the Snowden disclosures. NPR picked up the story a week later.

Former NSA Chief Council Stewart Baker uses this as evidence that Snowden has harmed America. Glenn Greenwald calls this “CIA talking points” and shows that al Qaeda was using encryption well before Snowden. Both quote me heavily, Baker casting me as somehow disingenuous on this topic.

Baker is conflating my stating of two cryptography truisms. The first is that cryptography is hard, and you’re much better off using well-tested public algorithms than trying to roll your own. The second is that cryptographic implementation is hard, and you’re much better off using well-tested open-source encryption software than you are trying to roll your own. Admittedly, they’re very similar, and sometimes I’m not as precise as I should be when talking to reporters.

This is what I wrote in May:

I think this will help US intelligence efforts. Cryptography is hard, and the odds that a home-brew encryption product is better than a well-studied open-source tool is slight. Last fall, Matt Blaze said to me that he thought that the Snowden documents will usher in a new dark age of cryptography, as people abandon good algorithms and software for snake oil of their own devising. My guess is that this an example of that.

Note the phrase “good algorithms and software.” My intention was to invoke both truisms in the same sentence. That paragraph is true if al Qaeda is rolling their own encryption algorithms, as Recorded Future reported in May. And it remains true if al Qaeda is using algorithms like my own Twofish and rolling their own software, as Recorded Future reported earlier this month. Everything we know about how the NSA breaks cryptography is that they attack the implementations far more successfully than the algorithms.

My guess is that in this case they don’t even bother with the encryption software; they just attack the users’ computers. There’s nothing that screams “hack me” more than using specially designed al Qaeda encryption software. There’s probably a QUANTUMINSERT attack and FOXACID exploit already set on automatic fire.

I don’t want to get into an argument about whether al Qaeda is altering its security in response to the Snowden documents. Its members would be idiots if they did not, but it’s also clear that they were designing their own cryptographic software long before Snowden. My guess is that the smart ones are using public tools like OTR and PGP and the paranoid dumb ones are using their own stuff, and that the split was the same both pre- and post-Snowden.

Posted on August 19, 2014 at 6:11 AM • View Comments

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Sidebar photo of Bruce Schneier by Joe MacInnis.