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[BrianEnigma]

If anyone wants to look at the actual RC5 spec (it is not too terribly long, but very mathematically/algorithmically dense and includes C samples), you can find it in RFC-2040.

Edit to add: I have a little piece of code from Schneier's most wonderful book that I have butchered up a little to try to solve this. My modifications are not the cleanest in the world and there are lots of bits commented out as I tried one thing, then another. Basically, going on the following assumptions, I was unable to come up with a solution.

1) That my code is correct. Most of the computer cryptography I have worked with in the past has been fairly high-level PKI. I believe I tweaked the correct constants to adjust for the proper key bytes, etc, but am willing to admit that I may be wrong.

2) That the key is ASCII text and in my system dictionary (while adding a few custom words like "PerplexC" and the street names found on the back of the card). This is probably not a valid assumption. It is likely 8 bytes of binary. I did try the key from the RSA challenge, but that did not turn up anything useful.

3) That the result is ASCII text (I also allow for carriage return and linefeed).

So, if anyone wants to take a look at the code, it is attached. There are three different rotateKey() functions, one to pull words from a dictionary file, one to rotate through all possible ASCII, and one to rotate through all possible alphas. I am running the plain-letter rotations now, but have not found anything of interest.

Also, it should be noted that as a time/processor saver, I am only calculating the first block of the cyphertext, not the whole thing. If the first block looks interesting (so far, none really have), then I will go back and decrypt the whole message.

Edit2: the last line of 8 bytes all by itself may be the initialization vector and not part of the code itself.

[BovineOne]

[BrianEnigma]

Yeah, I got home and was actually able to grab my copy of Applied Cryptography and discover that some of my assumptions were wrong and that the code I picked up was for RC5-32.

[Mosestrotsky]

Just want to say thanks for the description of what the code etc is. Good luck guys

[SteveC]

OK, some of my things are solved and I'm not being forced to look at RC5 (despite the fact that my computer has been crunching keys for it for the past 6 or so years).

1/ Are we sure that the IV set as:

WORD P = 0xb7e151628aed2a6bULL; /* magic constants */
WORD Q = 0x9e3779b97f4a7c15ULL; /* magic constants */

Is valid? I'm not so sure... How does that relate to an initial vector of say, "79 ce d5 d5 50 75 ea fc" ?

2/ What is the benefit of rotating the keys? Are we just taking the set point as a starting point and hitting it from there? That makes little sense, I'd have thought it preferable to be able to manually enter a key each time as we're clearly not going to dedicate the resources to hit this brute force.

[BovineOne]

[BovineOne]

[erekose]

[Scott]

[BrianEnigma]

BovineOne (or anyone, for that matter), do you have access to some sample RC5-64 decryption code with IVs that you could point us to? I can't seem to find much through Goggle and I'm a bit nervous about tweaking the 32 code to use 64 and would rather use something that is a known-good rather than introduce unnecessary unknowns. Even OpenSSL doesn't have RC5-64 algorithms yet.

The RC5 IV's don't seem to be touched on in Schneier's book (which only has about 5 paragraphs devoted to this.) So when decoding the first block, each byte of the output is XOR'ed with the 8-bit IV, right? From that point onward, when decoding block "n", after passing it to RC5_DECRYPT, you XOR it with the decode of block "n-1," correct?

[oliverkeers13]

I don't think that i understood one sentence of that Bri.

[SteveC]

Just picking nits a little bit, but both codes are RC5-64, the 64 is how we refer to the keysize, not the blocksize. Still means we have to decypher the meaning of the code in order to double the blocks it's dealing with though.

[diddymac]

I've googled the phrase "thirteenth labour" and it's come up with- get this- poems, puzzles and this:
http://www.mytholmroyd.net/tedhughes/alcestis2.html

[Night565]

Maybe

[BovineOne]