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What are the "standard procedures in cryptanalysis" to analyze unknown ciphertext?

In other words: Are there any protocols, officially acknowledged checklists or something like that which represent a "standard" approach in cryptanalysis to analyze unidentified ciphertext? If yes, which ones would that be and what would be the path to follow?


Since some don't seem to understand what I am asking...

I am well aware of cryptanalytic procedures like detecting algorythms etc. My question was: is there any “officially acknowledged protocol” in cryptography to follow when doing cryptanalysis?

To get one point very clear before anyone else goes down the wrong road: this has nothing to do with "only a bad guy would ask something like this..." since the situation is actually the other way around. That's why I asked for "officially acknowledged protocols" as in "legally accepted by authorities as evidence and/or proof". Think "whitepaper", not "reverse engineering".

The goal is the "capturing and identification of digital evidence coming from communication streams" to pass on that information to authorities in a way they will accept it. I know how to gain the rough information I want to pass on (like the identification of the crypto-algorithm used), but I want to follow the correct path/order/procedures which authorities accept as "correctly collected digital evidence".

There has to be some protocol that governments, (security) companies, and institutions follow. When it comes to "evidence", there's always a "specific way to do it"... question is: which one would that be when it comes to analyzing unknown ciphertext?

To be sure you understand me correctly: I'm not asking how to analyze unidentified ciphertext, I'm asking what protocol to follow when doing so because I need to pass on the collected evidence later on!


I think my comment-reply to @mikeazo wraps up my question rather perfectly:

Collected evidence was captured in internal company network. We secured full access logs and identified source and target of transmission. Currently, source denies it's anything else but random text. Doing a quick analysis of copies of the captured data shows different encryption algorithms. As proof it's not random text I would like to add my findings to the other evidence collected. Do do so, I want to be sure they accept the way I identify the algorithms etc. That's why I would like to know if there's an "official protocol" to follow that's "officially accepted" (by authorities).

Addendum to the answer I accepted…

After getting a tip via twitter, it turns out that there are several pointers in publications from the National Institute of Justice (NIJ):

  1. Electronic Crime Scene Investigation: A Guide for First Responders, Second Edition (April 2008, NCJ 219941)
  2. Forensic Examination of Digital Evidence: A Guide for Law Enforcement (April 2004, NCJ 199408)
  3. Digital Evidence in the Courtroom: A Guide for Law Enforcement and Prosecutors (January 2007, NCJ 211314)

Those provide initial hints, but even those don't really handle the details of standard procedures to follow in cryptanalysis.

Like the answer I accepted already implied, cryptanalysis obviously isn't enjoying any standard protocols yet that are known to the general public around here. That is, unless a cryptanalytic professional from some governmental institution raises his voice and enlightens everyone (me included).

share|improve this question
Ciphertext only cryptoanalysis has fallen out of fashion. Modern crypto needs to be secure against attackers who know everything about your system except the key. Even with Enigma the attackers stole some machines to learn how they work. So Step 1 is: Obtain the algorithm used to produce the ciphertext – CodesInChaos Jun 27 '13 at 15:41
Can you narrow the scope some? Are you interested in public key ciphers, symmetric key ciphers, or classical ciphers? – mikeazo Jun 27 '13 at 15:56
Officially acknowleged by whom? Once that list came out, all new ciphers would be specifically designed to make the checklist/standard irrelevant. – mikeazo Jun 27 '13 at 16:02
@e-sushi I think I get it now. Thanks for jumping through the hoops to help me. – mikeazo Jun 27 '13 at 16:29
@mikeazo Thanks for understanding what I'm after. To be honest, I was slowly losing hope anyone would actually understand why I'm looking for. Life is hard being a white-hat. ;) – e-sushi Jun 27 '13 at 16:34

2 Answers 2

up vote 5 down vote accepted

Unfortunately Crypto.SE probably has very few lawyers, law enforcement officers, public policy experts, etc. Furthermore, Legal.SE doesn't exist yet. To further complicate the situation (at least in the US) the answer will probably vary from state to state.

So, I'm going to do the best I can, but you are probably best off either calling police or a lawyer in your jurisdiction.

Here is the deal, it is hard to prove a negative. Thus, I don't feel sufficiently knowledgeable to say no such protocol/procedure exists. That said, I don't know of one. I did find this flowchart for collecting digital evidence if you are in the US. Step one seems to imply that some knowledge of local/agency policies must be followed. But this deals more with evidence collection and not analysis.

My guess is that (at least in US), once you have the relevant network traffic and logs, your job is done. This is especially true if you have any motive/interest in seeing the offending party get in trouble.

So, assuming evidence is collected properly (since that is beyond the scope of your question) my guess is that the prosecuting attorney will hire an expert witness to examine the evidence and give testimony at a trial. They would probably also call you up to testify as to the collection procedures, etc.

Some added thoughts
The answer probably also depends on the exact protocols/software used. For example, if after an expert analyzes the ciphertext, they can determine that PGP or some other piece of software was used in which a long-term secret is used for encryption, a judge could order the defendant to disclose the encryption password or long-term secrets. At this point, no additional cryptanalysis would be needed. This would become more difficult if say TLS is used as the client often has no long-term secrets. I suppose a judge could force the party owning the server to reveal long-term secrets so decryption could take place (assuming the initial handshake was captured). If TLS, or whatever protocol used, is used in such a way that ephemeral keys are used, then even this would not work.

share|improve this answer
Jurisdiction is Germany, Lawyer is on it, police too, yet police passes evidence on to their related departments... just wanted to be sure they get everything they need. But reading "once you have the relevant network traffic and logs, your job is done" sounds like I'm indeed trying to provide more than needed. Maybe you're right and my job is done. Nevertheless, I think the question made sense and - if it did - your answer fits the question best. Thanks for taking the time to get your head around the question in the first place. Much appreciated! – e-sushi Jun 27 '13 at 16:48
Your added thoughts provide even more confidence that you're correct about my job being done. The lawyer(s) already informed the "expert" (police). about my findings. I bet it will be easy for them to cross-check and confirm my recent findings and secure any related proof while they're at it. At least I provided them with some good hints on what to look for. Anyway, thanks again! – e-sushi Jun 27 '13 at 17:11

No there is not.

The reason is simple: If you do not know anything about the scheme used to create the ciphertext, you can not find out anything more than the maximum length of the message. The ciphertext could be created with a one time pad. The only information you would have is that that the message can be at most the size of the ciphertext.

In historical ciphers you could use certain assumptions of the plaintext (e.g. distribution of letters and bigrams in language) and statistical tools to identify the cipher. For example Caesar only shifts the distribution by a certain amount, while Vigenere also also flattens it.

Today's encryption schemes are designed differently, where statistical analysis doesn't help much any more. If you just have a bit-representation of a single ciphertext, you don't even know how many numbers/symbols you got.

However, not knowing the encryption scheme is no realistic scenario today. Cryptography abandoned "security by obscurity" decades ago (even if some laymen still think it would be a good idea). Unless you are the "bad guy", this entire question is pointless, as security should not be based on the obscurity of the cryptosystem.

share|improve this answer
My question is not related to "obscurity of the cryptosystem" or anything else you're talking about. My best guess is that you did not understand what I asked. Therefore, please note the EDIT to my question, which is partly directed to your "bad guy" remark which I find rather insulting to be honest. – e-sushi Jun 27 '13 at 16:04
First off, it was not meant as an insult to you personally. But cryptanalysis simply does not start at the point "I don't know the scheme". So no, there is no such official guideline in general. Maybe certain institutions have standard procedures, but that is not a question about cryptography. If you have such a case, maybe you could just ask a contact person. As a general rule of thumb, you don't look for clues on the cryptosystem in the ciphertext but rather the surrounding information. E.g. every protocol has some kind of handshake to agree on parameters. – tylo Jun 27 '13 at 16:33
Where do you get your "I don't know the scheme" from? If you would have taken the time to read my question and the edits to my question, you would have understood what I was asking. But meanwhile there's no need to anymore. I received an answer I could use and accepted accordingly. Therefore, all I can say is "Thanks for your efforts" with the hope that maybe next time, our communication level overlaps a bit more. – e-sushi Jun 27 '13 at 16:58
I disagree about the scheme not being known today. I navigated here looking for a general purpose technique for analyzing obfuscated text in malware. This is a realm where you do not know the cryptosystem (oftentimes weak) being used. – Fred Concklin Jul 16 '14 at 8:37

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