This lesson will cover symmetric encryption, a well-known standard for data encryption. It is a shared-key methodology, meaning the key used to encrypt the data is the same key used to decrypt it.
I was reading this interesting question: It shows a weak home-brew algorithm developed by 'Dave', and the answers discuss why this is a bad idea. (Actually hashing algorithm rather than encryption, but my question applies to both.) It makes sense to me that a home-brew algorithm is a very bad idea, but there's one thing I'm not understanding. Assume I'm an attacker, and I am faced with an weak-but-unknown encryption algorithm developed by 'Dave'. How would I crack it? I wouldn't even know where to begin.
It would be a seemingly meaningless string of characters. For example, say that the home-brew algorithm is like this:. Use a weak well-known encryption algorithm on the original data, then:. Do a bitwise-negative on any byte whose serial number in the file has a repeated digit sum which is prime. (Or any other such mathematical manipulation, this is just an example.) How would one hack a file produced by such an algorithm without knowing it in advance? Edit: Everybody, please don't try to convince me of how hard it is to keep an algorithm secret. Please answer this question on the assumption that the algorithm is kept completely secret, despite of how difficult that is to achieve in real life.
![Decryption Decryption](https://denuvo.com/img/infografik.png?v=2)
Also, assume that I have no access at all to the algorithm, only to the resulting data. In real-world scenarios, if the confidentiality of your data is dependant on the secrecy of your encryption algorithm, it would probably be easier to steal the details of that algorithm (assuming the algorithm is not trivial).
Weakest link, and all that. Moreover, in non-trivial real-world usage scenarios, it is nearly impossible to keep the algorithm to any level of secrecy, even without the attacker's involvement - whether it is other consumers of the algorithm, developers with access to the source code repository, or other inside attackers. – Mar 18 '13 at 20:53. Assume I'm an attacker, and I am faced with an weak-but-unknown encryption algorithm developed by 'Dave'. How would I crack it? I wouldn't even know where to begin. It would be a seemingly meaningless string of characters.
That's correct, you wouldn't. Here's some encrypted data (74588).
Got a clue what that means? Absolutely not. However, you're missing the core, fundamental most integrally important central pillar key to the universe that holds cryptography together. The idea is simple: the key is everything That's it. That's the bit you have to protect. The bit you must guard with your life and hope nobody is going to hit you with a hammer until you tell them what it is.
On this basis, you must assume that your algorithim can be read by the attacker. They know how it works.
They can document its process. If there are any weaknesses, they'll find them. And they'll exploit them. Like that angry CIA Dad from Taken. This, it turns out, is less of an assumption and more of the practical case in use.
Dave, the home brew cryptographer, wants to include an encryption algorithm in his program. Deciding to eschew all the testing and design work cryptographers have done for him for free over the years, he writes something involving the odd xor, compiles his program and helpfully gives it to friends. That algorithm is now in their hands. Now, you might ask 'can't I just keep the algorithm secret? That'll work, right?' Oh Dave, plz stop. The problem with secret algorithms is that they're much more likely to be stolen.
![Crack Crack](http://www.softnuke.com/wp-content/uploads/2014/09/previewlocked.jpg)
After all, the key is different for each user (actually, this is not a requirement, but, let's just assume it is for simplicity) but the algorithm remains unchanged. So you only need one of your implementations to be exposed to an attacker and it is game over again. Edit: Ok, in response to the OP's updated question.
Let us assume for a moment that the algorithm is totally unknown. Each of the two participants in an encrypted conversation have perfect security of their algorithm implementation. In this case, you've got data to analyse.
You could do any one of the following:. Analyze for. This is how you'd break a typical caesar-shift cipher. Attempt to guess the length of the key. With this information, you can move into looking for repeated ciphertext blocks which may correspond to the same plaintext. Attempt index of coincidence and other such measures used to break the vigenere cipher, since many polyalphabetic ciphers are (possibly) just variants of this.
Watch for patterns. Any pattern might give you the key. Look for any other clues. Do the lengths correspond to a certain measure, are they for example multiples of a certain value such as a byte boundary and so are (possibly) padded?. Attempt to analyze with one of the. These rely on knowing the algorithm in many cases, so may not apply here. If you believe the the data in question represents a key exchange, you can try one of the many techniques for breaking.
The fact is that a short piece of data from an unknown algorithm could well be undecryptable. However, this does not mean you should rely on this being the case. The more data a cryptanalyst can recover, the more likely they are to break your algorithm. You probably don't know without serious cryptanalysis what that boundary is - for example, it is reasonable to assume that one could bruteforce a caeser-cipher algorithm for three letter words, since there are few that make sense. You are up against re-use problems too. In WWII, the Engima overcame this problem by having programmable settings for their secret algorithm, but this was broken too. There is also the human element of cryptography to consider.
I realise the label on the tin says 'use once, do not digest' etc, but humans are humans and will likely use twice, three times etc. Any such behaviour plays into the hands of the cryptanalyst. If you don't know the algorithm, and depending on how good its design is, it is not trivial, but it has been proven you just need either enough ciphered messages or a few clear messages and their ciphered versions to remove the noise and infer the transformation by applying a correlation algorithm between ciphered messages and their most probable decryption (using the known messages as training sequence, if available). If you are interested on the maths involved, reading about information theory, signal processing and machine learning may help you. – Mar 18 '13 at 21:32. An unknown 'encryption' algorithm has been historically achieved at least once. I am speaking of, a writing method which was used in Crete around 1300 BC.
The method was lost a few centuries later, with the death of all practitioners and the overall collapse of civilization during the so-called. When archaeologists began sifting the earth around Knossos and other locations, at the end of the 19th century, all they got was a bunch of tablets with unknown signs, without a clue about the writing system which was used to produce them. The interesting story here is that Linear B was in 1950s, using the same analysis tools which were employed against encryption systems of that time. In effect, the writing was considered as an 'unknown encryption algorithm'.
It succumbed to statistical analyses, chained inferences, and some hypotheses on the plaintext (basically, the assumption that the base language for a variant of Greek). This is a classic and masterful illustration of how cryptanalysis works against 'manual cryptosystems'. Of course, assuming that a cryptographic algorithm can be in use and still remain secret, is implausible. By the same assumption, there is no piracy of video games or media contents. The real world implacably reminds us that this is not true.
The only known way by which an algorithm may remain secret is to kill its inventors and practitioners, destroy their apparatus, and wait for a few centuries. This has a few inconvenient side effects. And even if, in a given specific instance, details on an algorithm have not leaked yet, there is no way of quantifying how much secret the algorithm is, i.e. How much time it will take for reverse engineering, bribes or wholesome theft to rebuild the algorithm.
This is the main reason why cryptographers, about 40 years ago, decided that key and algorithm should be split, with the key being secret and the algorithm being non-secret: you can quantify the secrecy of a key, not the secrecy of an algorithm. This gives us an insight into your specific question.
Your 'secret algorithm' hinges on the notion of a 'mathematical manipulation'. How many of these are they?
Can you estimate or describe the set of 'mathematical manipulations'? You will find that an encryption algorithm is itself a 'mathematical manipulation', so your question is rather ill-defined. I don't really understand all this 'The real world implacably reminds us that this is not true.' In all the answers. Real life example: one uses a reversible encryption algorithm to protect sensitive user data on the server.
That means that it can't be a one-way algorithm like we can use to store passwords, so it must have a key. So now how exactly protecting this key is different from protecting the algorithm? Just assume that the guy who wrote this algorithm is the same guy who generates/manages the encryption keys. Bribes, stealing etc. Would apply in the same way to both methods. – Mar 19 '13 at 6:59.
The algorithm exists as compiled code in files, and also source code on developers' machines, revision control software, backups. And there are design documents, as printed paper, emails, and in the head of several people. It would be very hard to track them all and ensure secrecy. This contrasts with a key, which exists only in RAM or, at worst, in a single file, and not in all the other mediums I just listed. You can abduct all the developers, none of them has the slightest clue about the value of the key since it never entered their brain in the first place. – Mar 19 '13 at 12:16.
To attack a cryptographic protocol, you have the following attack methods. Known plaintext: Trying to find correlations between the plaintext you have and the corresponding ciphertext.
Chosen plaintext: Encrypting specific plaintext and studying the changes to the ciphertext as the plaintext changes. Choosen ciphertext: Decrypting specific ciphertext and studying the changes to the plaintext and the cipher text changes. Known cipher text: Where all you have is the cipher text, below is a simple example. Long time ago I took a cryptography class, in one the lectures we were taught the cryptonalysis of.
This is not how things are done now, but this is where the science of cryptography had started, and this is how cryptonalysis had begun. Let's say you can across this cipher text. Mx qeoiw wirwi xs qi xlex e lsqi-fvia epksvmxlq mw e zivc feh mhie, fyx xlivi'w sri xlmrk M'q rsx yrhivwxerhmrk. You don't know the algorithm, you don't know the key. How should you start?. Analyze the letter frequency: Total length is 87 letters. We see that i was used 12 times - 13%.
According to, this letter is likely to be e. Our cipher text is now: Mx qeoew werwe xs qe xlex e lsqe-fvea epksvmxlq mw e zevc feh mhee, fyx xleve'w sre xlmrk M'q rsx yrhivwxerhmrk. Now the second most frequent letter is x was used 11 times - 11%, so it's likely to be t. Our cipher text is now: Mt qeoew werwe ts qe tlet e lsqe-fvea epksvmtlq mw e zevc feh mhee, fyt tleve'w sre tlmrk M'q rst yrhivwterhmrk.
Now we're starting to see the patterns. Replacing i-e and x-t suggests that the key could be 4. Let's try it: It makes sense to me that a home-brew algorithm is a very bad idea, but there's one thing I'm not understanding. Now you've done your first cryptonalysis. This is one way the ciphertext could be analyzed. I think nobody has said it aloud here, so I will. If a cryptographer is given only one ciphertext with no means to get more, the ciphertext is short and no knowledge of the plaintext is given, it is near impossible to decrypt the text.
The only way this is still possible is if the cipher is around the difficulty level of a substitution cipher. Given the same algorithm, if there is a way to get more ciphertexts on demand, if the ciphertext is sufficiently long or if there are some known parts of the plaintext to help, it is likely that the algorithm can be cracked given enough effort. But even still, cryptanalysis takes a lot of effort compared to the effort of creating a simple cryptoalgorithm from scratch, so it is unlikely anyone will expend the effort unless there's a good reason for doing so. There are several ways. The first, and most obvious is that the attackers compromised your server to the extent where they managed to obtain your source code.
In that particular case, your homegrown scheme is as good as nothing. The second way is that the attacker might be able to submit his own values to your algorithm and see the before/after result. This is known as the. A good encryption scheme should not be vulnerable to it. A homegrown scheme probably is. Even without a chosen plaintext attack, a homegrown scheme is usually laughably weak. A laymen like you and I might not be able to make sense of the output of a homegrown scheme.
However, there are a class of very smart people who devote their time and effort to breaking such cryptographic schemes usually in return for a good paycheck. You might have heard of them, we call them Cryptographers.