I currently informed myself about encryption a lot.
And I wonder, whether it would be good to toggle some bits (XOR and bitmasks) at a known position in the encrypted byte array and to toggle them again before decrypting them.
Because even if you know the algorithm and the key it wouldn't be possible to decrypt them propably without knowing where to toggle the bits wouldn't it?
The bit-toggling becomes part of the algorithm, so if "they know the algorithm" comes to include which bits were toggled.
It does become marginally harder to find out that "the algorithm", but this gain is small. If they can get their hands on the key, I think your problem is somewhere else...
There are some disadvantages to this as well. First, you may introduce security flaws in the system. I don't think this will happen, but I don't know I won't, and in security you should assume you might cause security flaws unless you know you won't.
The second problem is that if you make a mistake here somewhere it is possible to corrupt data. Of course, rigid testing will make sure that a mistake won't make it to production, but it just isn't as safe as using the functionality of a security library.
Lastly, there is the problem that your code and data will be harder to work with. If you need to work with it in the future, or someone has to work with it, it'll probably take more effort than it otherwise would have.
Those aren't big things, but I'd say more than the gain. At the end of the day, this is little more than "security through obscurity", so no, I wouldn't say it is a good idea.
You need to look at Kerckhoff's Principle. Any crypto-system needs to be secure, even if the attacker knows the entire algorithm. Only the key must be kept secret. Your bit-twiddling is part of the algorithm, and so will be known to any enemy.
Consider AES. There are public papers describing in great detail exactly how AES works: NIST AES Description. That description of the algorithm is public, yet AES is still secure because if follows Kerckhoff. You need to make sure that your algorithm is secure. Simple bit-twiddling as you describe will not make an insecure algorithm secure. Any attacker will know what bits to twiddle and can untwiddle them and break the underlying insecure cypher.
As an alternative, you could add the bitmask you use to the key. This increases the key size and the processing time for very little security gain. There are usually better ways to increase the security of a cypher, such as adding more rounds.
Related
The IV used in schemes such as CBC has to be random and unpredictable. But at the same time it does not have to be kept secret.
If the IV does not have to be secret, why does it have to be random then? I fail to make sense out of these seemingly contradicting requirements.
I have seen descriptions of attacks which exploit the non-randomness. So, I would understand why randomness is needed. However, things get confusing when the requirements specify that the IV does not have to be secret !This seems to defeat the whole purpose of randomness.
Somebody help clarify this please.
I think you are reversing the roles.
When a cryptographic protocol is designed, it is designed with certain assumptions in mind. The more assumptions you use, the less useful the protocol is, as you are less likely to find scenarios in which the assumptions hold.
In the case of CBC, the IV was designed to not need to be secret. You can keep it a secret, if you like. The algorithm is definitely not less secure this way. It is not, however, a requirement.
Having a non-random IV, on the other hand, causes the entire protocol to be unsuitable for certain applications. When choosing between adding a requirement to the protocol and adding a requirement to its data, the right choice is obvious.
In other words, the IV does not need to be secret, merely because it can be non-secret.
I don't know much about the heavy math behind cryptosystems, I get stuck when it gets bad with the Z/nZ algebra, and sometimes with all these exponent of exponents. It's not I don't like it, it's just that the information you find on the web are not easy to follow blindly.
I was wondering: how reliable can a algorithm be when it encodes a message into plain binary. If my algorithm is arbitrary and known only to me, how can a cryptanalist study an encrypted file and decrypt it, with or without having the decoded file ?
I'm thinking about not using ASCII text to code my message, and I have some ideas to make this algorithm/program.
Attacking a AES or blowfish crypted file is more trivial for a cryptanalyst, than if the algorithm the file is encrypted with is unknown to him, but how does he do then ?
I don't know if I understanded well, but a CS teacher once told me that codes are harder to crack that crypted ciphers.
What do you think ?
Attacking a AES or blowfish crypted file is more trivial for a cryptanalyst, than if the algorithm the file is encrypted with is unknown to him...
What about:
Attacking an untested self written algorithm with no real research is more trivial for a cryptanalyst, than if the algorithm the file is encrypted with, is a well known and proofed one, that has been correctly used....
In short, DO NOT roll your own cryptography unless you're an expert, no unless you're part of an expert group in that field.
Nintendo failed when they implemented RSA on their own in the Wii, Sony failed too when using it in the PS3 (they pretty much used XKCD's random number function for M...)
And you really think you can win by using security by obscurity?
PS: That doesn't mean that you should take the Wikipedia entry on RSA and roll you own implementation from that one (that's exactly were Sony and Big-N failed), no use a tested, open source implementation.
You seem to be using two words interchangeably but remember that Encoding is Not Encryption
When the attacker has no idea which algorithm you used and it is safe, cryptoanalyst has a hard job. So it is unimportant if you use AES or your own cipher as long as it is as strong and safe as AES. Here is the but. Cryptography is a bit demanding and therefore you have many ways to shoot yourself in a foot without knowing it. I would suggest using standard algorithms, maybe with some safe variations.
Common wisdom is that you should not build your own algorithms, and especially not rely on these algorithms remaining secret.
The conceptual reason is that good encryption is about quantified confidentiality. We do not want our secrets to get cracked, but in a more precise way we want to be able to tell how much it would cost to crack our secrets (and hopefully show that the cost is way too high to be envisioned by any entity on Earth). This is the real advance which occurred a few years after World War II: to understand the distinction between key and algorithm. The key concentrates the secret. The algorithm becomes the implementation.
Since the implementation is, well, implemented, it exists as some code or a device, which is tangible and stored even when it is not used. Keeping an implementation secret requires keeping track of the hard disk on which the code resides at all times. If the attacker sees the binary code, he may be able to reverse-engineer it, something which depends on his wits and patience. The point here is that it is very difficult to be able to say: "it costs X dollars to recover a description of the algorithm".
On the other hand, the key is short. It can be stored safely much more easily; e.g. you could memorize it, and avoid committing it to any permanent storage device. You then have to worry about your key only at times when you use it (and not when you do not, e.g. in the middle of the night, when you sleep). The number of possible keys is a simple mathematical problem. You can easily and accurately estimate the average cost of enumerating the possible keys until your key is found. The key is a sturdy foundation for quantified security.
So you should not roll your own algorithms because then you do not know how much security you get.
Also, most people who rolled their own algorithms found out, usually the hard way, that they did not get much security at all. Designing a good encryption algorithm is hard, because it cannot be automatically tested. Your code may run, and properly decrypt data that it encrypted, but it tells you nothing about how secure the algorithm is. The design of the AES was the result of a process which took several years and involved hundreds of skilled cryptographers (most of whom had a PhD and years of experience in academic research on symmetric encryption). That a lone developer could do as well, let alone better, in the secrecy of his own workshop, looks kind of... implausible.
The biggest part of your strategy is called "security through obscurity." You're making the gamble that, since nobody knows the precise details of your little variation on an idea, they won't be able to figure it out.
I'm not a security expert, but I can tell you that you probably won't come up with something incredibly new. Cryptography has been studied by people for millenia and your idea is highly unlikely to be original. Even if you're a relatively good programmer and code something really tricky, the question will come down to who you're up against. If you're just trying to protect your data from your kid sister, then it will probably be fine. On the other hand, if you're using it to send credit card numbers across the internet, then you're doomed to fail. It will be analysed in ways you didn't think of or don't know, and ultimately cracked.
Another way to think of it: algorithms like AES have been extensively studied by professionals in the field and its level of security is pretty well understood. Anything you come up with by yourself will not have the benefit of having been attacked by the best and brightest minds out there. You will have almost no idea of how good it actually is until people start reporting identity theft.
I am reading about cryptography I was thinking about these properties of AES (that I use):
same message = same ouput
no message length secrecy
possible insecurity if you know the messages (does this actually apply to AES?)
I hear that AES is secure, but what if I want to theoritcaly improve these properties?
I was thinking I could do this:
apply encryption algorithm A
XOR with random data D (making sure the output looks random in case of any cipher)
generate random data that are longer than the original message
use hashing function F to allocate slots in random data (this scrambles the order bytes)
Inputs: Encryption algorith A, Data to XOR with D and a hashing function F
My questions are
does the proposed solution theoreticaly help with my concerns?
is this approach used somewhere?
Possible enhancements to this approach
I could also say that the next position chosen by hashing function will be altered using a checksum of the last decoded byte after the XOR step (that way the message has to be decoded from beginning to end)
If I was to use this to have conversation with someone, the data to XOR with could be the last message from the other person, but thats probably a vulnerability.
I am looking forward to your thoughts!
(This is only theoretical, I am not in need of more secure encryption, just trying to learn from you guys.)
Yeah.
Look. If you want to learn about cryptography, I suggest you read Applied Cryptography. Really, just do it. You will get some nice definitive learnings, and get an understanding of what is appropriate and what is not. It specifically talks about implementation, which is what you are after.
Some rules of thumb:
Don't make up your own scheme. This is almost universally true. There may be exceptions, but it's fair to say that you should only invent your own scheme if you've thoroughly reviewed all existing schemes and have specific quantifiable reasons for them not being good enough.
Model your attacker. Find out what scenarios you are intending to protect against, and structure your system so that it works to mitigate the potential attacks.
Complexity is your enemy. Don't make your system more complex then it needs to be.
Stay up to date. You can find a few mailing lists related to cryptography and (and hashing) join them. From there you will learn interesting implementation details, and be aware of the latest attacks.
As for specifically addressing your question, well, it's confusing. I don't understand your goal, nor do I understand steps 3 and 4. You might like to take a quick look here to gain an understanding of the different ways you can use a given encryption algorithm.
Hope this helps.
You assumptions are incorrect.
same message != same output
The output will not be the same if you encrypt the same message twice.
This is because you are suppsed to use different IVs'.
Message length can be hidden by adding random data to the plaintext.
Attacks have been demonstrated against AES with lesser number of rounds.
Full-round AES has not been compromised in any way.
Other than that I suggest you follow Noon Silks recommendation and read Applied Cryptography.
What's the point of the random data XOR? If it's truly random, how will you ever decrypt it? If you're saying the random data is part of the key, you might as well drop AES and use only the truly random key - as long as it's the same length (or longer than) the data and is never used more than once to encrypt. It's called a one-time pad, the only theoretically unbreakable encryption algorithm I know about.
If the random bits are pseudo-randomly generated, it's highly unlikely that your efforts will yield added security. Consider how many talented mathematicians were involved in designing AES...
EDIT: And I too highly recommend Applied Cryptography, it's an actually very readable and interesting book, not as dry as it may sound.
I realize this question might not be that programming related, and that it by many will sound like a silly question due to the intuitive logical fault of this idéa.
My question is: is it provable impossible to construct a cryptographic scheme (implementable with a turing-complete programming language) where the encrypted data can be decrypted, without exposing a decryption key to the decrypting party?
Of course, I can see the intuitive logical fault to such a scheme, but as so often with formal logic and math, a formal proof have to be constructed before assuming such a statement. Is such a proof present, or can it easely be constructed?
Thank you for advice on this one!
Edit: Thank you all for valuable input to this discussion!
YES!!! This already exists and are called zero knowledge protocols and zero knowledge proofs.
See http://en.wikipedia.org/wiki/Zero-knowledge_proof
However, you have to have a quite a good background in mathematics and crypto to understand the way it works and why it works.
One example of a zero knowledge protocol is Schnorr's ZK protocol
No; but I'm not sure you're asking what you want to be asking.
Obviously any person who is decrypting something (i.e. using a decryption key) must, obviously, have the key, otherwise they aren't decrypting it.
Are you asking about RSA, which has different keys for decrypting and encrypting? Or are you asking about a system where you may get a different (valid) result, based on the key you use?
If by "decrypted" you just mean arrive at the clear text in some way, then it is certainly possible to create such a cryptographic scheme. In fact it already exists:
Take an asymmetric encryption scheme, eg: RSA where you have the public key but not the private key. Now we get a message that's been encrypted with the public key (and therefore needs the private key to decrypt it). We can get the original message by "brute force" (yes, this'll take an enormously long time given a reasonable key/block size) going through all possible candidates and encrypting them ourselves until we get the same encrypted text. Once we get the same encrypted text we know what the decrypted text would be without ever having discovered the private key.
Yes.
Proof: Encryption can be considered as a black box, so you get an input and an output and you have no idea how the black box transforms the input to get the output.
To reverse engineer the black box, you "simply" need to enumerate all possible Turing machines until one of them does produce the same result as the one you seek.
The same applies when you want to reverse the encryption.
Granted, this will take much more time than the universe will probably live, but it's not impossible that the algorithm will find a match before time runs out.
In practice, the question is how to efficiently find the key that will decode the output. This is a much smaller problem (since you already know the algorithm).
It's called encoding.
But everyone with the encoding algorithm can "decrypt" the message. This is the only way of keyless encryption.
Triple DES or RC4?
I have the choice to employ either one.
As a high level view the following comments on both should be useful.
It is extremely easy to create a protocol based on RC4 (such as WEP) that is of extremely low strength (breakable with commodity hardware in minutes counts as extremely weak).
Triple DES is not great in that its strength comes though excessive cpu effort but it is of considerably greater strength (both theoretically in real world attacks) than RC4 so should be the default choice.
Going somewhat deeper:
In the field of encryption with no well defined target application then the definition of 'best' is inherently hard since the 'fitness' of an algorithm is multi variant.
Ease of implementation
Can you run it on commodity hardware?
Are implementations subject to accidental flaws that significantly reduce security while still allowing 'correctness' of behaviour.
Cost of implementation
Power/silicon/time to encode/decode.
Effort to break
Brute Force resilience. Pretty quantifiable
Resistance to cryptanalysis, less quantifiable, you might think so but perhaps the right person hasn't had a try yet:)
Flexibility
Can you trade off one of the above for another
What's the maximum key size (thus upper limits of the Brute Force)
What sort of input size is required to get decent encryption, does it require salting.
Actually working out the effort to break itself requires a lot of time and effort, which is why you (as a non cryptographer) go with something already done rather than roll your own. It is also subject to change over time, hopefully solely as a result of improvements in the hardware available rather than fundamental flaws in the algorithm being discovered.
The core overriding concern is of course just that, is it secure? It should be noted that many older algorithms previously considered secure are no longer in that category. Some are so effectively broken that their use is simply pointless, you have no security whatsoever simply obscurity (useful but in no way comparable to real security).
Currently neither of the core algorithms of RC4 and TDES is in that category but the naive implementation of RC4 is considered extremely flawed in protocols where the message data can be forced to repeat. RC4 has several more significant theoretical flaws than TDES.
That said TDES is NOT better than RC4 in all the areas listed above. It is significantly more expensive to compute (and that expensiveness is not justified, other less costly crypto systems exist with comparable security to TDES)
Once you have a real world application you normally get one or both of the following:
Constrains on your hardware to do the task
Constraints imposed be the data you are encrypting (this is used to transmit data which needs to be kept secret only for X days... for example)
Then you can state, with tolerances and assumptions, what can achieve this (or if you simply can't) and go with that.
In the absence of any such constraints we can only give you the following:
Ease of implementation
Both have publicly available secure free implementations for almost any architecture and platform available.
RC4 implementations may not be as secure as you think if the message can be forced to repeat (see the WEP issues). Judicious use of salting may reduce this risk but this will NOT have been subject to the rigorous analysis that the raw implementations have been and as such should be viewed with suspision.
Cost of implementation
I have no useful benchmarks for RC4 (it is OLD) http://www.cryptopp.com/benchmarks.html has some useful guides to put TDES in context with RC5 which is slower than RC4 (TDES is at least an order of magnitude slower than RC4) RC4 can encrypt a stream at approximately 7 cycles per byte in a fast implementation on modern x86 processors for comparison.
Effort to break
Brute Force resilience of TDES is currently believed to be high, even in the presence of many encryption outputs.
RC4 brute force resilience is orders of magnitude lower than TDES and further is extremely low in certain modes of operation (failure to discard initial bits of stream)
Resistance to cryptanalysis, There are publicly known flaws for Triple DES but they do not reduce the effectiveness of it to realistic attack in the next decade or two, the same is not true for RC4 where several flaws are known and combined they have produced reliable attacks on several protocols based on it.
Flexibility
TDES has very little flexibility (and your library may not expose them anyway)
RC4 has a lot more flexibility (the key used to initialize it can be arbitrarily long in theory, though the library may limit this.
Based on this and your statement that you must use one or the other you should consider the RC4 implementation only if the CPU cost of TripleDES makes it unrealistic to implement in your environment or the low level of security provided by RC4 is still considerably higher than your requirements specify.
I should also point out that systems exist which are empirically better in all areas than RC4 and TDES.
The eSTREAM project is evaluating various stream cyphers in the order of 5 or less cycles per byte though the cryptanalysis work on them is not really complete.
Many faster, stronger block cyphers exist to compete with TDES. AES is probably the best known, and would be a candidate since it is of comparable (if not better) security but is much faster.
Sorry - triple DES is no longer considered best practices. AES is simply a better algorithm so if you can use it then you should. For an easy implementation, go here.
I strongly suggest that you learn more by reading up on TDES on Wikipedia. The money quote is:
"TDES is slowly disappearing from use,
largely replaced by the Advanced
Encryption Standard (AES)."
RC4 is, honestly, just not an acceptable option for any application where security is important.
Agreed -- DES is largely outdated, so unless there is a good reason to use it, go with AES. If that's not an option, TDES would be the better choice, unless you're dealing with streaming data (ie, data which cannot be broken into blocks), then RC4 is the way to go (out of the given options).
Of course, I feel like I should mention... Cryptography is really, really hard to get right, and even the strongest algorithm can be broken easily if you get something even a little wrong (see, eg, older Kerberos or WEP).
This might not be the most informative answer, but during my 4 year employment term with a very large telco, Triple DES was the encryption standard for all sensitive applications, others were simply not allowed. It was Triple DES or the application does not go live. Hope that helps.
Both are secure, well... enough. RC4 is faster so if that's important to you...
After reading other peoples answers (which are all correct), it's clear that it really depends on your context. There are so many other questions that could influence your decision. If it just needs to be fool proof, if it's not really something sensitive and you have a lot of data and speed is the factor, go for RC4.
Otherwise, if you need something a bit more secure and easier to implement or as you say "tougher to screw up" :) then go for 3DES, which is, as far as I remember, secure enough (!) till 2020-2030, or something like that.
Are those your only two options? If you can use AES (also known as Rijndael) then use it instead. DES is slow, and now considered obsolete (AES is the replacement for it).
RC4 sucks, don't use it. It's a stream cipher but you can use a block cipher instead, just pad the final block of data (Google PKCS#5 padding scheme).
Lately I've only seen DES being used in embedded devices (firmware), because the implementation is simple and it uses very little memory. Even in JavaME you can use AES.
One factor in deciding between 3DES and RC4 is language support. Java doesn't natively support RC4 and you would need to grab an open source library such as BouncyCastle to implement. MS doesn't have this same challenge.