I am asked to add a new algorithm to ssh so data is ciphered in new algorithm, any idea how to add new algorithm to ssh ?
thanks
It is possible to add some new algorithm to SSH communication, and this is done from time to time (eg. AES was added later). But the question is that you need to modify both client and server so that they both support this algorithm, otherwise it makes no sense.
I assume that you were asked to add some custom, either home-made or non-standard algorithm. So first thing I'd like to do is to warn you that the added algorithm can be weak. You need to perform at least basic search for information about this algorithm, as if it's broken, you will do completely useless and even dangerous work.
As for software modification themselves - it's a rare job to do so most likely you won't find anybody with this experience there. However the code that handles various algorithms is typical and adding new algorithm is trivial - you add one source file with algorithm implementation and then modify a bunch of places by adding one more case to switch statement.
In my career I've worked on a private fork of ssh that was sold as closed-source commercial software. Even they in all their crazy stupidity (private fork? who in their right mind uses non-Open Source encryption software? I thought our customers were completely off their rockers.) didn't add a new encryption algorithm.
It can be done though. Adding the hooks to the ssh protocol to support it isn't hard. The protocol is designed to be extensible in that way. At the beginning the client and server exchange lists of encryption algorithms they're willing to use.
This means, of course, that only a modified client and modified server will talk to eachother.
The real difficulty is OpenSSL. ssh does not use TLS/SSL, but it does use the OpenSSL encryption library. You would have to add the new algorithm to that library, and that library is a terrible beast.
Though, I suppose you could add the algorithm without adding it to OpenSSL. That might be tricky though since I think openssh may rely heavily on the way the OpenSSL APIs work. And part of how they work allows you to pass around a constant representing which algorithm you want to use and then a standard set of calls for encryption and decryption that use the constant to decide on the algorithm.
Again though, if I recall correctly, OpenSSL has an API specifically for adding new algorithms to its suite. So that may not be so hard. You will have to make sure this happens when the OpenSSL library is being initialized.
Anyway, this is a fairly vague answer, but maybe it will point you in the right direction. You should make whoever is doing this pay enormous sums of money. Stupidity that requires this level of knowledge to pull off should never come cheaply.
Related
Do I need both MD5 and SHA-1 values to be sure the downloaded file is
a) Untouched by hackers. For example, when I need to download some app's .iso via torrents
and
b) Not corrupted during technical issues? For example, some unstable network connection during download.
Or, probably, SHA-1 value will be enough for both checks?
Also, is SHA-1 (without MD5) enough to be sure that some file downloaded years ago and stored somewhere on my HDD haven't degradated?
From a security perspective MD-5 is utterly broken.
SHA-1 is considered suspicious, and avoided for most uses if at all possible. For new projects: don't use it at all.
SHA-2 (aka SHA-256, SHA-512, etc.) is still widely used for fast hashes.
SHA-3 is the future since 2012, nothing is stopping you from using it already. I see little reason not to use it for new projects.
What's the problem with older ones:
Their resistance to finding collisions is below par: This is an attacker creating 2 contents that have the same hash. These are constructed at the same time. This problem is there for MD5 and SHA-1, and it's BAD, but requires the attacker creating both versions (and then they can do a switch at any time they want undetected).
Their resistance to length extension attacks is relatively weak. This is especially true for MD5, but SHA-1 and even SHA-2 to some degree suffer from it.
When is it not a problem: to ensure your disk has not produced an error: and hash will do, even a simple CRC32 will work wonders (and I'd recommend the simpler CRC check), or a RAID array, as these can fix errors, not just detect them.
Use both ?
Well if you have to find a collision on one hash and have that same set of plaintexts also produce a collision on another hash, is probably more difficult. This approach has been used in the past, The original PGP did something like it. If I'm not mistaken it had a number of things it calculated, one of them simply the length (which would prevent the extension attack above).
So yes, it likely adds something, but the way md5 and SHA-1 and SHA-2 work internally is quite similar, and that's the worrisome part: they are too much alike to be sure just how much it adds against a highly sophisticated attacker (think the level of the NSA and their counterparts).
So why not use one of the more modern versions of SHA-2, or even better SHA-3 ? They've no known weaknesses and have been peer-reviewed heavily. As such for any commercial level use, they should be more than enough.
Refs:
https://en.wikipedia.org/wiki/Length_extension_attack
https://en.wikipedia.org/wiki/Collision_attack
https://stackoverflow.com/questions/tagged/sha-3
I am developing a Windows Form Application in C#.I have heard that one should not use built in methods and functions in code since hackers have deep understanding of such built in methods and know how to fail them Instead one should always use his/her own functions and methods and if not then call built in functions intelligently from those newly made functions.How much is that true?
A supporting example in favour of my argument is that I have seen developer always develope there own made encryption algorithm like AES,DES,RC4 and Hash functions since they believe that built in encryption algorithm have many times backdoor in them.
What?! No, no, no! Whoever told you this is just wrong.
There is a common fallacy that published source code is more vulnerable to "h4ckerz" because it is available for anyone to spot the flaws in. However, I'm glad you mentioned crypto, because this is an area where this line of reasoning really stands out as the fallacy it is.
One of the most popular questions of all time on https://security.stackexchange.com/ is about a developer (in the OP he was given the pseudonym "Dave") who shared this fear of published code. Dave, like the developer you saw, was trying to homebrew his own encryption algorithm. Here's one of the most popular comments in that thread:
Dave has a fundamentally false premise, that the security of an algorithm relies on (even partially) its obscurity - that's not the case. The security of a hashing algorithm relies on the limits of our understanding of mathematics, and, to a lesser extent, the hardware ability to brute-force it. Once Dave accepts this reality (and it really is reality, read the Wikipedia article on hashing), it's a question of who is smarter - Dave by himself, or a large group of specialists devoted to this very particular problem. (emphasis added)
As a matter of fact, as it stands now, the top two memes on Security.SE are "Don't roll your own" and "Don't be a Dave".
While this has all been about crypto, this applies in general to most open-source software. The chance that a backdoor will get found and fixed goes up with each new set of eyes laid on the code. This should be a simple and uncontroversial premise: the more people are looking for something, the higher the chance it will be found. Yes, this applies to malicious users looking for exploits. However, it also applies to power users, white hat hackers, security researchers, cryptographers, professional developers, and others working for "good", which generally (hopefully) outnumber those working for "evil". This also implicitly relies on the false premise that hackers need to see the source code to do bad things. This should be obviously false based on the sheer number of proprietary systems whose source code has never been published (various Microsoft and Adobe programs come to mind) which have been inundated with vulnerabilities for years. Maybe having source code to read makes the hacker's job easier, but maybe not -- is it easier to pore over source code looking for an attack vector or to just use scanning tools and scripts against a compiled binary?
tl;dr Don't be a Dave. Rolling your own means you have to be the best at everything to succeed, instead of taking a sampling of the best the community has to offer.
Heartbleed
In your comment, you rebut:
Then why was the Heartbleed bug in openSSL not found and corrected [earlier]?
Because no one was looking at it. That's the sad truth. Here's the difference -- what happened once someone did find it? Now tens of thousands of security researchers, crypto experts, and others are looking at it. Suppose the same kind of vulnerability existed in one of the proprietary products I mentioned earlier, which it very well could. Once it's caught (if it's caught), ask yourself:
Could the team of programmers at the company responsible benefit from the help of the entire worldwide community of security experts, cryptographers, and other analysts right now?
If a bug this critical were discovered (and that's a big if!) in your software, would you be prepared to deal with the fallout caused by your custom implementation?
Unless you know of specific failure modes or weaknesses of the built-in methods your application would use and know how to minimize or eliminate them, it is probably better to use the methods provided by the language or library designers, which will often be both more efficient and more secure than what an average programmer would come up with on the fly for a particular project.
Your example absolutely does not support your view: developing your own encryption algorithm without some serious background in the domain and review by cryptanalysts, and then employing it in security-critical code, is a recipe for disaster. Even developing your own custom implementation of an industry standard encryption algorithm can present problems, and almost certainly will if you are inexperienced at it.
I've read quite a few times how I shouldn't use cryptography if I'm not an expert. Basically both Jeff and Eric tell you the same:
Cryptography is difficult, better buy the security solution from experts than doing it yourself.
I completely agree, for a start it's incredibly difficult to perceive all possible paths an scenario might take, all the possible attacks against it and against your solution... but then When should we use it?
I will face in a few months with the task of providing a security solution to a preexisting solution we have. That is, we exchange data between servers, second phase of the project is providing good security to it. Buying a third party solution will eat up the budget anyway so ... When is it good to use cryptography for a security solution? Even if you are not a TOP expert.
Edit: To clarify due to some comments.
The project is based on data transport across network locations, the current implementation allows for a security layer to be placed before transport and we can make any changes in implementation we like (assuming reasonable changes, the architecture is well design so changes should have an acceptable impact). The question revolves around this phrase from Eric Lippert:
I don’t know nearly enough about cryptography to safely design or implement a crypto-based security system.
We're not talking about reinventing the wheel, I had in mind a certain schema when I designed the system that implied secure key exchange, encryption and decryption and some other "counter measures" (man in the middle, etc) using C# .NET and the included cryptography primitives, but I'm by no means an expert in the field so when I read that, I of course start doubting myself. Am I even capable of implementing a secure system? Would it always be parts of the system that will be insecure unless I subcontract that part?
I think this blog posting (not mine!) gives some good guidelines.
Other than that there are some things you should never do unless you're an expert. This is stuff like implementing your own crypto algorithm (or your own version of a published algorithm). It's just crazy to do that yourself! (When there's CAPI, JCE, OpenSSL, ....)
Beyond that though if you're 'inventing' anything it's almost certainly wrong. In the Coding Horror post you linked to - the main mistake to my mind is that he's doing it a very low level and you just don't need to. If you were encrypting things in Java (I'm not so familiar with .NET) you could use Jasypt which uses strong default algorithms and parameters and doesn't require you to know about ECB and CBC (though, arguably, you should anyway just because...).
There is going to be a prebuilt system for just about anything you're going to want to do with crypto. If you're storing keys then theres KeyCzar, in other cases theres Jasypt. The point is if you're doing anything 'unusual' with crypto - you shouldn't be; if you're doing something not 'unusual' then you don't need to do the crypto yourself. Don't invent a new way to store keys, generate keys from passwords, verify signatures etc - it's not necessary, it's complicated and you'll almost certainly make a mistake unless you're very very careful...
So... I don't think you necessarily need to be afraid of encrypting things but be aware that if you're specifying algorithms and parameters to those algorithms directly in your code it is probably not good. There are exceptions to any rule but as in the blog post I linked above - if you type AES into your code you're doing it wrong!
The key "take-away" from the Matasano blog post is right at the end (note that TLS is a more precise name for SSL):
THOMAS PTACEK
GPG for data at rest. TLS for data in
motion.
NATE LAWSON
You can also use Guttman's cryptlib,
which has a sane API. Or Google
Keyczar. They both have really simple
interfaces, and they try to make it
hard to do the wrong thing. What we
need are fewer libraries with higher
level interfaces. But we also need
more testing for those libraries.
The rule of thumb with cryptography isn't that you shouldn't use it if you're not an expert; rather, it's that you shouldn't re-invent the wheel unless you're an expert. In other words, use existing implementations / libraries / algorithms as much as possible. For example, don't write your own cryptographic authentication algorithm, or come up with yet another way to store keys.
As for when to use it: whenever you have data that needs to be protected from having others see it. Beyond that, it comes down to which algorithms / approaches are best: SSL vs. IPsec vs. symmetric vs. PKI, etc.
Also, a word of advice: key management is often the most challenging part of any comprehensive cryptographic solution.
You have things backwards: first you must specify your actual requirements in detail ("provide a security solution" is meaningless marketing drivel). Then you look for ways to satisfy those specific requirements; croptography will satisfy some of them.
Example of requirements that cryptography can satisfy:
Protect data sent over publich channels from spying
Protect data against tampering (or rather, detect manipulated data)
Allow servers and clients as well as users to prove their identity to each other
You need to go through the same process as for any other requirement. What is the problem being solved, what is the outcome the users are looking for, how is the solution proposed going to be supported going forward, what are the timescales involved. Sometimes there is an off the shelf solution that does the job, sometimes what you want needs to be developed as a custom solution, and sometimes you'll choose a custom solution as it will work out more cost effective than an off the shelf one.
The same is true with security requirements, but the added complexity is that to do any sort of custom solution requires additional expertise in the technical teams (development, support etc). There is also the issue that the solution may need to be not only secure but recognised as secure. This may be far easier to achieve with an off the shelf solution.
And RickNZ is absolutely right - don't forget key management. Consider this right at the outset as part of the decision making process.
The question I would start by asking, is what are you trying to achieve.
If you are trying to just secure the transmission of the data from server a to server b, then there are a number of mechanisms you could use, which would require little work, such as SSL.
However if you are trying to secure all of the data stored in the application that is a far more difficult, although if it is a requirement, then I would suggest that any cryptography, regardless of how easy to break, is better than none.
As someone who has been asked to do similar things, you face a daunting number of questions in implementing your system. There are major difference between securing a system and implementing cryptography systems.
Implementing a cryptography system is very difficult and experts routinely get it wrong, both in theory and practice. A famous theoretical failure was the knapsack cryptosystem which has been largely abandoned due to the Lenstra–Lenstra–Lovász lattice basis reduction algorithm. On the other side, we saw in the last year how an incorrect seed in Debian's random number generator opened up any key generated by the OS. You want to use a prepackaged cryptosystem, not because its an "experts-only" field, but because you want a community tested and supported system. Almost every cryptographic algorithm I know of has bounds that assume certain tasks to be hard, and if those tasks turn out to be computable (as in the LLL algorithm) the whole system becomes useless over night.
But, I believe, the real heart of the question is how to use things in order to make a secure system. While there are many libraries out there to generate keys, cipher the text, and so on, there are very few systems that implement the entire package. But as always security boils down to two concepts: worth of protection and circle of trust.
If you are guarding the Hope diamond, you spend a lot of money designing a system to protect it, employ a constant force to watch it, and hire crackers to continually try to break in. If you are just discouraging bored teenagers from reading your email, you hack something up in an hour and you don't use that address for secret company documents.
Additionally managing the circle of trust is just as difficult of a task. If your circle includes tech savvy, like-minded friends, you make a system and give them a large amount of trust with the system. If it includes many levels of trust, such as users, admins, and so on, you have a tiered system. Since you have to manage more and more interactions with a larger circle, the bugs in the larger system become more weaknesses to hack and thus you must be extremely careful in designing this system.
Now to answer your question. You hire a security expert the moment the item you're protecting is valuable enough and your circle of trust includes those you cannot trust. You don't design cryptography systems unless you do it for a living and have a community to break them, it is a full time academic discipline. If you want to hack for fun, remember that it is only for fun and don't let the value of what you are protecting get too high.
Pay for security (of which cryptography is a part but only a part) what it is worth but no more. So your first task is to decide what your security is worth, or or how much various states of security are worth. Then invite whoever holds the budget to select which state to aim for and therefore how much to spend.
No absolutes here, it's all relative.
Why buy cryptography? It's one of the most developed area in open source software of great quality :) See for example TrueCrypt or OpenSSL
There is a good chance that whatever you need cryptography for there is already a good quality, reputable open source project for it! (And if you can see the source you can see what they did; I once saw an article about a commercial software supposed to "encrypt" a file that simply xorred every byte with a fixed value!)
And, also, why would you want to re-invent the wheel? It's unlikely that with no cryptography background you will do better or even come close to the current algorithms such as AES.
I think it totally depends on what you are trying to achieve.
Does the data need to be stored encrypted at either end or does it just need to be encrypted whilst in transit?
How are you transferring the data? FTP, HTTP etc?
Probably not a good idea to have security as a second phase as by that point presumably you've been moving data around insecurely for a period of time?
I'm modifying existing security code. The specifications are pretty clear, there is example code, but I'm no cryptographic expert. In fact, the example code has a disclaimer saying, in effect, "Don't use this code verbatim."
While auditing the code I'm to modify (which is supposedly feature complete) I ran across this little gem which is used in generating the challenge:
static uint16 randomSeed;
...
uint16 GetRandomValue(void)
{
return randomSeed++;/* This is not a good example of very random generation :o) */
}
Of course, the first thing I immediately did was pass it around the office so we could all get a laugh.
The programmer who produced this code knew it wasn't a good algorithm (as indicated by the comment), but I don't think they understood the security implications. They didn't even bother to call it in the main loop so it would at least turn into a free running counter - still not ideal, but worlds beyond this.
However, I know that the code I produce is going to similarly cause a real security guru to chuckle or quake.
What are the most common security problems, specific to cryptography, that I need to understand?
What are some good resources that will give me suitable knowledge about what I should know beyond common mistakes?
-Adam
Don't try to roll your own - use a standard library if at all possible. Subtle changes to security code can have a huge impact that aren't easy to spot, but can open security holes. For example, two modified lines to one library opened a hole that wasn't readily apparent for quite some time.
Applied Cryptography is an excellent book to help you understand crypto and code. It goes over a lot of fundamentals, like how block ciphers work, and why choosing a poor cipher mode will make your code useless even if you're using a perfectly implemented version of AES.
Some things to watch out for:
Poor Sources of Randomness
Trying to design your own algorithm or protocol - don't do it, ever.
Not getting it code reviewed. Preferably by publishing it online.
Not using a well established library and trying to write it yourself.
Crypto as a panacea - encrypting data does not magically make it safe
Key Management. These days it's often easier to steal the key with a side-channel attack than to attack the crypto.
Your question shows one of the more common ones: poor sources of randomness. It doesn't matter if you use a 256 bit key if they bits aren't random enough.
Number 2 is probably assuming that you can design a system better than the experts. This is an area where a quality implementation of a standard is almost certainly going to be better than innovation. Remember, it took 3 major versions before SSL was really secure. We think.
IMHO, there are four levels of attacks you should be aware of:
social engineering attacks. You should train your users not to do stupid things and write your software such that it is difficult for users to do stupid things. I don't know of any good reference about this stuff.
don't execute arbitrary code (buffer overflows, xss exploits, sql injection are all grouped here). The minimal thing to do in order to learn about this is to read Writing Secure Code from someone at MS and watching the How to Break Web Software google tech talk. This should also teach you a bit about defense in depth.
logical attacks. If your code is manipulating plain-text, certificates, signatures, cipher-texts, public keys or any other cryptographic objects, you should be aware that handling them in bad ways can lead to bad things. Minimal things you should be aware about include offline&online dictionary attacks, replay attacks, man-in-the-middle attacks. The starting point to learning about this and generally a very good reference for you is http://www.soe.ucsc.edu/~abadi/Papers/gep-ieee.ps
cryptographic attacks. Cryptographic vulnerabilities include:
stuff you can avoid: bad random number generation, usage of a broken hash function, broken implementation of security primitive (e.g. engineer forgets a -1 somewhere in the code, which renders the encryption function reversible)
stuff you cannot avoid except by being as up-to-date as possible: new attack against a hash function or an encryption function (see e.g. recent MD5 talk), new attack technique (see e.g. recent attacks against protocols that send encrypted voice over the network)
A good reference in general should be Applied Cryptography.
Also, it is very worrying to me that stuff that goes on a mobile device which is probably locked and difficult to update is written by someone who is asking about security on stackoverflow. I believe your case would one of the few cases where you need an external (good) consultant that helps you get the details right. Even if you hire a security consultant, which I recommend you to do, please also read the above (minimalistic) references.
What are the most common security problems, specific to cryptography, that I need to understand?
Easy - you(1) are not smart enough to come up with your own algorithm.
(1) And by you, I mean you, me and everyone else reading this site...except for possibly Alan Kay and Jon Skeet.
I'm not a crypto guy either, but S-boxes can be troublesome when messed with (and they do make a difference). You also need a real source of entropy, not just a PRNG (no matter how random it looks). PRNGs are useless. Next, you should ensure the entropy source isn't deterministic and that it can't be tampered with.
My humble advice is: stick with known crypto algorithms, unless you're an expert and understand the risks. You could be better off using some tested, publicly-available open source / public domain code.
I need to write a small program that can detect that it has been changed. Please give me a suggestion!
Thank you.
The short answer is to create a hash or key of the program and have the program encrypt and store that key within itself. From time to time the program would make a checksum of itself and compare it against that hash/key. If there is a difference then handle it accordingly.
There are lots and lots of ways to go about this. There are lots of very smart engineers out there that know how to work around it if that is what you are trying to avoid.
The simplest way would be to use a hash function to generate a short code which is a digest of the whole program and then check this.
It would be fairly easy to debug the code and replace the hash value to subvert this.
A better way would be to generate a digital signature using your private key and with the public key in the program to check it.
This would then require changing the public key and the hash as well as understanding the program, or changing the program code itself to subvert the check.
All you can do in the case described so far is make it more difficult to subvert but it will be possible with a certain amount of effort. I'd suggest looking into cryptographic techniques and copy protection for more information to suit your specific case.
Do you mean that program 'foo' should be able to tell if some part of it was modified prior to / during run time? That's not the responsibility of the program, its the responsibility of the security hooks in the target OS.
For instance, if the installed and trusted 'foo' has signature "xyz1234" , the kernel should refuse to run a modified (or completely new) 'foo'. The same goes for 'foo' while its currently running in memory. Look up 'Trusted Path Of Execution', aka TPE to start.
A better question to ask would be how to sign your released version of 'foo', which depends upon your target platform.
try searching for "code signing"
The easiest way would be for the program to detect its own md5 and store that in a separate file, but this isn't totally secure. An MD5 + CRC might work slightly better.
Or as others here have suggested, a sha1, sha2 or sha3 which are much more secure than md5 currently.
I'd ask an external tool to do the check. This problem reminds me of the challenge to write a program that prints itself. In Bash you could do something like this:
#!/bin/bash
cat $0
which really asks for an external tool to do the job. It's kind of solving the problem by getting away from solving the problem...
The best option is going to be code signing -- either using a tool supplied by your local friendly OS (For example, If you're targeting Windows, you probably want to take a look at Authenticode where the Operating System handles the tampering), or by rolling your own option storing MD5 hashes and comparing
It is important to remember that bets are off if someone injects a thread into your process (to potentially kill your ongoing checks, etc.), or if they tamper with your compiled application to bypass said checks.
An alternative way which wasn't mentioned is to use a binary packer such as UPX.
If the binary gets changed on the disk then the unpacking code is likely to fail.
This however doesn't protect you if someone changes the binary while it is in memory.