I got the following problem:
In an Java Application I want to store some configuration data in an encrypted, local file. This file might be used for confidential data, like user credentials.
This file should be accessible by using a password (and only a password).
Now most trustworthy people and reference implementations use random salts. I completely understand that this is a reasonable choice. But if my application terminates and will be started later, the random salt is not available anymore. This application is stand-alone so no additional database could be used as a salt store.
For my software the user shall only type in the password (means: no user name, no salt, no favorite animal or colour).
Now my idea was deriving the salt from the password (e.g. by using the first 16 bytes of SHA-256).
My questions are:
How (in)secure would this implementation be?
What is a common way of encrypting stuff with only a password and would be a better alternative?
What is not the aim of this question:
Where to store salts
Secure algorithms and crypto implementation (of course, I did not implement crypto by myself)
Architectural improvements (nop, I do not want a global database for storing stuff)
First, I strongly recommend against devising novel encryption formats if you can help it. It is very difficult to do them correctly. If you want an encryption format that does what you're describing, see JNCryptor, which is an implementation of the RNCryptor format. The RNCryptor format is designed precisely for this problem, so the spec is a good source of information on how you could create your own if you don't want to use it directly. (I'm the author of RNCryptor.)
See also libsodium. It's a better encryption format than RNCryptor for various technical reasons, but it's a bit harder to install and use correctly. There are several Java bindings for libsodium.
When you say "of course, I did not implement crypto by myself," that's what you're doing. Crypto schemes are more than just the AES code. Deciding how to generate the salts in a novel way is implementing crypto. There are many ways to put together secure primitives (like salts) in simple ways and make them wildly insecure. That's why you want to use something well established.
The key take-away is that you store the salt with the data. I know you said this isn't about storing the salt, but that's how you do this. The simplest way to do this is to just glue the salt onto the start of the cipher text and store that. Then you just read the salt from the header. Similarly, you could put the whole thing in an envelope if that's more convenient. Something as simple as JSON:
{ "salt": "<base64-salt>",
"data": "<base64-data>" }
It's not the most efficient way to store the data, but it's easy, standard, and secure.
Remember, salts are not secrets. It is fine that everyone can read the salt.
OK, enough of how to do it right. Let's get to your actual question.
Your salting proposal is not a salt. It's just a slightly different hashing function. The point of a salt is if the same password is used twice (without intending to be the same password), then they will have different hashes. Your scheme fails that. If I implement the same approach as you do, and I pick the same password as yours, then the hash will be the same. Rainbow tables win.
The way you fix that is with a static salt, not a modified hash function. You should pick a salt that represents your system. I usually like reverse DNS for this, because it leads to uniqueness. For example: "com.example.mygreatapp". Someone else would naturally pick "org.example.ourawesomedb". You also can pick a long random string, but the important thing is uniqueness, so I like reverse DNS. (Random strings tend to make people think the salt is a secret, and the salt is not a secret.)
That's the whole system; just pick some constant salt, unique to your system. (If you had a username, you'd add the username to the salt. This is a standard way to construct a deterministic salt.)
But for file storage, I'd never do it that way.
To encrypt data, one needs a key not a password. There are key-derivation-functions to get a key from a user password.
A salt can be used for password hashing, but it cannot be used for encrypting data. There is a similar concept for encryption though, the random values there are called IV or Nonce and are stored together with the encrypted data.
The best thing you can do is
use a key-derivation-function with a salt, to get a key from the password.
With the resulting key you can encrypt the data.
In this case the salt can be stored inside the encrypted data container (the IV is already there), so there is no need for a global database.
To answer your original question: Derriving a salt from the password negates the whole purpose of the salt, it just becomes a more complex hash function.
I'd like to incorporate the encryption and decryption of files in one of my C# .NET apps. The scenario is simple: User A sends an AES256-encrypted file to user B. The clear text password is exchanged on a different channel (e.g. phone call or whatever).
From what I understand I should use Rfc2898DeriveBytes for converting the user's clear text password into a more secure password using maybe 10,000 rounds. (see this article).
What I don't understand is the role of salt in my scenario. Usually salt is used in hashing passwords to prevent dictionary attacks. But in my scenario the PBKDF2 algo is used to compensate weaknesses of short or easy to guess clear text passwords by adding extra calculations required by the PBKDF2-rounds.
If I choose a random salt then the receiver will need to know that salt also in order to decrypt correctly. If I use a constant salt, then hackers can easily reverse engineer my code and run brute force attacks using my constant salt (although they'll be really slow thanks to the PBKDF2 iterations).
From what I understand I have no choice but to use a constant salt in my scenario and enforce a good clear text password rule to make up for the weakness of constant salt. Is my assumption correct?
Salts, in the context of password hashing (and key derivation), are used to prevent precomputation attacks like rainbow tables.
Note that the salt must be different and unpredictable (preferably random) for every password. Also note that salts need not be secret – that's what the password is for. You gain no security by keeping the salt secret.
The recommended approach in your case is to generate a random salt every time a file is encrypted, and transmit the salt along with the ciphertext.
Is there a specific reason you're using AES-256 by the way? It's around 40% slower than AES-128 due to the extra rounds, and it offers no practical security benefit (particularly not in the case of password-based encryption).
It's also worth considering using a well-established standard like PGP rather than building your own protocol from cryptographic primitives, because building secure protocols is so hard that even experts don't always get it right.
Your assumption is correct. If they have access to the password, they will also have access to the salt. The BCrypt implementations I've seen put the number of iterations, the hash, and the salt all in the same result string!
The idea is: your hash should be secure even if the salt and number if iterations is known. (If we could always know that the salt and number of iterations and even the algorithm would be unknown to attackers, security would get a whole heck of a lot easier! Until attackers politely decline to read our salts, we must assume they will have access to them in the event of a breach.) So you're right, they can brute force it - if they have a few supercomputers and a couple million years of computing time at their disposal.
We would like to cryptographically (SHA-256) hash a secret value in our database. Since we want to use this as a way to lookup individual records in our database, we cannot use a different random salt for each encrypted value.
My question is: given unlimited access to our database, and given that the attacker knows at least one secret value and hashed value pair, is it possible for the attacker to reverse engineer the cryptographic key? IE, would the attacker then be able to reverse all hashes and determine all secret values?
It seems like this defeats the entire purpose of a cryptographic hash if it is the case, so perhaps I'm missing something.
There are no published "first pre-image" attacks against SHA-256. Without such an attack to open a shortcut, it is impossible for an attacker to the recover a secret value from its SHA-256 hash.
However, the mention of a "secret key" might indicate some confusion about hashes. Hash algorithms don't use a key. So, if an attacker were able to attack one "secret-value–hash-value" pair, he wouldn't learn a "key" that would enable him to easily invert the rest of the hash values.
When a hash is attacked successfully, it is usually because the original message was from a small space. For example, most passwords are chosen from a relatively short list of real words, perhaps with some simple permutations. So, rather than systematically testing every possible password, the attacker starts with an ordered list of the few billion most common passwords. To avoid this, it's important to choose the "secret value" randomly from a large space.
There are message authentication algorithms that hash a secret key together with some data. These algorithms are used to protect the integrity of the message against tampering. But they don't help thwart pre-image attacks.
In short, yes.
No, a SHA hash is not reversible (at least not easily). When you Hash something if you need to reverse it you need to reconstruct the hash. This is usually done with a private (salt) and public key.
For example, if I'm trying to prevent access based off my user id. I would hash my user id and the salt. Let say MD5 for example. My user id is "12345" and the salt is "abcde"
So I will hash the string "12345_abcde", which return a hash of "7b322f78afeeb81ad92873b776558368"
Now I will pass to the validating application the hash and the public key, "12345" which is the public key and the has.
The validating application, knows the salt, so it hashes the same values. "12345_abcde", which in turn would generate the exact same hash. I then compare the hash i validated with the one passed off and they match. If I had somehow modified the public key without modifying the hash, a different has would have been generated resulting in a mismatch.
Yes it's possible, but not in this lifetime.
Modern brute-force attacks using multiple GPUs could crack this in short order. I recommend you follow the guidelines for password storage for this application. Here are the current password storage guidelines from OWASP. Currently, they recommend a long salt value, and PBKDF2 with 64,000 iterations, which iteratively stretches the key and makes it computationally complex to brute force the input values. Note that this will also make it computationally complex for you to generate your key values, but the idea is that you will be generating keys far less frequently than an attacker would have to. That said, your design requires many more key derivations than a typical password storage/challenge application, so your design may be fatally flawed. Also keep in mind that the iteration count should doubled every 18 months to make the computational complexity follow Moore's Law. This means that your system would need some way of allowing you to rehash these values (possibly by combining hash techniques). Over time, you will find that old HMAC functions are broken by cryptanalysts, and you need to be ready to update your algorithms. For example, a single iteration of MD5 or SHA-1 used to be sufficient, but it is not anymore. There are other HMAC functions that could also suit your needs that wouldn't require PBKDF2 (such as bcrypt or scrypt), but PBKDF2 is currently the industry standard that has received the most scrutiny. One could argue that bcrypt or scrypt would also be suitable, but this is yet another reason why a pluggable scheme should be used to allow you to upgrade HMAC functions over time.
Someone told me that he has seen software systems that:
retrieve MD5 encrypted passwords from other systems;
decrypt the encrypted passwords and
store the passwords in the database of the system using the systems own algorithm.
Is that possible? I thought that it wasn't possible / feasible to decrypt MD5 hashes.
I know there are MD5 dictionaries, but is there an actual decryption algorithm?
No. MD5 is not encryption (though it may be used as part of some encryption algorithms), it is a one way hash function. Much of the original data is actually "lost" as part of the transformation.
Think about this: An MD5 is always 128 bits long. That means that there are 2128 possible MD5 hashes. That is a reasonably large number, and yet it is most definitely finite. And yet, there are an infinite number of possible inputs to a given hash function (and most of them contain more than 128 bits, or a measly 16 bytes). So there are actually an infinite number of possibilities for data that would hash to the same value. The thing that makes hashes interesting is that it is incredibly difficult to find two pieces of data that hash to the same value, and the chances of it happening by accident are almost 0.
A simple example for a (very insecure) hash function (and this illustrates the general idea of it being one-way) would be to take all of the bits of a piece of data, and treat it as a large number. Next, perform integer division using some large (probably prime) number n and take the remainder (see: Modulus). You will be left with some number between 0 and n. If you were to perform the same calculation again (any time, on any computer, anywhere), using the exact same string, it will come up with the same value. And yet, there is no way to find out what the original value was, since there are an infinite number of numbers that have that exact remainder, when divided by n.
That said, MD5 has been found to have some weaknesses, such that with some complex mathematics, it may be possible to find a collision without trying out 2128 possible input strings. And the fact that most passwords are short, and people often use common values (like "password" or "secret") means that in some cases, you can make a reasonably good guess at someone's password by Googling for the hash or using a Rainbow table. That is one reason why you should always "salt" hashed passwords, so that two identical values, when hashed, will not hash to the same value.
Once a piece of data has been run through a hash function, there is no going back.
You can't - in theory. The whole point of a hash is that it's one way only. This means that if someone manages to get the list of hashes, they still can't get your password. Additionally it means that even if someone uses the same password on multiple sites (yes, we all know we shouldn't, but...) anyone with access to the database of site A won't be able to use the user's password on site B.
The fact that MD5 is a hash also means it loses information. For any given MD5 hash, if you allow passwords of arbitrary length there could be multiple passwords which produce the same hash. For a good hash it would be computationally infeasible to find them beyond a pretty trivial maximum length, but it means there's no guarantee that if you find a password which has the target hash, it's definitely the original password. It's astronomically unlikely that you'd see two ASCII-only, reasonable-length passwords that have the same MD5 hash, but it's not impossible.
MD5 is a bad hash to use for passwords:
It's fast, which means if you have a "target" hash, it's cheap to try lots of passwords and see whether you can find one which hashes to that target. Salting doesn't help with that scenario, but it helps to make it more expensive to try to find a password matching any one of multiple hashes using different salts.
I believe it has known flaws which make it easier to find collisions, although finding collisions within printable text (rather than arbitrary binary data) would at least be harder.
I'm not a security expert, so won't make a concrete recommendation beyond "Don't roll your own authentication system." Find one from a reputable supplier, and use that. Both the design and implementation of security systems is a tricky business.
Technically, it's 'possible', but under very strict conditions (rainbow tables, brute forcing based on the very small possibility that a user's password is in that hash database).
But that doesn't mean it's
Viable
or
Secure
You don't want to 'reverse' an MD5 hash. Using the methods outlined below, you'll never need to. 'Reversing' MD5 is actually considered malicious - a few websites offer the ability to 'crack' and bruteforce MD5 hashes - but all they are are massive databases containing dictionary words, previously submitted passwords and other words. There is a very small chance that it will have the MD5 hash you need reversed. And if you've salted the MD5 hash - this won't work either! :)
The way logins with MD5 hashing should work is:
During Registration:
User creates password -> Password is hashed using MD5 -> Hash stored in database
During Login:
User enters username and password -> (Username checked) Password is hashed using MD5 -> Hash is compared with stored hash in database
When 'Lost Password' is needed:
2 options:
User sent a random password to log in, then is bugged to change it on first login.
or
User is sent a link to change their password (with extra checking if you have a security question/etc) and then the new password is hashed and replaced with old password in database
Not directly. Because of the pigeonhole principle, there is (likely) more than one value that hashes to any given MD5 output. As such, you can't reverse it with certainty. Moreover, MD5 is made to make it difficult to find any such reversed hash (however there have been attacks that produce collisions - that is, produce two values that hash to the same result, but you can't control what the resulting MD5 value will be).
However, if you restrict the search space to, for example, common passwords with length under N, you might no longer have the irreversibility property (because the number of MD5 outputs is much greater than the number of strings in the domain of interest). Then you can use a rainbow table or similar to reverse hashes.
Not possible, at least not in a reasonable amount of time.
The way this is often handled is a password "reset". That is, you give them a new (random) password and send them that in an email.
You can't revert a md5 password.(in any language)
But you can:
give to the user a new one.
check in some rainbow table to maybe retrieve the old one.
No, he must have been confused about the MD5 dictionaries.
Cryptographic hashes (MD5, etc...) are one way and you can't get back to the original message with only the digest unless you have some other information about the original message, etc. that you shouldn't.
Decryption (directly getting the the plain text from the hashed value, in an algorithmic way), no.
There are, however, methods that use what is known as a rainbow table. It is pretty feasible if your passwords are hashed without a salt.
MD5 is a hashing algorithm, you can not revert the hash value.
You should add "change password feature", where the user gives another password, calculates the hash and store it as a new password.
There's no easy way to do it. This is kind of the point of hashing the password in the first place. :)
One thing you should be able to do is set a temporary password for them manually and send them that.
I hesitate to mention this because it's a bad idea (and it's not guaranteed to work anyway), but you could try looking up the hash in a rainbow table like milw0rm to see if you can recover the old password that way.
See all other answers here about how and why it's not reversible and why you wouldn't want to anyway.
For completeness though, there are rainbow tables which you can look up possible matches on. There is no guarantee that the answer in the rainbow table will be the original password chosen by your user so that would confuse them greatly.
Also, this will not work for salted hashes. Salting is recommended by many security experts.
No, it is not possible to reverse a hash function such as MD5: given the output hash value it is impossible to find the input message unless enough information about the input message is known.
Decryption is not a function that is defined for a hash function; encryption and decryption are functions of a cipher such as AES in CBC mode; hash functions do not encrypt nor decrypt. Hash functions are used to digest an input message. As the name implies there is no reverse algorithm possible by design.
MD5 has been designed as a cryptographically secure, one-way hash function. It is now easy to generate collisions for MD5 - even if a large part of the input message is pre-determined. So MD5 is officially broken and MD5 should not be considered a cryptographically secure hash anymore. It is however still impossible to find an input message that leads to a hash value: find X when only H(X) is known (and X doesn't have a pre-computed structure with at least one 128 byte block of precomputed data). There are no known pre-image attacks against MD5.
It is generally also possible to guess passwords using brute force or (augmented) dictionary attacks, to compare databases or to try and find password hashes in so called rainbow tables. If a match is found then it is computationally certain that the input has been found. Hash functions are also secure against collision attacks: finding X' so that H(X') = H(X) given H(X). So if an X is found it is computationally certain that it was indeed the input message. Otherwise you would have performed a collision attack after all. Rainbow tables can be used to speed up the attacks and there are specialized internet resources out there that will help you find a password given a specific hash.
It is of course possible to re-use the hash value H(X) to verify passwords that were generated on other systems. The only thing that the receiving system has to do is to store the result of a deterministic function F that takes H(X) as input. When X is given to the system then H(X) and therefore F can be recalculated and the results can be compared. In other words, it is not required to decrypt the hash value to just verify that a password is correct, and you can still store the hash as a different value.
Instead of MD5 it is important to use a password hash or PBKDF (password based key derivation function) instead. Such a function specifies how to use a salt together with a hash. That way identical hashes won't be generated for identical passwords (from other users or within other databases). Password hashes for that reason also do not allow rainbow tables to be used as long as the salt is large enough and properly randomized.
Password hashes also contain a work factor (sometimes configured using an iteration count) that can significantly slow down attacks that try to find the password given the salt and hash value. This is important as the database with salts and hash values could be stolen. Finally, the password hash may also be memory-hard so that a significant amount of memory is required to calculate the hash. This makes it impossible to use special hardware (GPU's, ASIC's, FPGA's etc.) to allow an attacker to speed up the search. Other inputs or configuration options such as a pepper or the amount of parallelization may also be available to a password hash.
It will however still allow anybody to verify a password given H(X) even if H(X) is a password hash. Password hashes are still deterministic, so if anybody has knows all the input and the hash algorithm itself then X can be used to calculate H(X) and - again - the results can be compared.
Commonly used password hashes are bcrypt, scrypt and PBKDF2. There is also Argon2 in various forms which is the winner of the reasonably recent password hashing competition. Here on CrackStation is a good blog post on doing password security right.
It is possible to make it impossible for adversaries to perform the hash calculation verify that a password is correct. For this a pepper can be used as input to the password hash. Alternatively, the hash value can of course be encrypted using a cipher such as AES and a mode of operation such as CBC or GCM. This however requires the storage of a secret / key independently and with higher access requirements than the password hash.
MD5 is considered broken, not because you can get back the original content from the hash, but because with work, you can craft two messages that hash to the same hash.
You cannot un-hash an MD5 hash.
There is no way of "reverting" a hash function in terms of finding the inverse function for it. As mentioned before, this is the whole point of having a hash function. It should not be reversible and it should allow for fast hash value calculation. So the only way to find an input string which yields a given hash value is to try out all possible combinations. This is called brute force attack for that reason.
Trying all possible combinations takes a lot of time and this is also the reason why hash values are used to store passwords in a relatively safe way. If an attacker is able to access your database with all the user passwords inside, you loose in any case. If you have hash values and (idealistically speaking) strong passwords, it will be a lot harder to get the passwords out of the hash values for the attacker.
Storing the hash values is also no performance problem because computing the hash value is relatively fast. So what most systems do is computing the hash value of the password the user keyed in (which is fast) and then compare it to the stored hash value in their user database.
You can find online tools that use a dictionary to retrieve the original message.
In some cases, the dictionary method might just be useless:
if the message is hashed using a SALT message
if the message is hash more than once
For example, here is one MD5 decrypter online tool.
The only thing that can be work is (if we mention that the passwords are just hashed, without adding any kind of salt to prevent the replay attacks, if it is so you must know the salt)by the way, get an dictionary attack tool, the files of many words, numbers etc. then create two rows, one row is word,number (in dictionary) the other one is hash of the word, and compare the hashes if matches you get it...
that's the only way, without going into cryptanalysis.
The MD5 Hash algorithm is not reversible, so MD5 decode in not possible, but some website have bulk set of password match, so you can try online for decode MD5 hash.
Try online :
MD5 Decrypt
md5online
md5decrypter
Yes, exactly what you're asking for is possible.
It is not possible to 'decrypt' an MD5 password without help, but it is possible to re-encrypt an MD5 password into another algorithm, just not all in one go.
What you do is arrange for your users to be able to logon to your new system using the old MD5 password. At the point that they login they have given your login program an unhashed version of the password that you prove matches the MD5 hash that you have. You can then convert this unhashed password to your new hashing algorithm.
Obviously, this is an extended process because you have to wait for your users to tell you what the passwords are, but it does work.
(NB: seven years later, oh well hopefully someone will find it useful)
No, it cannot be done. Either you can use a dictionary, or you can try hashing different values until you get the hash that you are seeking. But it cannot be "decrypted".
MD5 has its weaknesses (see Wikipedia), so there are some projects, which try to precompute Hashes. Wikipedia does also hint at some of these projects. One I know of (and respect) is ophrack. You can not tell the user their own password, but you might be able to tell them a password that works. But i think: Just mail thrm a new password in case they forgot.
In theory it is not possible to decrypt a hash value but you have some dirty techniques for getting the original plain text back.
Bruteforcing: All computer security algorithm suffer bruteforcing. Based on this idea today's GPU employ the idea of parallel programming using which it can get back the plain text by massively bruteforcing it using any graphics processor. This tool hashcat does this job. Last time I checked the cuda version of it, I was able to bruteforce a 7 letter long character within six minutes.
Internet search: Just copy and paste the hash on Google and see If you can find the corresponding plaintext there. This is not a solution when you are pentesting something but it is definitely worth a try. Some websites maintain the hash for almost all the words in the dictionary.
MD5 is a cryptographic (one-way) hash function, so there is no direct way to decode it. The entire purpose of a cryptographic hash function is that you can't undo it.
One thing you can do is a brute-force strategy, where you guess what was hashed, then hash it with the same function and see if it matches. Unless the hashed data is very easy to guess, it could take a long time though.
It is not yet possible to put in a hash of a password into an algorithm and get the password back in plain text because hashing is a one way thing. But what people have done is to generate hashes and store it in a big table so that when you enter a particular hash, it checks the table for the password that matches the hash and returns that password to you. An example of a site that does that is http://www.md5online.org/ . Modern password storage system counters this by using a salting algorithm such that when you enter the same password into a password box during registration different hashes are generated.
No, you can not decrypt/reverse the md5 as it is a one-way hash function till you can not found a extensive vulnerabilities in the MD5.
Another way is there are some website has a large amount of set of password database, so you can try online to decode your MD5 or SHA1 hash string.
I tried a website like http://www.mycodemyway.com/encrypt-and-decrypt/md5 and its working fine for me but this totally depends on your hash if that hash is stored in that database then you can get the actual string.