I understand that hash functions like md5 can be used to tell if two files(or sets of data) are similar or not. Even changing a single bit changes the hash value of any file. Apart from this information is there any other information when comparing two hash function like to what degree are the two files different or the location of the changes. Are there any hash functions that can used to get these information
None if the hash function is cryptographically secure.
If you are presented with two hashes coming from two files, the only thing you can tell is if the files are exactly, bit for bit, identical (same hash) or not.
Some properties of a hash function is that any final bit of the hash depends on multiple bits of the message, and that a change in a single bit in the message will result in a completely different hash, to the extent that this second hash cannot be distinguished from any other possible hash.
Even with a somewhat vulnerable hash function like md5, the main thing an attacker could do is create a second document hashing to the same final hash (a collision). Not really infer the relatedness of two documents. For this to be possible, the hash function would have to be quite weak.
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Decrypt MD5 hash [duplicate]
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Does MD5 hashes the string or it encrypt it? If it hashes it, then it's as they say a one-way hash function and original string (or data) are non-recoverable by the hash produced because it's only for authentication. Then how can we explain the online websites for MD5 decryption? I actually tried it, it gets back the original string. And here's a site that does this: http://www.md5decrypter.co.uk/
How is this possible?
MD5 is a hash algorithm, meaning that it maps an arbitrary-length string to a string of some fixed length. The intent is to make it so that it is hard to start with the output of an MD5 hash and to recover some particular input that would hash to that output. Because there are infinitely many strings and finitely many outputs, it is not an encryption function, and given just the output it's impossible to determine which input produced that output.
However, MD5 has many cryptographic weaknesses and has been superseded by a variety of other hash functions (the SHA family). I would strongly suggest not using MD5 if cryptographic security is desired, since there are much better algorithms out there.
Hope this helps!
MD5 is a cryptographic hash function. It maps a variable-length string to a 128-bit hash value. It's one-way but the code can be cracked quickly using Rainbow Tables. Not to mention the site you posted says it has
a total of just over 8.7 billion unique decrypted MD5 hashes...
so it can check against those first before it even needs to try to crack it.
They don't "decrypt", they find a string that matches your hash, which is not the same thing but when you limit yourself to common English words it could very well be.
To understand what's going on you have to consider the count of possible MD5 hashes - 2^128, which is more than the count of words in English (2^16?) but much less than all possible string values 2^(number of bits the internet has and then some)
When you convert from a smaller set into a bigger one (english->MD5) it's likely all values will be different, but the other way around isn't true.
Bottom line: use a password that isn't a string that can be found by google anywhere on the net.
Does the MD5 algorithm always generate the same output for the same string?
Is using a salt the only to produce different output?
Yes, otherwise MD5 would be useless for things like file verification. What reason would you have for non deterministic output?
Yes, MD5 always outputs the same given the same input. That's how it's used for passwords. You store the hash in the database, then when the user types their password in, it's hashed again and the two hashes are compared.
NOTE: MD5 is not recommended for hashing passwords because it's cryptographically weak. There are more suitable cryptographic hashes available, such as bcrypt. However, historically, it has been used for this purpose.
Yes, a hash algorithm always produces the same output. If you use the same salt, this will also always produce the same output for a given input.
Yes MD5 is deterministic, and this is considered a desirable characteristic for many applications of message digest functions.
As for using a salt, by that you really mean 'changing the input string in some subtle way' don't you ? And, of course, it is also a desirable characteristic of message digests that they produce (with very high probability) a different digest for a different message.
Yes. MD5 is a hash function.
This does not mean that an MD5 is unique. Multiple inputs could map to the same hash, but any given input has only one hash.
Yes. Although in some cases it creates the same hash for different strings.
I remember a guy telling me that if I let him change 4 bytes he can make a file have any checksum he wants (CRC-32).
I heard mention of salting a hash. I am wondering if someone had his file match my file would salting the MD5 or SHA-1 hash change the result so both files no longer collide? Or does it change the end hash value only?
You are mixing up two different uses of hash values:
Checksumming for guarding against random (non-malicious) errors.
Computing cryptographical message digests for storing passwords, signing messages, certificates ...
CRCs are a good choice for the first application, but totally unsuited for the second, because it is easy to compute a collision (in math-speak: CRCs are linear). This is what your friend is essentially telling you.
MD5 and SHA1 are cryptographic hashes intended for the second kind of application. However, MD5 has been cracked and SHA1 is considered weak these days. Still, even though MD5 can be cracked it takes a long time to find MD5 collisions (days to weeks).
As for salt, it makes the computation of the cryptographic hash local by mixing in some random non-secret value, this value is called the salt. This prevents computing global tables which make it easy to compute possible values (e.g. passwords) from the hash value. The computation of the tables is extremely expensive, but without salt the cost would be amortized over many cracked passwords.
The attack (against CRC-32) is irrelevant if the hash you are using is not CRC-32 - MD5 and SHA-1 are not vulnerable to that kind of attack (yet).
The current attacks against MD5 are where an attacker creates two documents with the same hash.
Salts are used for password verification - they prevent an attacker performing an offline attack against the password database - each user's password has a salt attached to the plain-text before the hashing - then a pre-computed rainbow table of plaintext <-> hashed text is useless.
Adding salt to your hash function doesn't really serve any purpose if the digest function has been compromised, because the salt will have to be made public to be used, and the attacker can adjust their file to factor this in too.
The solution to this problem is to use a secure hash function. MD5 has shown to be vulnerable to hash collision, but I believe SHA-1 has not (so far).
Salting is usually used in password hashes to avoid dictionary attacks. There are plenty of web based reverse hash dictionaries where you enter the hash (say: 1a79a4d60de6718e8e5b326e338ae533) and get back the text: "example". With salt, this becomes next to impossible. If you prepend a password with random salt, the dictionary attack become more difficult.
As for collisions, I don't think you need to worry about entire files having the same md5 or sha1 hash. it's not important. The important use of the hash is to prove the file you receive is the same as the file that was approved by someone who is an authority on the file. If you add salt to the file, you need to send the salt so the user can verify the hash.
This actually makes it easier for the attacker to spoof your file because he can provide a false salt along with the false file. The user can usually tell if the file is faked because it no longer serves the purpose it is supposed to. But how is the user supposed to know the difference between the correct salt and the attacker's salt?
One concept I've always wondered about is the use of cryptographic hash functions and values. I understand that these functions can generate a hash value that is unique and virtually impossible to reverse, but here's what I've always wondered:
If on my server, in PHP I produce:
md5("stackoverflow.com") = "d0cc85b26f2ceb8714b978e07def4f6e"
When you run that same string through an MD5 function, you get the same result on your PHP installation. A process is being used to produce some value, from some starting value.
Doesn't this mean that there is some way to deconstruct what is happening and reverse the hash value?
What is it about these functions that makes the resulting strings impossible to retrace?
The input material can be an infinite length, where the output is always 128 bits long. This means that an infinite number of input strings will generate the same output.
If you pick a random number and divide it by 2 but only write down the remainder, you'll get either a 0 or 1 -- even or odd, respectively. Is it possible to take that 0 or 1 and get the original number?
If hash functions such as MD5 were reversible then it would have been a watershed event in the history of data compression algorithms! Its easy to see that if MD5 were reversible then arbitrary chunks of data of arbitrary size could be represented by a mere 128 bits without any loss of information. Thus you would have been able to reconstruct the original message from a 128 bit number regardless of the size of the original message.
Contrary to what the most upvoted answers here emphasize, the non-injectivity (i.e. that there are several strings hashing to the same value) of a cryptographic hash function caused by the difference between large (potentially infinite) input size and fixed output size is not the important point – actually, we prefer hash functions where those collisions happen as seldom as possible.
Consider this function (in PHP notation, as the question):
function simple_hash($input) {
return bin2hex(substr(str_pad($input, 16), 0, 16));
}
This appends some spaces, if the string is too short, and then takes the first 16 bytes of the string, then encodes it as hexadecimal. It has the same output size as an MD5 hash (32 hexadecimal characters, or 16 bytes if we omit the bin2hex part).
print simple_hash("stackoverflow.com");
This will output:
737461636b6f766572666c6f772e636f6d
This function also has the same non-injectivity property as highlighted by Cody's answer for MD5: We can pass in strings of any size (as long as they fit into our computer), and it will output only 32 hex-digits. Of course it can't be injective.
But in this case, it is trivial to find a string which maps to the same hash (just apply hex2bin on your hash, and you have it). If your original string had the length 16 (as our example), you even will get this original string. Nothing of this kind should be possible for MD5, even if you know the length of the input was quite short (other than by trying all possible inputs until we find one that matches, e.g. a brute-force attack).
The important assumptions for a cryptographic hash function are:
it is hard to find any string producing a given hash (preimage resistance)
it is hard to find any different string producing the same hash as a given string (second preimage resistance)
it is hard to find any pair of strings with the same hash (collision resistance)
Obviously my simple_hash function fulfills neither of these conditions. (Actually, if we restrict the input space to "16-byte strings", then my function becomes injective, and thus is even provable second-preimage resistant and collision resistant.)
There now exist collision attacks against MD5 (e.g. it is possible to produce a pair of strings, even with a given same prefix, which have the same hash, with quite some work, but not impossible much work), so you shouldn't use MD5 for anything critical.
There is not yet a preimage attack, but attacks will get better.
To answer the actual question:
What is it about these functions that makes the
resulting strings impossible to retrace?
What MD5 (and other hash functions build on the Merkle-Damgard construction) effectively do is applying an encryption algorithm with the message as the key and some fixed value as the "plain text", using the resulting ciphertext as the hash. (Before that, the input is padded and split in blocks, each of this blocks is used to encrypt the output of the previous block, XORed with its input to prevent reverse calculations.)
Modern encryption algorithms (including the ones used in hash functions) are made in a way to make it hard to recover the key, even given both plaintext and ciphertext (or even when the adversary chooses one of them).
They do this generally by doing lots of bit-shuffling operations in a way that each output bit is determined by each key bit (several times) and also each input bit. That way you can only easily retrace what happens inside if you know the full key and either input or output.
For MD5-like hash functions and a preimage attack (with a single-block hashed string, to make things easier), you only have input and output of your encryption function, but not the key (this is what you are looking for).
Cody Brocious's answer is the right one. Strictly speaking, you cannot "invert" a hash function because many strings are mapped to the same hash. Notice, however, that either finding one string that gets mapped to a given hash, or finding two strings that get mapped to the same hash (i.e. a collision), would be major breakthroughs for a cryptanalyst. The great difficulty of both these problems is the reason why good hash functions are useful in cryptography.
MD5 does not create a unique hash value; the goal of MD5 is to quickly produce a value that changes significantly based on a minor change to the source.
E.g.,
"hello" -> "1ab53"
"Hello" -> "993LB"
"ZR#!RELSIEKF" -> "1ab53"
(Obviously that's not actual MD5 encryption)
Most hashes (if not all) are also non-unique; rather, they're unique enough, so a collision is highly improbable, but still possible.
A good way to think of a hash algorithm is to think of resizing an image in Photoshop... say you have a image that is 5000x5000 pixels and you then resize it to just 32x32. What you have is still a representation of the original image but it is much much smaller and has effectively "thrown away" certain parts of the image data to make it fit in the smaller size. So if you were to resize that 32x32 image back up to 5000x5000 all you'd get is a blurry mess. However because a 32x32 image is not that large it would be theoretically conceivable that another image could be downsized to produce the exact same pixels!
That's just an analogy but it helps understand what a hash is doing.
A hash collision is much more likely than you would think. Take a look at the birthday paradox to get a greater understanding of why that is.
As the number of possible input files is larger than the number of 128-bit outputs, it's impossible to uniquely assign an MD5 hash to each possible.
Cryptographic hash functions are used for checking data integrity or digital signatures (the hash being signed for efficiency). Changing the original document should therefore mean the original hash doesn't match the altered document.
These criteria are sometimes used:
Preimage resistance: for a given hash function and given hash, it should be difficult to find an input that has the given hash for that function.
Second preimage resistance: for a given hash function and input, it should be difficult to find a second, different, input with the same hash.
Collision resistance: for a given has function, it should be difficult to find two different inputs with the same hash.
These criterial are chosen to make it difficult to find a document that matches a given hash, otherwise it would be possible to forge documents by replacing the original with one that matched by hash. (Even if the replacement is gibberish, the mere replacement of the original may cause disruption.)
Number 3 implies number 2.
As for MD5 in particular, it has been shown to be flawed:
How to break MD5 and other hash functions.
But this is where rainbow tables come into play.
Basically it is just a large amount of values hashed separetely and then the result is saved to disk. Then the reversing bit is "just" to do a lookup in a very large table.
Obviously this is only feasible for a subset of all possible input values but if you know the bounds of the input value it might be possible to compute it.
Chinese scientist have found a way called "chosen-prefix collisions" to make a conflict between two different strings.
Here is an example: http://www.win.tue.nl/hashclash/fastcoll_v1.0.0.5.exe.zip
The source code: http://www.win.tue.nl/hashclash/fastcoll_v1.0.0.5_source.zip
The best way to understand what all the most voted answers meant is to actually try to revert the MD5 algorithm. I remember I tried to revert the MD5crypt algorithm some years ago, not to recover the original message because it is clearly impossible, but just to generate a message that would produce the same hash as the original hash. This, at least theoretically, would provide me a way to login to a Linux device that stored the user:password in the /etc/passwd file using the generated message (password) instead of using the original one. Since both messages would have the same resulting hash, the system would recognize my password (generated from the original hash) as valid. That didn't work at all. After several weeks, if I remember correctly, the use of salt in the initial message killed me. I had to produce not only a valid initial message, but a salted valid initial message, which I was never able to do. But the knowledge that I got from this experiment was nice.
As most have already said MD5 was designed for variable length data streams to be hashed to a fixed length chunk of data, so a single hash is shared by many input data streams.
However if you ever did need to find out the original data from the checksum, for example if you have the hash of a password and need to find out the original password, it's often quicker to just google (or whatever searcher you prefer) the hash for the answer than to brute force it. I have successfully found out a few passwords using this method.
Now a days MD5 hashes or any other hashes for that matter are pre computed for all possible strings and stored for easy access. Though in theory MD5 is not reversible but using such databases you may find out which text resulted in a particular hash value.
For example try the following hash code at http://gdataonline.com/seekhash.php to find out what text i used to compute the hash
aea23489ce3aa9b6406ebb28e0cda430
f(x) = 1 is irreversible. Hash functions aren't irreversible.
This is actually required for them to fulfill their function of determining whether someone possesses an uncorrupted copy of the hashed data. This brings susceptibility to brute force attacks, which are quite powerful these days, particularly against MD5.
There's also confusion here and elsewhere among people who have mathematical knowledge but little cipherbreaking knowledge. Several ciphers simply XOR the data with the keystream, and so you could say that a ciphertext corresponds to all plaintexts of that length because you could have used any keystream.
However, this ignores that a reasonable plaintext produced from the seed password is much, much more likely than another produced by the seed Wsg5Nm^bkI4EgxUOhpAjTmTjO0F!VkWvysS6EEMsIJiTZcvsh#WI$IH$TYqiWvK!%&Ue&nk55ak%BX%9!NnG%32ftud%YkBO$U6o to the extent that anyone claiming that the second was a possibility would be laughed at.
In the same way, if you're trying to decide between the two potential passwords password and Wsg5Nm^bkI4EgxUO, it's not as difficult to do as some mathematicians would have you believe.
By definition, a cryptographic hash function should not be invertible and should have the least collisions possible.
Regarding your question: it is a one way hash. The input (irrespective of length) will generate a fixed size output, which will be padded based on algo (512 bit boundary for MD5). The information is compressed (lost) and practically not possible to generate from reverse transforms.
Additional info on MD5: it is vulnerable to collisions. I have gone through this article recently,
http://www.win.tue.nl/hashclash/Nostradamus/
Open source code for crypto hash implementations (MD5 and SHA) can be found at Mozilla code.
(freebl library).
I like all the various arguments.
It is obvious the real value of hashed values is simply to provide human-unreadable placeholders for strings such as passwords.
It has no specific enhanced security benefit.
Assuming an attacker gained access to a table with hashed passwords, he/she can:
Hash a password of his/her own choice and place the results inside the password table if he/she has writing/edit rights to the table.
Generate hashed values of common passwords and test the existence of similar hashed values in the password table.
In this case weak passwords cannot be protected by the mere fact that they are hashed.