If SHA-1 was tried on each and every possible 160-bit string, could it happen that it produces a hash twice, i.e. could it happen that it wont produce a certain hash? In other words, for every 160-bit string, does SHA-1 produce exactly one hash?
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I'm doing a CTF challenge where you have to try and figure out the key used for a HMAC, you get given two example strings and their hashes (which are 512 bit) and also a hint to use a number between 40 and 50. What sort of attacks can you do on HMAC, and is it possible to recover the key?
If the hash function that is being used by HMAC is cryptographically secure then the only way to solve this is to brute force the key. Hashes are one-way function where it is easy to apply the hash but very hard to do the opposite. Since the search space is indefinite, this is infeasible in the general case but you seem to have a hint about the key.
Perhaps this should be seen as a literal hint and you write a loop that starts with a key of "40" and iterate up to "50" and check if any of the the example strings match with any of the hashes. If you found a match then you check if the other pairing also leads to a match with the same key.
I encoded(sha512 hash)the password string "hello" using the salt string "world" and saved the string in a file.
hex: 2b83319d3e78544e4430c4f5621968fee8b6ffa1254678b2c6fb98f7f79ff16afee2da909a7bb741488ca3bacbbf6cec8fd226c5a52eef805ea65a352e2ece8e
base64: K4MxnT54VE5EMMT1Yhlo/ui2/6ElRniyxvuY9/ef8Wr+4tqQmnu3QUiMo7rLv2zsj9ImxaUu74Beplo1Li7Ojg==
Now in my program i have the above encoded value of salted "hello" and the fresh password string "hello". I have to again encode "hello" using same salt and compare the output. Is it possible to extract the salt from the above output?
You cannot retrieve the "salt" from a hash. A hash function is a one-way function that cannot be reversed (only brute-forced).
Since you're using SHA-512 and the output is 512-bit long (128 hex-encoded bytes), there is simply no room where something like a salt is stored. When you create hashes using additional data such as a salt, you need to either store it yourself or use a function that produces a string that encodes such additional data into the output.
If you're hashing passwords or other easily brute-forceable data, use many iterations of such a hash function, because only one iteration is not enough. It is common to use PBKDF2, bcrypt or scrypt for these use cases.
The salt, when concatenated with the user's password, in essence becomes part of the password. There is the part of this compound password you know (the salt) and the part the user knows. There is no known way to identify even a single bit of the password or the salt from the output of the better hash functions. They are, for any outside party, supposed to be indistinguishable from randomness and are not reversible.
So if you have good salt and did not store it, you will never find the concatenated string that became the compound seed for password generation. Or at least not without brute forcing it, which would take nearly forever to do.
Wikipedia says:
preimage resistance: for essentially all pre-specified outputs, it is computationally infeasible to find any input which hashes to that output, i.e., it is difficult to find any preimage x given a "y" such that h(x) = y.
second-preimage resistance: it is computationally infeasible to find any second input which has the same output as a specified input, i.e., given x, it is difficult to find a second preimage x' ≠ x such that h(x) = h(x′).
Yet, I don't understand it. Doesn't h(x′) (where x' is input) generate that y (the output), which is then compared to the same h(x)?
Say, I have a string "example". It generates the MD5 "1a79a4d60de6718e8e5b326e338ae533". Why is it different to just use the MD5 compared to doing the MD5(example)?
Ideal hashing is like taking the fingerprint of a person, it is unique, it is non-reversible (you can't get the whole person back just from the fingerprint) and it can serve as a short and simple identifier for the given person.
If we bring some of the terminology you introduced into our analogy, we see that preimage resistance refers to the hash function's ability to be non-reversible. Imagine if you could generate the likeness of a whole person from their fingerprint, aside from being really cool, this could also be very dangerous. For the same reason, hash functions must be made so that an attacker cannot find the original message that generated the hash. In that sense, hash functions are one-way in that the message generates the hash and not the other way round.
Second preimage resistance refers to a given hash function's ability to be unique. Forensic fingerprinting would be a gross waste of time if any number of individuals could share the same fingerprint (lets exclude identical twins for now. Edit: See Det's comment below). If a given hash was used for verification against data corruption, it would quite pointless if there is a good chance corrupt data can generate the same hash.
To have both preimage resistance and second preimage resistance hash functions adopt several traits to help them. One trait very common for hash functions is where the given input has no correspondence to the output. A single bit change can produce a hash that has completely no bytes shared with the hash of the original input. For this reason, a good hash function is commonly used in message authentication.
Whilst you are right comparing the original message directly would be functionally equivalent to comparing the hashes, it is simply not feasible in the majority of cases. For example:
If party A wanted to reliably send a message to party B, party A/B would need to agree upon a scheme to detect data corruption during transfer. Note: party B does not have the original message until party A sends it.
A possible scheme of transfer could be to transfer the message twice such that party B can verify if the second message equals the first. The problem with this is that there is a chance that corruption can occur twice in the same place (as well as the significantly higher bandwidth). This can only be reduced by sending the messages even more times, incurring severe bandwidth costs.
As an alternative, party A can pass his/her long message into a hash function and generate a short hash which he/she sends to party B, followed by the original message. Party B can then take the received message and pass it into the hash function and match the hashes. If either the message or the hash got corrupted even by a single bit during transfer, the resultant hashes will not match, thanks to second preimage resistance (no two plaintext should have same hash).
Preimage Resistance in this case would be useful if the message is encrypted during transfer but the hash was taken prior encryption (whether this is appropriate is another discussion). If the hash was reversible, a eavesdropper could intercept the hash and reverse to find the original message.
All hash functions are not equal, that's why its important to consider their preimage resistance/second preimage resistance when choosing which ones to use, which ones are secure and which ones should be deprecated and replaced.
You understood preimage and second preimage resistance? It says the output of a hash function is unique, at least in theory.. And obtaining the original string from a hash is "computationally" in-feasible. It is possible (brute-force) though but takes up a lot of time and resources.
Now, output of a hash function and the string itself are different.. For example, consider a website with a dashboard. You provide your user_id and password at the time of signing up. If the website stores your password as such in their database, it is accessible to a hacker. He can access your account. But if a hash of your password is stored, even if he manages to hack down the server, that hash is of no use to him. Because, he cannot access your account without your password, and it is computationally in-feasible to obtain your password from the hash (preimage resistance). Comparing md5 (yourpassword) with the hash stored in the db is different. Each time you enter your password, it is hashed with the sampe hash function and compared to the existing hash. According to second-preimage resistance, if you entered an incorrect password, the hashes won't match.
Another example of hashing is in the version control or source control mechanisms. To track down changes in a file, hashing is used. They hash the entire file and keeps it. If a file is modified, its hash changes accordingly.
These are all examples explaining what you asked.
A friend of me ask this, and i was thinking of asking this here too..
"What kind of data are this, how are they encrypted, or decrypted?"
My friend told me he got this from facebook.
d9ca6435295fcd89e85bd56c2fd51ccc
It looks like it could be an md5 hash.
Basically a hash is a one-way function. The idea is that you take some input data and run it through the algorithm to create a value (such as the string above) that has a low probability of collisions (IE, two input values hashing to the same string).
You cannot decrypt a hash because there is not enough information in the resultant string to go back. However, it may be possible for someone to figure out your input values if you use a 'weak' hashing algorithm and do not do proper techniques such as salting a hash, etc.
I don't know how FaceBook uses hashes, but a common use for a hash might be to uniquely identify a page. For example, if you had a private image on a page, you might ask to generate a link to the image that you can email to friends. That link might use a hash as part of the URL since the value can be computed quickly, is reasonably unique, and has a low probability of a third party figuring it out.
This is actually a large topic that I am by no means doing justice to. I suggest googling around for hash, md5, etc to learn more, if you are so inclinded.
It is a sequence of 128 bits, encoded as a lower-case hex string.
If you are talking about a Facebook API key, there is no deeper meaning to decode from the bits. The keys are created at random by Facebook and assigned to a particular application to identify it. Each application gets a different set of random bits for its API key.
This appears the be the...
hexadecimal representation for...
- ... a 16 bytes encryption block or..
- ... some 128 bits hash code or even
- ... just for some plain random / identifying number.
(Hexadecimal? : note how there are only 0 thru 9 digits and a thru f letters.)
While the MD5 Hash guess suggested by others is quite plausible, it could be just about anything...
If it is a hash or a identifying / randomly assigned number, its meaning is external to the code itself.
For example it could be a key to be used to locate records in a database, or a value to be compared with the result of the hash function applied to the user supplied password etc.
If it is an encrypted value, its meaning (decrypted value) is directly found within the code, but it could be just about anything. Also, assuming it is produced with modern encryption algorithm, it could take a phenomenal amount of effort to crack the code (if at all possible).
My original Text : "sanjay"
SHA-1 Text : "25ecbcb559d14a98e4665d6830ac5c99991d7c25"
Now how can i get original value - "sanjay" from this hash value ?
is there any code or algorithm or method?
No. That's usually the point -- the process of hashing is normally one-way.
This is especially important for hashes designed for passwords or cryptology -- which differ from hashes designed, for say, hash-maps. Also, with an unbounded input length, there is an infinite amount of values which result in the same hash.
One method that can be used is to hash a bunch of values (e.g. brute-force from aaaaaaaa-zzzzzzz) and see which value has the same hash. If you have found this, you have found "the value" (the time is not cheap). "Rainbow tables" work on this idea (but use space instead of time), but are defeated with a nonce salt.
From what I've been taught on the subject, if you were the one that turned your value into a hash value, chances are you have full access to the hash function, and would be able to reverse it in the same way. If you only have the original value and the end value, and don't know what hash function was used, you can't really reverse it without doing what was said above (going over every possibility).