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Is it possible to reverse a SHA-1?
I'm thinking about using a SHA-1 to create a simple lightweight system to authenticate a small embedded system that communicates over an unencrypted connection.
Let's say that I create a sha1 like this with input from a "secret key" and spice it with a timestamp so that the SHA-1 will change all the time.
sha1("My Secret Key"+"a timestamp")
Then I include this SHA-1 in the communication and the server, which can do the same calculation. And hopefully, nobody would be able to figure out the "secret key".
But is this really true?
If you know that this is how I did it, you would know that I did put a timestamp in there and you would see the SHA-1.
Can you then use those two and figure out the "secret key"?
secret_key = bruteforce_sha1(sha1, timestamp)
Note1:
I guess you could brute force in some way, but how much work would that actually be?
Note2:
I don't plan to encrypt any data, I just would like to know who sent it.
No, you cannot reverse SHA-1, that is exactly why it is called a Secure Hash Algorithm.
What you should definitely be doing though, is include the message that is being transmitted into the hash calculation. Otherwise a man-in-the-middle could intercept the message, and use the signature (which only contains the sender's key and the timestamp) to attach it to a fake message (where it would still be valid).
And you should probably be using SHA-256 for new systems now.
sha("My Secret Key"+"a timestamp" + the whole message to be signed)
You also need to additionally transmit the timestamp in the clear, because otherwise you have no way to verify the digest (other than trying a lot of plausible timestamps).
If a brute force attack is feasible depends on the length of your secret key.
The security of your whole system would rely on this shared secret (because both sender and receiver need to know, but no one else). An attacker would try to go after the key (either but brute-force guessing or by trying to get it from your device) rather than trying to break SHA-1.
SHA-1 is a hash function that was designed to make it impractically difficult to reverse the operation. Such hash functions are often called one-way functions or cryptographic hash functions for this reason.
However, SHA-1's collision resistance was theoretically broken in 2005. This allows finding two different input that has the same hash value faster than the generic birthday attack that has 280 cost with 50% probability. In 2017, the collision attack become practicable as known as shattered.
As of 2015, NIST dropped SHA-1 for signatures. You should consider using something stronger like SHA-256 for new applications.
Jon Callas on SHA-1:
It's time to walk, but not run, to the fire exits. You don't see smoke, but the fire alarms have gone off.
The question is actually how to authenticate over an insecure session.
The standard why to do this is to use a message digest, e.g. HMAC.
You send the message plaintext as well as an accompanying hash of that message where your secret has been mixed in.
So instead of your:
sha1("My Secret Key"+"a timestamp")
You have:
msg,hmac("My Secret Key",sha(msg+msg_sequence_id))
The message sequence id is a simple counter to keep track by both parties to the number of messages they have exchanged in this 'session' - this prevents an attacker from simply replaying previous-seen messages.
This the industry standard and secure way of authenticating messages, whether they are encrypted or not.
(this is why you can't brute the hash:)
A hash is a one-way function, meaning that many inputs all produce the same output.
As you know the secret, and you can make a sensible guess as to the range of the timestamp, then you could iterate over all those timestamps, compute the hash and compare it.
Of course two or more timestamps within the range you examine might 'collide' i.e. although the timestamps are different, they generate the same hash.
So there is, fundamentally, no way to reverse the hash with any certainty.
In mathematical terms, only bijective functions have an inverse function. But hash functions are not injective as there are multiple input values that result in the same output value (collision).
So, no, hash functions can not be reversed. But you can look for such collisions.
Edit
As you want to authenticate the communication between your systems, I would suggest to use HMAC. This construct to calculate message authenticate codes can use different hash functions. You can use SHA-1, SHA-256 or whatever hash function you want.
And to authenticate the response to a specific request, I would send a nonce along with the request that needs to be used as salt to authenticate the response.
It is not entirely true that you cannot reverse SHA-1 encrypted string.
You cannot directly reverse one, but it can be done with rainbow tables.
Wikipedia:
A rainbow table is a precomputed table for reversing cryptographic hash functions, usually for cracking password hashes. Tables are usually used in recovering a plaintext password up to a certain length consisting of a limited set of characters.
Essentially, SHA-1 is only as safe as the strength of the password used. If users have long passwords with obscure combinations of characters, it is very unlikely that existing rainbow tables will have a key for the encrypted string.
You can test your encrypted SHA-1 strings here:
http://sha1.gromweb.com/
There are other rainbow tables on the internet that you can use so Google reverse SHA1.
Note that the best attacks against MD5 and SHA-1 have been about finding any two arbitrary messages m1 and m2 where h(m1) = h(m2) or finding m2 such that h(m1) = h(m2) and m1 != m2. Finding m1, given h(m1) is still computationally infeasible.
Also, you are using a MAC (message authentication code), so an attacker can't forget a message without knowing secret with one caveat - the general MAC construction that you used is susceptible to length extension attack - an attacker can in some circumstances forge a message m2|m3, h(secret, m2|m3) given m2, h(secret, m2). This is not an issue with just timestamp but it is an issue when you compute MAC over messages of arbitrary length. You could append the secret to timestamp instead of pre-pending but in general you are better off using HMAC with SHA1 digest (HMAC is just construction and can use MD5 or SHA as digest algorithms).
Finally, you are signing just the timestamp and the not the full request. An active attacker can easily attack the system especially if you have no replay protection (although even with replay protection, this flaw exists). For example, I can capture timestamp, HMAC(timestamp with secret) from one message and then use it in my own message and the server will accept it.
Best to send message, HMAC(message) with sufficiently long secret. The server can be assured of the integrity of the message and authenticity of the client.
You can depending on your threat scenario either add replay protection or note that it is not necessary since a message when replayed in entirety does not cause any problems.
Hashes are dependent on the input, and for the same input will give the same output.
So, in addition to the other answers, please keep the following in mind:
If you start the hash with the password, it is possible to pre-compute rainbow tables, and quickly add plausible timestamp values, which is much harder if you start with the timestamp.
So, rather than use
sha1("My Secret Key"+"a timestamp")
go for
sha1("a timestamp"+"My Secret Key")
I believe the accepted answer is technically right but wrong as it applies to the use case: to create & transmit tamper evident data over public/non-trusted mediums.
Because although it is technically highly-difficult to brute-force or reverse a SHA hash, when you are sending plain text "data & a hash of the data + secret" over the internet, as noted above, it is possible to intelligently get the secret after capturing enough samples of your data. Think about it - your data may be changing, but the secret key remains the same. So every time you send a new data blob out, it's a new sample to run basic cracking algorithms on. With 2 or more samples that contain different data & a hash of the data+secret, you can verify that the secret you determine is correct and not a false positive.
This scenario is similar to how Wifi crackers can crack wifi passwords after they capture enough data packets. After you gather enough data it's trivial to generate the secret key, even though you aren't technically reversing SHA1 or even SHA256. The ONLY way to ensure that your data has not been tampered with, or to verify who you are talking to on the other end, is to encrypt the entire data blob using GPG or the like (public & private keys). Hashing is, by nature, ALWAYS insecure when the data you are hashing is visible.
Practically speaking it really depends on the application and purpose of why you are hashing in the first place. If the level of security required is trivial or say you are inside of a 100% completely trusted network, then perhaps hashing would be a viable option. Hope no one on the network, or any intruder, is interested in your data. Otherwise, as far as I can determine at this time, the only other reliably viable option is key-based encryption. You can either encrypt the entire data blob or just sign it.
Note: This was one of the ways the British were able to crack the Enigma code during WW2, leading to favor the Allies.
Any thoughts on this?
SHA1 was designed to prevent recovery of the original text from the hash. However, SHA1 databases exists, that allow to lookup the common passwords by their SHA hash.
Is it possible to reverse a SHA-1?
SHA-1 was meant to be a collision-resistant hash, whose purpose is to make it hard to find distinct messages that have the same hash. It is also designed to have preimage-resistant, that is it should be hard to find a message having a prescribed hash, and second-preimage-resistant, so that it is hard to find a second message having the same hash as a prescribed message.
SHA-1's collision resistance is broken practically in 2017 by Google's team and NIST already removed the SHA-1 for signature purposes in 2015.
SHA-1 pre-image resistance, on the other hand, still exists. One should be careful about the pre-image resistance, if the input space is short, then finding the pre-image is easy. So, your secret should be at least 128-bit.
SHA-1("My Secret Key"+"a timestamp")
This is the pre-fix secret construction has an attack case known as the length extension attack on the Merkle-Damgard based hash function like SHA-1. Applied to the Flicker. One should not use this with SHA-1 or SHA-2. One can use
HMAC-SHA-256 (HMAC doesn't require the collision resistance of the hash function therefore SHA-1 and MD5 are still fine for HMAC, however, forgot about them) to achieve a better security system. HMAC has a cost of double call of the hash function. That is a weakness for time demanded systems. A note; HMAC is a beast in cryptography.
KMAC is the pre-fix secret construction from SHA-3, since SHA-3 has resistance to length extension attack, this is secure.
Use BLAKE2 with pre-fix construction and this is also secure since it has also resistance to length extension attacks. BLAKE is a really fast hash function, and now it has a parallel version BLAKE3, too (need some time for security analysis). Wireguard uses BLAKE2 as MAC.
Then I include this SHA-1 in the communication and the server, which can do the same calculation. And hopefully, nobody would be able to figure out the "secret key".
But is this really true?
If you know that this is how I did it, you would know that I did put a timestamp in there and you would see the SHA-1. Can you then use those two and figure out the "secret key"?
secret_key = bruteforce_sha1(sha1, timestamp)
You did not define the size of your secret. If your attacker knows the timestamp, then they try to look for it by searching. If we consider the collective power of the Bitcoin miners, as of 2022, they reach around ~293 double SHA-256 in a year. Therefore, you must adjust your security according to your risk. As of 2022, NIST's minimum security is 112-bit. One should consider the above 128-bit for the secret size.
Note1: I guess you could brute force in some way, but how much work would that actually be?
Given the answer above. As a special case, against the possible implementation of Grover's algorithm ( a Quantum algorithm for finding pre-images), one should use hash functions larger than 256 output size.
Note2: I don't plan to encrypt any data, I just would like to know who sent it.
This is not the way. Your construction can only work if the secret is mutually shared like a DHKE. That is the secret only known to party the sender and you. Instead of managing this, a better way is to use digital signatures to solve this issue. Besides, one will get non-repudiation, too.
Any hashing algorithm is reversible, if applied to strings of max length L. The only matter is the value of L. To assess it exactly, you could run the state of art dehashing utility, hashcat. It is optimized to get best performance of your hardware.
That's why you need long passwords, like 12 characters. Here they say for length 8 the password is dehashed (using brute force) in 24 hours (1 GPU involved). For each extra character multiply it by alphabet length (say 50). So for 9 characters you have 50 days, for 10 you have 6 years, and so on. It's definitely inaccurate, but can give us an idea, what the numbers could be.
I'm trying to store user passwords in my DB using Argon2 algorithm.
This is what I obtain by using it:
$echo -n "password" | argon2 "smallsalt" -id -t 4 -m 18 -p 4
Type: Argon2id
Iterations: 4
Memory: 262144 KiB
Parallelism: 4
Hash: cb4447d91dd62b085a555e13ebcc6f04f4c666388606b2c401ddf803055f54ac
Encoded: $argon2id$v=19$m=262144,t=4,p=4$c21hbGxzYWx0$y0RH2R3WKwhaVV4T68xvBPTGZjiGBrLEAd34AwVfVKw
1.486 seconds
Verification ok
In this case, what should I store in the DB?
The "encoded" value as shown above?
The "hash" value as shown above?
Neither, but another solution?
Please, could you help me? I'm a newbie with this and I'm a little bit lost.
I'm a bit late to the party, but I disagree with the previous answers.
You should store the field: Encoded
The $argon2id$.... value.
(At least if you are using normal Argon2 libraries having the verify() function.
It does not look like the man-page for argon2 command does this, however.
Only if you are stuck with the command line, you should consider storing each field individually.)
The $argon2id$ encoded hash
The argon2 encoded hash follows the same as its older cousin bcrypt's syntax.
The encoded hash includes all you ever need to verify the hash when the user logs in.
It is most likely more future proof. When a newer and better argon2 comes along: You can upgrade your one column hashed passwords. Just like you could detect bcrypt's $2a$-hashes, and re-hash them as $argon2id$-hashes, next time the user logs in. (If you were moving from bcrypt to agron2.)
TL;DR
Store the $-encoded value encoded_hash in your database.
Use argon2.verify(password, encoded_hash) to verify that the password is correct.
Don't bother about all the values inside the hash. Let the library do that for you. :)
Neither. Save following as a single value:
algorithm ID (e.g. argon2id)
salt
number of iterations (4)
memory usage factor (18)
parallelism (4)
The output of the field "encoded" is misleading because you cannot use it as is for password check (i.e. for hash generation), e.g. m=262144 where as for password check you need the original factor m=18.
Are you going to launch an OS process each time you check password? I would discourage you from doing this. I'd suggest you use a library (C++, Java, ...). They produce a string that contains all these data concatenated and separated with "$".
I'd put the type, iterations, memory, parallelism, hash, salt and corresponding user id into separate columns and leave the encoded bit out, because it's just all the attributes joined together. If they're in separate columns then you can reference the attributes more easily than having to split and index the encoded string.
The other option is to just store the encoded string in 1 column, but as I said its more tedious to look at certain attributes, as you'd have to split the encoded string and then index it.
I had the same question and read this post while gathering some information. Now after some days and thoughts about all this, I'll personally take a different route than the accepted answer and therefore slightly disagree with it. I thought I would share my perspective so that it might help others as well.
I suppose it will depend on everyone's context. I don't think there is a one size fits all answer here. I'm sure there are situations where it is perfectly valid and even better/simpler to store the encoded string ($argon2...).
However, I would argue that depending on the context, storing the encoded string doesn't seem to be the right approach.
First of all, it makes the hashing method very obvious. It is probably not that important but for some reasons it makes me a bit more comfortable not having it ^^. But, more importantly, it means that implementation details are stored in your persistence layer (db or else). At the time of writing, argon2id is the recommended hashing mechanism by OWASP but these things can change (eventually do change...). Some day, it might be considered unsecure, or another function will be considered more secure.
As a result, I would suggest this more function "agnostic" starting point:
The hash (for argon2 -> the hex string)
The salt
The last_modified date
A string with hashing parameters (for argon2, you could put the parameters here in the form of your choosing)
The last_modified allows to know if the hash needs updating or not and the parameters allows to support the verification and update of "old" hashes.
Of course this means that you have to work a bit more in the code and can't simply use every libraries shortcuts straight away. But, I would say that this increased complexity offer more flexibility in other circumstances (like moving away from a given hashing function). As always there are no free lunch.
That's why I suppose it depends on your context and why personally I wouldn't go with the accepted answer in my situation.
PS: I'm no cryptography expert nor some devsecop guru. So feel free to contradict, enrich, agree or disagree. I just like to keep my implementation details contained ;)
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).
I am learning MD5. I found a term 'hash' in most description of MD5. I googled 'hash', but I could not find exact term of 'hash' in computer programming.
Why are we using 'hash' in computer programming? What is origin of the word??
I would say any answer must be guesswork, so I will make this a community wiki.
Hash, or hash browns, is breakfast food made from cutting potatoes into long thin strips (smaller than french fries, and shorter, but proportionally similar), then frying the mass of strips in animal or vegetable fat until browned, stuck together, and cooked. By analogy, 'hashing' a number meant turning it into another, usually smaller, number using a method which still deterministically depending on the input number.
I believe the term "hash" was first used in the context of "hash table", which was commonly used in the 1960's on mainframe-type machines. In these cases, usually an integer value with a large range is converted to a "hash table index" which is a small integer. It is important for an efficient hash table that the "hash function" be evenly distributed, or "flat."
I don't have a citation, that is how I have understood the analogy since I heard it in the 80's. Someone must have been there when the term was first applied, though.
A hash value (or simply hash), also
called a message digest, is a number
generated from a string of text. The
hash is substantially smaller than the
text itself, and is generated by a
formula in such a way that it is
extremely unlikely that some other
text will produce the same hash value.
You're refering to the "hash function". It is used to generate a unique value for a given set of parameters.
One great use of a hash is password security. Instead of saving a password in a database, you save a hash of the password.
A hash is supposed to be a unique combination of values from 00 to FF (hexadecimal) that represents a certain piece of data, be it a file or a string of bytes. It is used primarily for password storage and verification, and to test if a file is the same as another file (i.e. you hash two files, if they match, they're the same file).
Generally, any of the SHA algorithms are preferred over MD5, due to hash collisions that can occur when using it. See this Wikipedia article.
According to the Wikipedia article on hash functions, Donald Knuth in the Art of Computer Programming was able to trace the concept of hash functions back to an internal IBM memo by Hans Peter Luhn in 1953.
And just for fun, here's a scrap of overheard conversation quoted in Two Women in the Klondike: the Story of a Journey to the Gold Fields of Alaska (1899):
They'll have to keep the hash table going all day long to feed us. 'T will be a short order affair.
the hash function hashes input to a value, requires a salt value and no proof salt is needed to store. Indications are everybody says we must store the salt same time match and new still work. Mathematically related concept is bijection
adding to gabriel1836's answer, one of the important properties of hash function is that it is a one way function, which means you cannot generate the original string from its hash value.