Related
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.
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 want to store and index all of my historical e-mail and news as individual message files, using some computed hash code based on the message body+headers. Then I'll index on other things as well -- for searching.
For the primary index key, my thought is to use SHA-1 for the hash algorithm and assume that there will never be any collisions (although I know that there theoretically could be).
Besides the body, what headers should I index? Or more generally, what transformations should I apply to an in-memory copy of the message prior to hashing?
Should I ignore "ReSent-*:" headers? Should I join line-broken headers into single-line headers and remove extraneous whitespace?
(The reason I want to index the messages based on some head instead of on the Message-ID header is because Message-ID headers aren't uniformly formatted.)
You should hash precisely that which constitutes uniqueness of the message. If two messages may differ by the presence of "ReSent-*:" headers but still must be considered to be the "same" message, then those headers must not be part of what is hashed. Similarly, if equal messages may differ in header syntax then you should normalize header syntax. Hash functions such as SHA-1 return the same output only if the input is eaxctly the same, every single bit of it.
Now if using Message-IDs are just enough for you, save for the formatting issue, then there is a simple way: just hash the Message-IDs. A hashed Message-ID will have your regular, fixed-size, randomized format on which you can index.
We have a system where we want to prevent the same credit card number being registered for two different accounts. As we don't store the credit card number internally - just the last four digits and expiration date - we cannot simply compare credit card numbers and expiration dates.
Our current idea is to store a hash (SHA-1) in our system of the credit card information when the card is registered, and to compare hashes to determine if a card has been used before.
Usually, a salt is used to avoid dictionary attacks. I assume we are vulnerable in this case, so we should probably store a salt along with the hash.
Do you guys see any flaws in this method? Is this a standard way of solving this problem?
Let's do a little math: Credit card numbers are 16 digits long. The first seven digits are 'major industry' and issuer numbers, and the last digit is the luhn checksum. That leaves 8 digits 'free', for a total of 100,000,000 account numbers, multiplied by the number of potential issuer numbers (which is not likely to be very high). There are implementations that can do millions of hashes per second on everyday hardware, so no matter what salting you do, this is not going to be a big deal to brute force.
By sheer coincidence, when looking for something giving hash algorithm benchmarks, I found this article about storing credit card hashes, which says:
Storing credit cards using a simple single pass of a hash algorithm, even when salted, is fool-hardy. It is just too easy to brute force the credit card numbers if the hashes are compromised.
...
When hashing credit card number, the hashing must be carefully designed to protect against brute forcing by using strongest available cryptographic hash functions, large salt values, and multiple iterations.
The full article is well worth a thorough read. Unfortunately, the upshot seems to be that any circumstance that makes it 'safe' to store hashed credit card numbers will also make it prohibitively expensive to search for duplicates.
People are over thinking the design of this, I think. Use a salted, highly secure (e.g. "computationally expensive") hash like sha-256, with a per-record unique salt.
You should do a low-cost, high accuracy check first, then do the high-cost definitive check only if that check hits.
Step 1:
Look for matches to the last 4 digits (and possibly also the exp. date, though there's some subtleties there that may need addressing).
Step 2:
If the simple check hits, use the salt, get the hash value, do the in depth check.
The last 4 digits of the cc# are the most unique (partly because it includes the LUHN check digit as well) so the percentage of in depth checks you will do that won't ultimately match (the false positive rate) will be very, very low (a fraction of a percent), which saves you a tremendous amount of overhead relative to the naive "do the hash check every time" design.
Do not store a simple SHA-1 of the credit card number, it would be way too easy to crack (especially since the last 4 digits are known). We had the same problem in my company: here is how we solved it.
First solution
For each credit card, we store the last 4 digits, the expiration date, a long random salt (50 bytes long), and the salted hash of the CC number. We use the bcrypt hash algorithm because it is very secure and can be tuned to be as CPU-intensive as you wish. We tuned it to be very expensive (about 1 second per hash on our server!). But I guess you could use SHA-256 instead and iterate as many times as needed.
When a new CC number is entered, we start by finding all the existing CC numbers that end with the same 4 digits and have the same expiration date. Then, for each matching CC, we check whether its stored salted hash matches the salted hash calculated from its salt and the new CC number. In other words, we check whether or not hash(stored_CC1_salt+CC2)==stored_CC1_hash.
Since we have roughly 100k credit cards in our database, we need to calculate about 10 hashes, so we get the result in about 10 seconds. In our case, this is fine, but you may want to tune bcrypt down a bit. Unfortunately, if you do, this solution will be less secure. On the other hand, if you tune bcrypt to be even more CPU-intensive, it will take more time to match CC numbers.
Even though I believe that this solution is way better than simply storing an unsalted hash of the CC number, it will not prevent a very motivated pirate (who manages to get a copy of the database) to break one credit card in an average time of 2 to 5 years. So if you have 100k credit cards in your database, and if the pirate has a lot of CPU, then he can can recover a few credit card numbers every day!
This leads me to the belief that you should not calculate the hash yourself: you have to delegate that to someone else. This is the second solution (we are in the process of migrating to this second solution).
Second solution
Simply have your payment provider generate an alias for your credit card.
for each credit card, you simply store whatever you want to store (for example the last 4 digits & the expiration date) plus a credit card number alias.
when a new credit card number is entered, you contact your payment provider and give it the CC number (or you redirect the client to the payment provider, and he enters the CC number directly on the payment provider's web site). In return, you get the credit card alias! That's it. Of course you should make sure that your payment provider offers this option, and that the generated alias is actually secure (for example, make sure they don't simply calculate a SHA-1 on the credit card number!). Now the pirate has to break your system plus your payment provider's system if he wants to recover the credit card numbers.
It's simple, it's fast, it's secure (well, at least if your payment provider is). The only problem I see is that it ties you to your payment provider.
Hope this helps.
PCI DSS states that you can store PANs (credit card numbers) using a strong one-way hash. They don't even require that it be salted. That said you should salt it with a unique per card value. The expiry date is a good start but perhaps a bit too short. You could add in other pieces of information from the card, such as the issuer. You should not use the CVV/security number as you are not allowed to store it. If you do use the expiry date then when the cardholder gets issued a new card with the same number it will count as a different card. This could be a good or bad thing depending on your requirements.
An approach to make your data more secure is to make each operation computationally expensive. For instance if you md5 twice it will take an attacker longer to crack the codes.
Its fairly trivial to generate valid credit card numbers and to attempt a charge through for each possible expiry date. However, it is computationally expensive. If you make it more expensive to crack your hashes then it wouldn't be worthwhile for anyone to bother; even if they had the salts, hashes and the method you used.
#Cory R. King
SHA 1 isn't broken, per se. What the article shows is that it's possible to generate 2 strings which have the same hash value in less than brute force time. You still aren't able to generate a string that equates to a SPECIFIC hash in a reasonable amount of time. There is a big difference between the two.
I believe I have found a fool-proof way to solve this problem. Someone please correct me if there is a flaw in my solution.
Create a secure server on EC2, Heroku, etc. This server will serve ONE purpose and ONLY one purpose: hashing your credit card.
Install a secure web server (Node.js, Rails, etc) on that server and set up the REST API call.
On that server, use a unique salt (1000 characters) and SHA512 it 1000 times.
That way, even if hackers get your hashes, they would need to break into your server to find your formula.
Comparing hashes is a good solution. Make sure that you don't just salt all the credit card numbers with the same constant salt, though. Use a different salt (like the expiration date) on each card. This should make you fairly impervious to dictionary attacks.
From this Coding Horror article:
Add a long, unique random salt to each password you store. The point of a salt (or nonce, if you prefer) is to make each password unique and long enough that brute force attacks are a waste of time. So, the user's password, instead of being stored as the hash of "myspace1", ends up being stored as the hash of 128 characters of random unicode string + "myspace1". You're now completely immune to rainbow table attack.
Almost a good idea.
Storing just the hash is a good idea, it has served in the password world for decades.
Adding a salt seems like a fair idea, and indeed makes a brute force attack that much harder for the attacker. But that salt is going to cost you a lot of extra effort when you actually check to ensure that a new CC is unique: You'll have to SHA-1 your new CC number N times, where N is the number of salts you have already used for all of the CCs you are comparing it to. If indeed you choose good random salts you'll have to do the hash for every other card in your system. So now it is you doing the brute force. So I would say this is not a scalable solution.
You see, in the password world a salt adds no cost because we just want to know if the clear text + salt hashes to what we have stored for this particular user. Your requirement is actually pretty different.
You'll have to weigh the trade off yourself. Adding salt doesn't make your database secure if it does get stolen, it just makes decoding it harder. How much harder? If it changes the attack from requiring 30 seconds to requiring one day you have achieved nothing -- it will still be decoded. If it changes it from one day to 30 years you have achived someting worth considering.
Yes, comparing hashes should work fine in this case.
A salted hash should work just fine. Having a salt-per-user system should be plenty of security.
SHA1 is broken. Course, there isn't much information out on what a good replacement is. SHA2?
If you combine the last 4 digits of the card number with the card holder's name (or just last name) and the expiration date you should have enough information to make the record unique. Hashing is nice for security, but wouldn't you need to store/recall the salt in order to replicate the hash for a duplicate check?
I think a good solution as hinted to above, would be to store a hash value of say Card Number, Expiration date, and name. That way you can still do quick comparisons...
Sha1 broken is not a problem here.
All broken means is that it's possible to calculate collisions (2 data sets that have the same sha1) more easily than you would expect.
This might be a problem for accepting arbitrary files based on their sha1 but it has no relevence for an internal hashing application.
If you are using a payment processor like Stripe / Braintree, let them do the "heavy lifting".
They both offer card fingerprinting that you can safely store in your db and compare later to see if a card already exists:
Stripe returns fingerprint string - see doc
Braintree returns unique_number_identifier string - see doc