问题
So I'm working on a supplemental web-based system required by an HR department to store and search records of former personnel. I fought the requirement, but in the end it was handed down that the system has to both enable searching by full SSN, and retrieval of full SSN. My protestations aside, taking some steps to protect this data will actually be a huge improvement over what they are doing with it right now (you don't want to know).
I have been doing a lot of research, and I think I have come up with a reasonable plan -- but like all things crypto/security related there's an awful lot of complexity, and it's very easy to make a mistake. My rough plan is as follows:
- On first time run of the application, generate a large random salt, and a 128bit AES key using RijndaelManaged
- Write out both of these into a plaintext file for emergency recovery. This file will be stored offline in a secure physcial location. The application will check for the presence of the file, and scream warnings if it is still sitting there.
- Store the salt and key securely somewhere. This is the part I don't have a great answer for. I was planning on using DPAPI -- but I don't know how secure it really is at the end of the day. Would I be better off just leaving it in plaintext and restricting filesystem access to the directory its stored in ?
- When writing a record to the database, hash the SSN along with the large salt value above to generate a field that is searchable (but not recoverable without obtaining the salt and brute forcing all possible SSNs), and AES encrypt the raw SSN value with a new IV (stored alongside) to generate a field that is retrievable (with the key/iv) but not searchable (because encrypting the same SSN twice should yield different output).
- When searching, just hash the search value with the same salt and look it up in the DB
- When retrieving, decrypt the value from the DB using the AES key/iv
Other than needing a way to store the keys in a relatively secure way (number 3 above) it seems solid enough.
Things that won't work for us:
- "Don't do any of this" Is not an option. This needs to be done, and if we don't do it they'll a) get mad at us and b) just pass all the numbers around in a plaintext document over email.
This will be internal to our network only, so we have that layer of protection at least on top of whatever is implemented here. And access to the application itself will be controlled by active directory.
Thank you for reading, and for any advice.
Update #1: I realized from the comments that it makes no sense to keep a private IV for the SSN retrieval field. I updated the plan to properly generate a new IV for each record and store it alongside the encrypted value.
Update #2: I'm removing the hardware stuff from my list of stuff we can't do. I did a bit of research, and it seems like that stuff is more accessible than I thought. Does making use of one of those USB security token things add meaningful security for key storage?
回答1:
I've had to solve a similar problem recently and have decided to use an HMAC for the hashing. This would provide more security than a simple hash, especially as you can't salt the value (otherwise it wouldn't be searchable).
Then as you say, use AES with a random salt for the reversible encryption.
It maybe that you don't need to encrypt this data but I had no choice and this seemed like a reasonable solution.
My question on IT Security https://security.stackexchange.com/questions/39017/least-insecure-way-to-encrypt-a-field-in-the-database-so-that-it-can-still-be-in
回答2:
With respect to key storage there are two methods you can use if you choose to store your AES key in the web.config. First method is to use DPAPI as you mentioned. This will encrypt your web.config application setting for that box. The other method you can use is via RSA key (check out this MSDN tutorial), this will encrypt your web.config just like DPAPI however you can use the RSA key on multiple boxes, so if the application is clustered then RSA key is better (just more complicated to setup).
As far as generating the key before you run your application not on the machine running the app this way there's no chance you're going to leave the text file in the directory. You should generate the key as follows.
- Generate a random value using RngCryptoServiceProvider
- Generate a random salt value using RngCryptoServiceProvider
- Hash the two values with PBKDF2 (Rfc2898DeriveBytes)
The reason you use the key derivation method is it protects you in case RngCryptoServiceProvider was found to be insecure for some reason which happens with random number generators.
Use AES 256 instead of AES 128, reason is these algorithms are extremely fast anyway so get the higher security it's almost free. Also make sure you're using the algorithm in CBC or CTR mode (CTR is available in the BouncyCastle library).
Now this will not give your key absolute protection if someone were able to put up a aspx file in your directory. Because that file will become part of your application it would have access to your decrypted values including your key. The reason I'm mentioning this is your network and server security will have to be top notch, so I would highly recommend you work hand-in-hand with your network security team to ensure that nobody has access to that box except the parties in the HR department that need access (Firewall not Active directory). Do NOT make this application publically accessible from the internet in any way shape or form.
You also cannot trust your HR department, someone could become a victim of a social engineering attack and end up giving away their login thus destroying your security model. So in addition to working with your network team you should integrate a two factor authentication mechanism to get into the system, highly recommend going with an actual RSA key or something similar rather than implementing TOTP. This way even if someone from the dept gives away their password because they thought they were winning a free ipad, the attacker would still need a physical device to get into the application.
Log Everything, any time someone sees a SSN make sure to log it somewhere that will be part of a permanent record that's archived on a regular basis. This will allow you to mitigate quickly. I would also put limits on how many records a person can see in a particular time frame, this way you know if someone is mining data from within your application.
Create a SQL user specifically to access this table, do not let any other user have access to the table. This will ensure that only with a particular user id and password can you view the table data.
Before deploying to a production environment you should hire a penetration testing team to test the application and see what they can get, this will go a long way to harden the application from potential attackers, and they can give you great advice on how to harden the security of the application.
回答3:
Create a new salt and IV for each record. If you need to dump the data into a report for some reason (hopefully without my SSN in it), you would be able to use the method you describe with the unique salt and IV. If you only need to search on an SSN, you could actually hash it instead of using a reversible encryption (more secure).
回答4:
I think I read somewhere once that hashing a limited set of inputs gets you absolutely nothing. A quick google turned up this SO post with similar warnings:
Hashing SSNs and other limited-domain information
I must admit that I am also no security expert, but given that the possible number of inputs is much smaller than 10^9 which any decent hacker should be able to breeze through in a matter of hours, hashing a SSN seems like you are adding a small layer of annoyance rather than an actual security/difficulty barrier.
Rather than doing it this way, could you do something else? For example, SSN's only have value to an attacker if they can associate a name to a number (since anyone can enumerate out all numbers easily enough). In that case, could you encrypt the user id that the SSN links to in such a way that would be impractical to attack? I am assuming your employees table has some sort of ID, but maybe instead of that do a hash on their email or some sort of guid? That way, even if they do get your SSN data, they would not be able to tell which employee's it is until they managed to brute force that link.
Then again, that approach is also flawed since your company may not have that many employees total. At that point it would be a relatively simple matter of guessing and checking against a company directory to attain everything. No matter how you slice it, this security flaw is going to exist if SSN's must be stored with other identifying data.
来源：https://stackoverflow.com/questions/17732933/securely-storing-and-searching-by-social-security-number