Encrypt RSA Private Key with AES 256 in Java - java
I am writing a secure file sharing application in Java. The general architecture looks like this:
User wishes to encrypt a file for secure sharing between multiple users.
The application generates a random UUID on the client and uses this as the AES 256 password, and encrypts the data with the UUID.
The UUID is then RSA encrypted with each person's public key. Once per shared user.
Each encrypted UUID packet is stored as part of the file in a custom file header.
The file is then uploaded to a server where others can access it.
The user's can each use their private key to read the AES encryption key and decrypt the file.
Here is the catch. The user's private key must be encrypted and stored on our servers in our database so that the files can be accessed from multiple locations. The private key will be encrypted with a user selected password on the client prior to being uploaded to the server.
I would like to do this using AES 256 bit encryption. And I would like to do the entire thing without relying on BouncyCastle libraries or any 3rd party libraries. It needs to use the standard Java 5 libraries, which is why I have chosen to use AES 256 encryption and RSA rather than something like PGP.
Can anyone find anything inherently insecure with this approach, or think of a more efficient way to do this?
Edit:
OK, I'm updating the question because all of the answers I am getting are suggesting that I not transmit the private key to the server. The reason I need the private key on the server is because the user's need to be able to access their data from multiple clients and multiple locations (ie: their iphone, their ipad, their work laptop, their home pc). They do not want to have to manage and copy their keys from device to device, which is even more insecure than storing their keys on our server because they would just end up emailing them to themselves at that point.
The big problem with this is using UUIDs. Although UUIDs are (sort of) guaranteed to be unique, quite a bit of what they contain is quite predictable; substantial amounts remain constant across all the UUIDs generated on a single machine. As such, if a person gets access to (for example) their own key, they can probably guess many other people's keys fairly easily.
The other part that's problematic is storing user's private keys on the server. This makes the whole rest of the scheme relatively fragile, since access to those keys obviously gives access to all the rest of the data. It also (apparently) means you'll normally be decrypting the data on the server, so when the user accesses that data across the network, it'll either need to be re-encrypted for transmission, and decrypted on the users's machine, or else you'll be transmitting the data in the clear (thus rendering most of the encryption useless).
Edit: As to how I think I'd do this:
I'd have a list of public keys on the server. When a client wants to share a file with some other clients, it obtains the public keys for those clients from the server. It then generates a secure random key, and encrypts the data with that key. It then encrypts the random key with the public keys of all the other clients that are supposed to be able to access the data. Put those together into a stream, and transmit them to the server. The other clients can then download the stream, decrypt the key with their private key, and use that to decrypt the data itself.
This means each client's private key remains truly private -- it never has to leave their machine in any form. All they ever have to share with the rest of the world is their public key (which, by definition, shouldn't cause a security problem).
With that, the two obvious lines of attack are against the random number generator, and against RSA itself. For the random number generator, I'd use Java's SecureRandom -- this is exactly the sort of purpose for which it's intended, and if memory serves it's been pretty carefully examined and significant breaks against it seem fairly unlikely.
I won't try to comment on the security of RSA itself. For now, I think your primary concern is with the protocol, not the encryption algorithm proper. Suffice to say that if RSA were significantly broken, you'd obviously need to change your code, but you'd have a lot of company.
With this, it's pretty much up to the client to store their private keys securely. I like smart cards for that job, but there are quite a few alternatives. From the viewpoint of the server and protocol, it's no longer really a factor at all though.
Edit 2: As for dealing with multiple devices, I think I'd simply treat each device as a separate user, with its own public/private key pair. I'd then (probably) group those together by the actual users, so I can easily choose "Joe Blow" to give him access on all his devices -- but with a hierarchical display, I could also pretty easily restrict access to a subset of those, so if I want to share it with Joe on his office machine, but it's sensitive enough that I don't want it going where somebody might look over his shoulder while he looks at it, I can pretty easily do that too.
This keeps life simple for the users, but retains the same basic security model (i.e., private keys remain private).
The scheme you outline is equivalent to CMS (the standard underlying S/MIME) and PGP; fundamentally, it is secure. In CMS, this mode is called "key transport". You could also use multi-party "key agreement," with an algorithm like DH or ECDH.
The only problem is that you are using poorly chosen keys for AES.
I can't think of any reason to use a random UUID, which contains non-random bits. Just use the normal key generation mechanism of the Java Cryptography Architecture. Keys, plaintext, and ciphertext should all be represented as byte sequences, unless you need to accommodate some external storage or transport that only accommodates text.
Iterable<Certificate> recipients = null;
KeyGenerator gen = KeyGenerator.getInstance("AES");
gen.init(256);
SecretKey contentEncryptionKey = gen.generateKey();
Initialize the AES cipher and let the provider choose an IV.
Cipher contentCipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
contentCipher.init(Cipher.ENCRYPT_MODE, contentEncryptionKey);
AlgorithmParameters params = contentCipher.getParameters();
byte[] iv = params.getParameterSpec(IvParameterSpec.class).getIV();
For each recipient, initialize the RSA cipher and encrypt the AES key.
Cipher keyEncryptionCipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
for (Certificate recipient : recipients) {
keyEncryptionCipher.init(Cipher.WRAP_MODE, recipient);
byte[] encryptedKey = keyEncryptionCipher.wrap(contentEncryptionKey);
/* Store the encryptedKey with an identifier for the recipient... */
}
/* Store the IV... */
/* Encrypt the file... */
Having users select and remember passwords that give 256 bits of effective strength is unreasonable. To get that strength, you'd have to randomly choose passwords, encode them as text, and have users write them down on a card. If you really need that much strength, you could check out a smart-card–based solution for storing the users' RSA keys.
I'd highly recommend using a CMS library to store your files. It will increase your chances that the protocol you're using is safe, the code you are using has had more review, and that other tools, libraries, and systems can inter-operate with the encrypted messages. BouncyCastle's API is a little obscure, but it might be worth learning it.
(I can't remember if Java 5 supports "RSA/ECB/OAEPWithSHA-512AndMGF1Padding"; if it does, you should use that instead of PKCS1Padding.)
OK, this question is asking for a protocol discussion, so it is not completely according to stackoverflow's standards. That said, let's see if we can make some remarks anyway :) :
The Bouncy Castle PGP libraries have a very permissive license, so you can even copy them into a sub-package within your code;
Besides PGP there are also other standard container formats such as CMS or XML encryption, although the latter might not be such a good general purpose format;
Instead of UUID's, I would strongly suggest to use a well seeded PRNG such as the Java JCE "SHA1PRNG" to create the AES keys - I don't see any strong reason why you should rely on something like an UUID in your scheme;
AES keys are supposed to consist of random bits to have enough entropy, thinking of them as "passwords" is leading into a trap: you cannot use a String as a secure AES key;
The user will have to trust your application and server, you are acting as a trusted third party: you can send user's passwords to your server, you can send incorrect public keys to the users etc. etc. etc.
Your scheme is not protected against any man in the middle attacks (and many argue this cannot be done without using SSL)
Instead of directly encrypting with a password, you should look into something like Password Based Encryption PBKDF2 to encrypt the RSA private key;
Try and add integrity protection when encrypting, from Java 7 onwards you may use AES in GCM mode, it's well worth it.
It all depends on how "secure" you want the encryption to be. Obviously RSA is a well document/accepted standard for PKI. That being said, any time you provide the plaintext as well as the encrypted text, it makes it significantly easier for a hacker to decrypt the ciphertext knowing part of the plaintext. Here, you are doing precisely that. Although you are only transmitting the encrypted UUID, by having the same plaintext encrypted with multiple keys gives an attacker significant insight into the payload. Furthermore, if the hacked is actually one of the recipients, he is able to decode the UUID, and thereby automatically knows the plaintext that is being encrypted by the other users' public keys.
This is not likely a critical issue for you, but just thought I would point out a security risk.
I am not entirely sure why you need to store the user's private key, however. Furthermore, by using a simple password to encrypt the private key, you have basically reduced the overall security of the entire system to the strength of the user's password. Finally, if the user loses his password, he is toast; no way to recover any data.
I did something similar in the past but stored the results in a DB. However, I used the BouncyCastle libraries at the time, so am not sure how to accomplish this without them.
Related
Elliptic Curve Cryptography Shared Secret Key Derivation
I am implementing ECDH and wanted to ask how the shared secret that is derived from one's private key and another's public key should be best used. I got the fact that both parties using this end up with the same secret, which is fine. The secret gets compromised once an attacker gets hold of a private key of any of the two parties involved in a communication (Alice or Bob). It appears that the (shared) secret is always the same when calculating the key agreement using a certain algorithm (in my case AES). Then you use the secret and use AES/GCM to encrypt whatever one wants. I think that this is flawed as you always use the same key for communication between the same partner making it possible to record the secret communication and there might be a chance to derive the secret by just cracking AES or whatever is used. So what I want to do is deriving keys from it instead without ever using the entire secure key as is. So instead of using the secret key both are able to generate independently, I would like to use the secret key to derive a number of keys passing along kind of an offset both sides use to create unlimited one time keys. My question is what Key Derivation function is usually used (as it is key based)? I currently implemented a SipHash128 as it promises what I need - keeping the actual secret secret but allow for infinite symmetric secrets by exchanging random numbers as messages (one part from the device and one part from the server) to derive the sip hash from by using the shared secret key avoiding the device being able to always send the same 'random' messages. PS: I want a Java Backend talking to Flutter and JavaScript apps. PSS: Yes I have a Master (a colleague challenged me on this already) in Computer Science but feel kind of stupid doing it this way, but it was/is the easiest solution I could come up with.
Storing public key locally on android device
I want to use A-symmetric encryption for my application. The application basically takes a photo and sends it to a server, I haven't written any code yet and am just going through this as a POC. I came across a problem that I solved before but now it's puzzling me again. Like I said I want to use Asymmetric encryption where the private key is located on my server, and the public key is stored locally and 'shipped' with each android app (I.E. including in its resources). I want to use the public key along with a ByteOutputStream or EncryptedOutputStream and send the information already encrypted to the server for decryption on site. My question is basically how safe is the process of encrypting the data i'm sending out to the server? Obviously I understand that there is no such thing as completely safe, but is this a good practice, is there something that's sort of an industry-standard?
Copypaste from this answer regarding asymmetric cryptography. The currently largest broken RSA key is a 768-bit modulus, and it took some huge effort (four years, and really big brains). 1024-bit keys are considered usable for short term security, although larger keys are encouraged. 2048-bit keys are appropriate. Using a key twice larger means 8 times more work for signing or decryption, so you do not want to overdo it. See this site for a survey of how RSA key length can be related to security. ECDSA over a 256-bit curve already achieves an "unbreakable" level of security (i.e. roughly the same level than AES with a 128-bit key, or SHA-256 against collisions). Note that there are elliptic curves on prime fields, and curves on binary fields; which kind is most efficient depends on the involved hardware (for curves of similar size, a PC will prefer the curves on a prime field, but dedicated hardware will be easier to build with binary fields; the CLMUL instructions on the newer Intel and AMD processors may change that).
Secure link between two peers without certificates?
I am writing software in Java that will run on Android, Windows, Linux and OSX and want the devices to all talk to each other securely.
My initial thought was just to use a asymmetric algorithm (eg. RSA) and share the public keys manually. Then sessions would be encrypted just using RSA. I hit problems with this due to buffer size, then I read about CBC etc and the problems of data leakage if some form of XOR of data was not performed.
So...I looked at using AES/CBC/{padding}. Initially this sounded good: just share the key, an IV and away we go.
But these apps all talk to each other in both directions at any time, so keeping the IV in sync did not seem possible, resulting in a new IV being sent with every message. Not a big deal, but one of the advantages of AES over RSA is data size, and now I'm going to be adding 32 bytes to every message. Though I guess keeping a 'receiver' and a 'sender' Cipher would probably work.
Now I am back to considering alternatives, and trying to avoid too much roll-your-own.
Is there any substantial reason not to use RSA and CBC (or similar) done manually? ie. break data into chunks, pad as necessary, and encrypt with RSA, doing whatever XOR strategy seems most reliable.
Is there a better was to keep secure and trusted comms between pairs of peers without creating an SSL CA? Or, is there a way of hooking into the certificate verifier so that I can use my pre-shared public keys to validate the peer connections?
Any other suggestions/examples for best/simple multi-platform peer-peer secure comms?
You should look into implementing a hybrid cryptosystem. If I can guess your knowledge about the subject correctly you should really be using a pre-existing one like TLS, or DTLS, as you're not going to design a cryptographically safe protocol out of the blue. Note that the certificates are required to create a PKI, and the asymmetric crypto of course comes with it. You should however only have to use the asymmetric crypto during the handshake which includes the initial authentication and session key negotiation.
Private key encryption method wanted
I am programming some server-client software and wanted some encryption. Both the server and client share a private key which is fixed length. Haven't decided what length I am going to have the key, but I do know that the key will be between 1 and 32 characters. The key is predetermined (so for instance I may decide I want the key to be abc1234) I am programming in Java and need the algorithm to be a quick as possible as the clients are most likely to be mobile devices. I don't need any compression but I would prefer if the encrypted string wasn't larger. I am not looking for top-notch encryption here obviously, but it is my understanding that any key based encryption can't be broken without knowledge of the private key anyway. Can anyone recommend me an algorithm/method for encryption/decryption with a shared private key?
it is my understanding that any key based encryption can't be broken without knowledge of the private key anyway That's not even close to true. A lousy algorithm absolutely can be broken without knowledge of the private key. Anyhow, if there's a shared key, consider AES. And read up on the JCE/JCA as Java can do AES and other types of encryption out of the box. Much better than rolling your own.
Confused about encryption with public and private keys (which to use for encryption)
I am making a licensing system when clients ask my server for a license and I send them a license if they are permitted to have one. On my current system I encrypt the license using a single private key and have the public key embedded into the client application that they use to decrypt the license. It works! Others have told me that I should be encrypting with the public key on the server and distributing the private key to clients. I have searched the web and can see that sometimes they use the private key to encrypt and other times they use the public key to encrypt. In this case what am I supposed to do?
Others have told me that I should be encrypting with the public key on the server and distributing the private key to clients. Those people are wrong. The name private key implies that it is private meaning that only you should have access to it. In this case what am I supposed to do? Use digital signatures. Sign the license file with your private key and use your public key in your application to verify that the signature on the license came from you.
Congratulations, you just invented the RSA signature. (Which is what you should be using, anyway.) To communicate with a public key system, you need to use the private key once and the public key once, but RSA supports two different orders: 1) Encrypt with the public key, decrypt with the private: The recipient doesn't know anything about the source of the message, but the sender knows that only the recipient (the holder of the private key) can read it. This is classical "encryption". 2) "Encrypt" with the private key, then "decrypt" with the public. This is a digital signature, and provides authentication. Anyone can read the message, but only the private key holder could have sent it. Assuming your license is customized to the client (which could be as simple as including a copy of a client-generated random number), then it's useless to anyone else, but the client can be sure that the server sent it. The symmetry isn't quite that neat in practice; the different modes of operation have different weaknesses and gotchas, so the implementation is typically significantly different, but that's the general idea. One of the first and most important lessons in cryptology is understanding authentication and when to use it. It's needed at least as often as encryption, and not knowing when to use it leaves you in a Midvale School for the Gifted situation.
If you are encrypting something that is only to be read by a single recipient, then you encrypt with that recipients public key and they use their private key to read it. If you are encrypting for multiple recipients, then you can encrypt with your private key and distribute your public key to those which you want to be able to read it. This is usually called "signing" as anyone who has access to your public key can read it, so it's not really a form of private communication. An overall more robust solution for you would be for your app to generate a key pair per installation, send the public key that it generated back to the server, which you would then use to encrypt so that only that single install could use the license that you created (by decrypting it with its private key).
At least in a typical public key encryption algorithm (e.g., RSA) there's not really a major difference between the public and the private key. When you generate keys, you get two keys. You keep one private and publish the other -- but it doesn't matter much which one you publish and which one you keep private. Anything you encrypt with one key can be decrypted with the other key. For normal purposes, you publish one key, which lets anybody encrypt something that only you can decrypt. From a technical viewpoint, the reverse works fine though: if you encrypt something with your private key, anybody with the public key can decrypt it. This is typically used for things like signature verification (i.e., anybody with the public key can verify that the signature had to have been created with the private key). You usually want to use separate key pairs for encryption and signing though. For your case, it's open to some question what you're really going to accomplish. You can certainly encrypt some data necessary to use the program, so the user needs the key to decrypt it and use the program -- but if the user is willing to give a copy of the code to an unauthorized person, they probably won't hesitate at giving a copy of the key to them as well. As such, even though the encryption/decryption will do it's job, it's unlikely to provide any real protection. A more typical licensing scheme is tied to something like a specific IP address, so you do something like encrypting the IP address, then use the result as a key to decrypt data necessary to use the program. If the IP address is wrong, the data isn't decrypted correctly, and the program doesn't work. As long as the user has a static IP address this can work well -- but will cause problems in conjunction with DHCP. My immediate advice would to just not do this at all. If you insist on doing it anyway, don't do it yourself -- get something like FlexNet to handle it for you. You're better off without it, but at least this way you'll get something that sort of works, and you won't waste time and effort on it that could be put to better purposes like improving your software.
If you are using public-private (asymmetric) encryption, you always encrypt with the recipient's public key, who decrypts with their private key. For digital signatures, you sign with your private key and the recipient verifies the signature with their public key. The question then arises, how do you make a secure DRM system? If you use encryption, and give the recipients a private key, they can distribute either the key or the decrypted content. If you use signatures, they can simply strip out the signature verification part of your program. The answer is that it's impossible. The concept of DRM is fundamentally flawed.
Hope this link from wikipedia helps. PKI is based on mutual trust. However the private key has to be protected by the owner. Public as the name implies is open to all. The entire architecture is made inorder to help the scenario as defined above in your question.