I have a question relating to the use of an Initialization Vector in AES encryption. I am referencing the following articles / posts to build encryption into my program:
[1] Java 256-bit AES Password-Based Encryption
[2] http://gmailassistant.sourceforge.net/src/org/freeshell/zs/common/Encryptor.java.html
I was originally following erickson's solution from the first link but, from what I can tell, PBKDF2WithHmacSHA1 is not supported on my implementation. So, I turned to the second link to get an idea for my own iterative SHA-256 hash creation.
My question comes in how the IV is created. One implementation ([1]) uses methods from the Cypher class to derive the IV where are the other ([2]) uses the second 16 bytes of the hash as the IV. Quite simply, why the difference and which is better from a security standpoint? I am kinda confused to the derivation and use of IVs as well (I understand what they are used for, just not the subtler differences), so any clarification is also very welcome.
I noticed that the second link uses AES-128 rather than AES-256 which would suggest to me that I would have to go up to SHA-512 is I wanted to use this method. This seems like it would be an unfortunate requirement as the user's password would have to be 16 characters longer to ensure a remotely secure hash and this app is destined for a cell phone.
Source is available on request, though it is still incomplete.
Thank you in advance.
The IV should not be generated from the password alone.
The point of the IV that even with the same key and plaintext is re-used, a different ciphertext will be produced. If the IV is deterministically produced from the password only, you'd get the same ciphertext every time. In the cited example, a salt is randomly chosen, so a new key is generated even with the same password.
Just use a random number generator to choose an IV. That's what the cipher is doing internally.
I want to stress that you have to store either the IV (if you use the first method) or a salt (if you use the second method) together with the ciphertext. You won't have good security if everything is derived from the password; you need some randomness in every message.
Cryptographers should generate IVs using a secure pseudo-random random number generator.
Application developers should use existing, off the shelf cryptography. I suggest that you use SSL with certificates to secure your network traffic and GPG to secure file data.
There are so many details that can make an implementation insecure, such as timing attacks. When an application developer is making decisions between AES 128 and AES 256 it is nearly always pointless since you've likely left a timing attack that renders the extra key bits useless.
My understanding is that Initialization Vector is just random input to encryption algorithm, otherwise you would always get same result for same input. Initialization Vector is stored together with cipher text, it's not secret in any way. Just use secure random function to generate initialization vector. PBKDF* algorithms are used to derive secret keys of desired length for encryption algorithms from user-entered passwords.
First implementation that you link to simply lets Cipher object to generate Initialization Vector. Then it fetches this generated IV to store it together with cipher text.
Second one uses part of hash bytes. Any approach that generates non-repeating IVs is good enough.
Most important property of IV is that it doesn't repeat (very often).
The IV is just a consequence of the use of block chaining. I presume that this is more than a simple API design question. I assume that you know that the reasoning for using it is so that the same plaintext will not show up as the same ciphertext in multiple blocks.
Think about recursion from the last block where the Nth ciphertext block depends in some way on the (N-1)th block, etc. When you get to the first block, 0th block, you need some data to get started. It doesn't matter what that data is as long as you know it before you attempt to decrypt. Using non-secret random data as an initialization vector will cause identical messages encrypted under the same key to come out as completely different ciphertext.
It's similar in concept to salting a hash. And that source code looks a little fishy to me. An IV should simply be fresh-at-encryption-time random bits dependent upon nothing, like a nonce. The IV is basically part of the encrypted message. If you re-encrypt identical data with identical key, you should not be able to correlate the messages. (Hey, think about the consequences of correlating by ciphertext length as well.)
As with everyone else here, I've always known IVs to be just chosen randomly using the standard algorithms for doing so.
The second reference you provided, though, doesn't seem to be doing that. Looks like he salts a password and hashes it. Then takes that hash and splits it up into halves. One half is the encryption key, one is the IV. So the IV is derived from the password.
I don't have any strong breaks for such a method, but it's bad design. The IV should be independent and random all on its own. Maybe if there's a weakness in the hashing algorithm or if you choose a weak password. You don't want to be able to derive the IV from anything else or it's conceivable to launch pre-computation attacks.
Related
I am trying to implement RSA encryption using blocks to cut down the run time of decrypting with large bits lengths. The way my program is now, it works but reads individual characters and decrpyts them. So as you might imagine, the run time for large bit lengths is long.
Is there a way to easily implement decryption using blocks so that it works faster for these large bit lengths. Examples of code with an implementation would be nice if it is easy and feasible. Thanks.
From your description, it appears that you're using the following encryption scheme:
for i=0 to length(input):
output(RSA_encrypt(key, input[i]))
This is not a secure encryption scheme. You appear to be asking for a way to do something similar to
for i=0 to blocks(input):
output(RSA_encrypt(key, block(i, input)))
That is likewise not secure. Secure RSA-based encryption schemes generally involve encrypting a unique session key with RSA then encrypting the message using a symmetric cipher such as AES. For instance, see RSAES-OAEP. Don't try implementing it yourself, because you're likely to get it wrong. Instead, use a reputable cryptographic library.
Always remember the Rules of Crypto:
Never design your own crypto.
Never implement your own crypto.
Anyone can design crypto that they can't break themselves.
I am implementing RSA manually on java (yes i know not the best idea but it is for university purposes). My algorithm is working good enough with small texts but when big texts come into consideration the algorithm breaks as the number of bits is superior than my key.
I am looking into the possibility to implement a padding scheme in order to fragment my plain text into small ones and then encrypt them.
Is there any suggestion on how to the padding?
Thanks in advance.
I'm sorry that I'm writing an answer that contains basically the same information as divanov's answer, but an edit to add all the little details that I think are important would be a complete rewrite of the answer.
Generally you don't want to asymmetrically encrypt your data directly, but instead use RSA to exchange a symmetric key that is used to encrypt your data symmetrically. You can do it like this (idea taken from "Cryptography Engineering" by Ferguson, Kohno and Schneier, a book I can wholeheartedly recommend):
Assuming that l is the bit length of your modulus n, generate an l-1 bit long random number r. Encrypt r with the RSA public key.
Use a cryptographic hash function to generate the symmetric key k out of r. I would advise the use of sha256: k=sha256(r)
Encrypt you data with a block cipher like AES256 using a proper "mode" like CBC.
The advantage of this procedure is that you do not have to care about RSA paddings at all (and there is a lot of stuff that can go wrong with them). Please don't check the structure of r after decrypting, though, and just stuff if into the hash function as you otherwise might open yourself up to padding oracle attacks (akin to this one) that are beyond the scope of my answer though.
Note that for a real world scenario you have to care about authenticity of the data, too. The only common use case where encryption is mostly enough is "data at rest", i.e. if no data is transmitted over the network and you only care about physical theft of your data.
When one needs to encrypt longer plain text than a assymetric key typically random symmetric cipher block key is generated, for example, AES128 and then it is used to encrypt the data. At the end of a process symmetric key is encrypted with RSA public key and saved along with the cipher text.
A decryption consists of recovering symmetric key with a private RSA key and then using the former to decrypt the long message.
One of the reasons to do so is that RSA is much slower than, for example, AES. Another one is that block cipher has no limitation for a size of a message.
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.
I want to know if our data is encrypted with one encryption algorithm(AES, DES, etc.) and then we transfer our data in open network, can anyone get real data or do some thing if the encryption algorithm is known even though the hacker doesn't know about the private keys, public key or PV?
can anyone get real data or do some thing if the encryption algorithm is known
If the attacker knows the encryption algorithm, it's a start, because now all they need to do is to find out what was the key used to encrypt it. But established encryption algorithms like AES have no known weaknesses. Thus an attacker would be forced to bruteforce it to gain access to the data.
If you are using keys of an appropriate size (eg: AES 256 bits or more), this would be a very difficult task. DES also has no known weaknesses, but its small key size (56 bits) allows for a bruteforce attack to succeed in a reasonable timeframe, (eg: days). That's why DES is not widely used any more.
even though the hacker doesn't know about the private keys, public key or PV?
Note that public keys are only relevant in the context of asymmetrical encryption. In this case, the public key is usually publicly available (hence, the name "public key"). But asymmetric encryption is designed so that even if you know the public key, you can't decrypt it unless you have the private key.
In summary, encryption algorithms like AES have stood the test a time and proven to be secure enough. As David Schwartz points out in his answer, if you have a problem, (usually) your implementation is the thing to blame, not the encryption algorithm.
Almost by definition, if the encryption is implemented properly and part of a sensibly-designed system, no. That's the whole point of encryption.
Note that encryption is not magic. It must be used precisely correctly to provide useful security. There are a lot of ways to do it wrong.
If you're not using a widely respect product (like TrueCrypt, Firefox, or GPG) and using it precisely how it's intended to be used, there's a very good chance you aren't getting real security. For example, Dropbox used AES, but a security flaw in another part of their system allowed one user to decrypt another user's data. So it didn't help that it was encrypted.
Yes, keeping the algorithm secret helps security marginally. If an attacker knows that you used DES (which isn't terrifically hard to break) they may be more likely to try to break it.
I think the core of your question is about statistical attacks, which tries to see through the encryption to decipher the nature of the data. Any reasonably modern algorithm is mathematically designed to thwart any attempts to guess what the data is.
However David makes a very good point. Even perfect encryption (if it existed) would be vulnerable to the human factor. These algorithms are worthless if you don't dot your i's and cross your t's, and have absolute (and justified) faith in those who can decrypt the data.
I am using PBE encryption to encrypt and decrypt some text on an Android application but I get the BadPaddingException: with the "pad block corrupted" message when I use the wrong private key to decrypt the text.
My question, since I am not well versed with encryption in Java, is if this is the normal behavior of the encryption API, because I need to do some logic in the case when the wrong key is entered, but I do not know the private key, nor do I store it anywhere (storing just the encrypted and decrypted check text).
Thanks,
Mihai
It is normal that most key mismatches result in a "bad padding error". But this is not 100% foolproof either. For instance, in the case of symmetric encryption with PKCS#5 padding (a very common way to pad data), about 0.4% of wrong keys will not result in a bad padding. The decrypted data will still be garbage, but, out of freak chance, that garbage turned out to end with a valid padding. Your application must not make it apparent whether a decryption failure is due to bad padding, or to garbage with freakishly valid padding: that information (whether the key is part of the 0.4% of keys which yield a proper padding) is a leak which can have severe consequences. There have been some attacks against SSL connections that way.
Yeah, less then ideal ( http://developer.android.com/reference/javax/crypto/BadPaddingException.html ). The decryption logic needs to strip the padding before it gets to the actual cypher-text and things go bad in that early stage.
In short, yes, BadPaddingException is what you should expect if the wrong password/key was used during decryption.
Edit: But as others have pointed out, this isn't something you should communicate out of your decryption code. It's simply a way of knowing that an incorrect key was used.