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What would be the best way of encrypting small mesasages to be stored in encrypted form? (This sounds like somthing a textbook would cover, but I haven't found any references).

Let's say I have many systems producing messages (log-lines) which I would like to transfer and store centrally in a secure manner. Using off-the-shelf products I could secure the transfer and storing of these messages independently, but I would like to investigate another approach: encrypting each message at the sendig system with the public key of the reciever, so that the messages can be stored as-is, and thus provide end-to-end encryption from the sender down to the storage medium.

First some definitions:

  • RSA(PublicKey, Message) means: RSA-encrypt Message with the given Key
  • AES(Key, IV, Message) means: AES-encrypt Message in CBC-mode with the given Key and IV
  • Message = gzip(plaintext)

I have come up with the following options:

I can encrypt each message with RSA:

  • Ciphertext = RSA(PublicKey, Message)

However, this would fail of the message is too long.

Or I could create a random AES-key for each message (and also use this as IV):

  • Key = Random()
  • EncryptedSessionKey = RSA(PublicKey, Key)
  • Ciphertext =EncryptedSessionKey, AES(Key, EncryptedSessionKey, Message)

However, RSA-computation is heavy and I probably cannot afford to do this for every message. Thus, I can re-use the key for, say, 100 messages and do:

  • Key = Random() (re-key every 100 messages)
  • EncryptedSessionKey = RSA(PublicKey, Key)
  • IV = Random() (new IV for each message)
  • Ciphertext = EncryptedSessionKey, AES(Key, IV, Message)

The last one seems to be both secure and efficient. However, I really don't want to invent a system like this myself. Is there any best practices, research or textbook-examples I should look into?

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Generally sound reasoning, but: First and foremost, the schemes give no integrity protection, only confidentiality. RSA-encrypt is not well defined; one should hardly ever use textbook RSA, see PKCS#1, and its two variants of RSAES. RSA encryption is not very heavy with short $e$ (decryption is). Use of IV=EncryptedSessionKey is needlessly complicated: when key is single use, IV can be 0. I let others answer the question. –  fgrieu Mar 12 '13 at 8:25
    
Great input. I won't implement RSA or AES myself, I'll use javax.crypto. I will probably benchmark to find out if the re-use of keys is neccessary. –  Rolf Rander Mar 12 '13 at 8:36
    
Hm, looking more at this, I guess I need an IV in the first example as well. –  Rolf Rander Mar 12 '13 at 9:57
    
If you implement it like this, I think there will be another problem: How do the logging sub systems receive the "reused-key"? I think the intent of public key cryptography in this scenario should be that these systems only need the public key to add log messages. –  Ekris Mar 12 '13 at 9:58
    
The AES "session key" will be included in the log-message, encrypted with RSA. I believe this is the common mode of operation when using asymetric encryption: use RSA to encode a session key, include the encoded key in the message. –  Rolf Rander Mar 12 '13 at 11:55
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1 Answer 1

up vote 1 down vote accepted

Ok, answering myself then, trying to summarize all the good input from above.

First of all, this design will provide confidenciality to each log message, but it will not:

  • ensure the integrity of the log as a whole, it will be possible to insert or remove log-entries unnoticed
  • authenticate the sender of log-messages, a rouge client or a man-in-the-middle could insert fake log-messages

However, if this is OK (or you have other ways of solving these issues), this scenario is probably secure.

As for performance, on my machine I get the following numbers:

  • Just RSA: just below 4000 messages pr second
  • RSA and AES with a new key each time: just above 4K msg/sec
  • RSA and AES with a new key each 10 messages: 17 K msg/sec
  • RSA and AES with a new key each 100 messages: 25K msg/sec

Just RSA seems to have no advantage over RSA+AES, and there is an upper limit to message size.

Now, this just encrypts the same string over and over. For a more real-life test I will make new strings (serialized xml) and write these to disk, to se if the difference is still significant.

As for the use of gzip, I have tried a 218-byte xml-message which gzipped into 164 bytes (25% reduction in size). This is a relevant testcase in my scenario, but of course your milage may vary.

Edit: I have re-tested with different values for key-size and exponent. All of these are using RSA to encrypt an 128-bit AES session key, with a new key for each message (I guess in effect I am measuring the throughput of RSA-encryption of 128-bit random numbers).

  • keysize=1024 bits, $e = 2^1+1$: 7 tx/millisecond
  • keysize=1024 bits, $e = 2^{16}+1$: 10 tx/millisecond
  • keysize=2048 bits, $e = 2^1+1$: 11 tx/millisecond
  • keysize=2048 bits, $e = 2^{16}+1$: 4 tx/millisecond

This is using the default crypto-provider in java 7. I let each configuration run 20 iterations before starting the measurement to "warm up" the jit-compiler.

My environment:

  • java.vm.version: 23.7-b01
  • java.vm.vendor: Oracle Corporation
  • java.vm.name: Java HotSpot(TM) 64-Bit Server VM
  • file.encoding.pkg: sun.io
  • java.runtime.version: 1.7.0_15-b03
  • os.arch: amd64
  • os.name: Windows 7
  • sun.management.compiler: HotSpot 64-Bit Tiered Compilers
  • os.version: 6.1
  • sun.arch.data.model: 64
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Perhaps, indicate what was the bit size of the modulus, and the value of the public exponent $e$, and (concisely) what sort of environment that was (e.g. crypto library built in JRE 6); this will make the numbers more relevant. Perhaps try RSA with e=3 instead of 65537 and I guess you'll get much better speed. –  fgrieu Mar 13 '13 at 16:27
    
This was javax.crypto with default settings, I was not aware of the possibility (or consequences) of changing the key parameters. I will update with more info. –  Rolf Rander Mar 14 '13 at 8:55
    
A small update on this: I took a look in Practical Cryptography (Ferguson/Schneier) which actually recommends using an encrypted message counter as IV, because you typically need a message counter anyway to prevent message replay or message deletion, and for a counter 48 or 64 bits would be sufficient. –  Rolf Rander Apr 8 '13 at 10:01
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