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I have an application which uses SSL (JSSE Implementation). Currently from my client side, the "Client Hello" message suggests a list of cipher suites that consists of both 128 & 256 bits key length encryption algorithm ( e.g. Cipher Suites: TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA (0xc00a), TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (0xc013) ) I want to filter out cipher suites which use AES 128 bits key length in the Client Hello message so I put this line in java.security file of client side

jdk.tls.disabledAlgorithms=DSA, RSA keySize < 2048, AES keySize < 256

However it does not work. Cipher suites like TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA are still there in the Client Hello message.

If I configure the parameter like this:

jdk.tls.disabledAlgorithms=DSA, RSA keySize < 2048, AES

then all the cipher suites which use AES encryption algorithm will be disabled.

I don't understand why the keySize constraint does not work in my case.

  • My guess is that key size restrictions are enforced only for asymmetric algorithms (RSA, DH... but not AES). Anyway, from a practical security point of view, AES-256 is not stronger than AES-128, since AES-128 is already quite far in the "cannot break it with existing or foreseeable technology" zone. Thus, removing AES-128 support does not make a lot of sense. (Also, AES-256 is about 40% slower than AES-128, so there are actually good reasons to prefer AES-128.) – Tom Leek Oct 21 '14 at 14:38
  • Thank you Tom. I already changed the strategy to implement the SSL cipher suites restriction using code instead :) – ducnh Oct 29 '14 at 4:32
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Rather old question, but just happened to come aross the same issue. Didn't have luck trying to avoid 128 bit AES ciphers by specifying "keySize < 256". What finally seemed to work was:

jdk.tls.disabledAlgorithms=..., AES_128_CBC, AES_128_GCM

This should force Java to only allow 256 bit AES ciphers - provided that the JRE allows using strong cryptography in the first place (which most recent JREs do out of the box, otherwise you still have to download and install the "Unlimited Strength Jurisdiction Policy").

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