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    <h1>"Is SHA256 sufficient as a commit function?"</h1> <p>The use of SHA256 should be just fine. The only issue I have heard of is that it has a hash extension vulnerability. If you produce multiple hashes using the same data don't simply concat more data to the end of the data you already hashed. In your post have have the two hashes "sha256( UUID || DH pub || Chall)" and "sha256( UUID || DH pub || DH sess || Chall)". If that second one was "sha256( UUID || DH pub || Chall || DH sess)" then there would be a relation between the hash values if UUID, DH pub, and Chall were all the same values as before. You should either take care to avoid the hash extension issue or include a salt value into the data to be hashed, either by communicating the salt across the link or having differing vales for each code path.</p> <p>On a side note: is it really necessary to transmit a Chall when you already have a previous session key saved and don't need to ask the user to manually confirm the challenge code?</p> <h1>"Is the addition of the shared secret as an authentication info in the commit string safe?"</h1> <p>I'm guessing you mean to ask "Is it safe to include secret information in a hash that is to be made public?" If the secret is something that is really hard to guess and would take a really long time to perform a bruteforce attack against, then yes it is safe. If the secret is something easy to guess or has only a few possible values, then no, it's not safe unless you, at the same time, include some hard to guess secret to force a potential eavesdropper to have to guess all such secrets simultaneously. For a large, effectively random number like a DH shared secret then it should be just fine.</p> <h1>"What is the overall security of the 1024 bit group DH"</h1> <p>I'm not sure if DH group 1024 is what you want to use. A key exchange that is considered to be close to being as effective as the SHA256 hash algorithm you're using would be 521 bit ECDH. The cryptographic strength of ECDH is considered to be 1/2, so if you want 256 bit security, you want 521 bit ECDH. Unfortunately, I'm not certain about the security of the many individual 521 bit ECDH groups that have been published.</p> <h1>"I assume at most 2^-24 bit probability of succesful MITM attack (because of 24 bit challenge). Is this plausible?"</h1> <p>There is more than one way perform a MITM attack. Eve will use every resource at her disposal to perform her attacks and if you are not careful she will exploit something you didn't think of. This is why peer review is necessary in cryptography.</p>
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    1. COIs it necessary to add the challenge - i simply don't know. I've added only stuff to reduce the risk of damaging the original algorithm. The thing is, i understand the spirit of the papers i've read, but not the security proofs :-) So you think i can skip this?
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    2. COWhat's the issue with DH 1024? My intention is to have a basic toolset that will allow me later to get an encryption key with strengths from AES 128 to AES 256 (maybe 3DES). I will try to learn something about ECDH... I assume this is faster?
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    3. COThe attack i didn't understand. Sure the MITM can make his key exchange, but the commit forces him to select a 24 bit challenge before. Afterwards he can not forward the remote challenge. Perhaps i was unclear in the Question, i do some edit...
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