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I wonder if there is any practical attack on MD5(key || fixed-length-message).

edo1
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    Related question. Brute forcing with GPUs, FPGAs, ASICs is perhaps the most practical we have, and with enough resources could be carried above 88 bit of key. – fgrieu Dec 21 '22 at 05:28
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    I don't think we can easily attack that with current tech, but the MD5 algorithm is pretty weak - so we don't know if this will be secure towards the future. Saying that the message size is fixed is fine, but beware that you could create a length extension attack if you don't verify the size before you or directly after you verify the authentication tag that you generate. In general you would try and not operate on the message before verification, so beware of implementation issues. – Maarten Bodewes Dec 21 '22 at 21:26
  • @MaartenBodewes I will not use this cipher in my program, of course. I'm just wondering if it's been hacked already. – edo1 Dec 21 '22 at 21:52

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