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I'm currently taking a computer security module in University and as part of a problem have been asked:

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My thought for the question is that no it does not provide 64 bits of security strength. This is as (IV,C) is transmitted over the network meaning that if the signal were intercepted by an adversary IV can easily be obtained as it has not been encrypted and hence is not confidential meaning that it doesn't add to the overall security. Rather 40 bits of security are enforced as K is not sent with the signal so the adversary cannot know K.

If anyone could comment on my answer or provide any alternative insight, it would be greatly appreciated.

Many thanks

NOTE: Edited answer to comment on how many bits I believe are actually enforced

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You are correct. The strength of a cipher is bounded by its key length, and while the IV may matter that changes how much data can safely be encrypted before needing to change the key, not the overall strength.

Additionally, RC4 is broken, and doesn't even provide as much strength as what its key length would imply. So RC4 with a 40-bit key isn't even 40 bits of security. This is part of why WEP is deprecated in favor of WPA2 (WPA1 had some other issues).

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  • $\begingroup$ As far as I'm aware, 40-bit RSA does provide 40-bit security (unless a related key attack is relevant), but the distinguishability of the keystream allows for plaintext recovery in some cases. $\endgroup$
    – forest
    Jan 15, 2023 at 22:38
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    $\begingroup$ WPA1 was designed as a stop gap measure; one requirement was that it had to be implementable as a software upgrade on everything implementing WEP, that severely limited the options the WPA1 designers had. $\endgroup$
    – poncho
    Jan 16, 2023 at 3:37
  • $\begingroup$ That plaintext recovery takes less than 2^40 operations: a non-negligible advantage in the IND-CPA game can be achieved with less work than brute-force. I'd say that's enough to classify as "less than 40 bits of security". It's not key recovery, If all you care about is avoiding key recovery, then RC4 is secure. If you care about the plaintext, then it's very risky. Not always guaranteed broken, but certainly not safe. $\endgroup$ Jan 16, 2023 at 3:42
  • $\begingroup$ @SAIPeregrinus You're right. I was only talking about key recovery. Even without that, it's really insecure. $\endgroup$
    – forest
    Jan 16, 2023 at 3:49

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