# Why RC4 used constant 24bits for initial vector

Why RC4 used 24-bits constant value + 40 bits random values to generate key?

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We must not be looking at the same stream cipher, to my knowledge RC4 uses no constant value other than the identity permutation (0, 1, 2, ..., 255), and has a key up to 2048 bits long. If this is a custom implementation which uses an IV, we need to have references. You might be talking about the WEP protocol, which uses a 24-bit IV, if so, mention it. –  Thomas Sep 5 '12 at 2:55
Also note 40-bit keys are way too short to be secure nowadays, I could break one in a couple hours on the computer I'm on right now. You want 80 bits at the very least and 128-bit to be comfortable. –  Thomas Sep 5 '12 at 2:55

What is apparent is that, even if the 24 bit constant value is not public information, if that attacker can get his hands on $2^N$ ciphertexts were the plaintexts has known linear relationships (for example, the MSBits are known to be all clear), then he can recover one of the keys with $2^{64-N}$ effort (by trying random 64 bit keys, generating the keystream, and seeing if the linear relationships work out); once he has that, he has the 24 bit constant value; he can recover the rest of the keys with $2^{40}$ work.