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I would like to ask if it's insecure to encrypt many different messages sent over the Internet encrypted by RC4 with the same password.

If it is insecure, then if I encrypt the first message with the password 100, second message with the password 101, third message with the password 102 and so on, would that make it secure?

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RC4 is not considered as secure anymore, since there are several security vulnerabilities found in it. More about RC4 on Wikipedia ...

Using the same password for all messages is also insecure, since if the password was discovered, all messages are compromised. Using pattern like 100, 101, 102 will not add much security to it, since attacker can guess what will be the next and previous password.

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    Also not secure without unique IV:s as it is a stream cipher – Natanael Nov 22 '15 at 2:02
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If you use different IVs for each message, then each is indeed as "secure" as RC4 itself. Be warned though: some IVs are weak! You should switch to a modern cipher, like AES or Salsa20.

(If you do not use unique IVs for each message, or use no IVs at all, your security is broken after the first re-use.)

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