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6

One problem with RC4 is that, while it does take a variable length input (up to 256 bytes), it's known not to be great at mixing those bytes together. Specifically, we see correlations between the RC4 key and the RC4 output stream. My first recommendation to you would be to use something other than RC4. About the only advantage RC4 has over most other ...


4

Poncho does a good job of explaining why you could use a KDF before RC4. But you are talking about a password and a PBKDF. A PBKDF does more than just provide a good way to extract entropy from the given input (the password): It uses a salt, which can be used to protect against rainbow tables (which could be created for known plaintext). This salt could ...


3

Yes, this should be secure, as it is largely compatible with KDF1 and KDF2 which basically use a 4 byte big endian encoding of the counter instead of a direct ASCII conversion to a byte. Note that this construct works fine for master keys (short length, high entropy) but may be vulnerable to length extension attacks if larger input is allowed. However, if ...


2

Depends on what you mean by Keccak. There is actually a slight issue here that not all 256-bit Keccak variants have 256-bit preimage resistance. SHA3-256 (in the current SHA-3 draft) does have 256-bit preimage, but if you are using Keccak with 256-bit capacity it only has 128-bit preimage resistance. At least some of the earlier documents had 256-bit output ...


2

The algorithm produces a password based on the value of the time that is input as an argument. That value does not have to be the current time. For the purposes for which TOTPs are generally used, there is no value in producing the password for a time other than the current time step - it won't be recognized by the validator.


2

Probably because a simple cascade would only be stronger against some attacks, while opening the door to more implementation bugs. While bcrypt and scrypt are (password based) key-derivation functions, much of what is in the answers to this question about combining hash functions applies here. Different constructions give preimage resistance and PRF-ness, ...



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