The RC6 symmetric block cipher is parameterized with three parameters :
- W : The number of bits in a machine word (8 , 16 , 32 , 64)
- B : The number of bytes specifying key length (0 to 255)
- R : The number of rounds to execute (1 to 255)
A data block is specified as 'uint_t data ' where 'uint_t' can be 8, 16, 32, or 64 bits resulting in a 32, 64, 128, or 256 bit data block size respectively.
These parameters allow the end user to select block size, key size, and to trade off performance for security.
The question I have is : Given parameters W and B, along with a desired security strength (not sure how to specify this exactly, but it depends on R), how does one determine the minimum value of R to ensure this minimum security bound?
ie: I desire to parameterize an instance of RC6 for a 256 bit data block (W = 64) and a 256 bit key (B = 32). I would like the cipher to have 256 bits of security (that of the key B) which clearly depends on the value of R. Assuming a full 256-bit key is used, what is the minimum number of rounds (R) required to ensure full diffusion and security equivalent to a brute-force search of the 256 key?
I have researched the original RC6 specification, alternative research papers that I could find which discuss various attacks on RC6, etc. but I have not yet seen any summary, equation, or guidelines on how to select values of R for tailored instances of RC6.
It seems to me that without an equation or clear instructions on how to select R, the specification is not 100% complete and layman implementing the cipher for a practical application will be left guessing, which will cause bad things to happen.
Note : I am not an expert cryptographer, but I do know enough not to "roll my own" and follow a standard to-the-letter. This standard, while awesome, seems to be missing a letter... :)