CPTR 427 Programming DES. Outline Top Level Design Design Decisions Operations to write Putting it together.

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Presentation transcript:

CPTR 427 Programming DES

Outline Top Level Design Design Decisions Operations to write Putting it together

Top Level Design Depending on whether we are encrypting or decrypting we set the input files accordingly. Create a “Keys” object that generates all the round keys used by DES in the order needed to encrypt or decrypt This is the hard part! Let’s delve into the design decisions that have to be made first.

Design Decisions Representing Data consistently BitSets (Java) BitArray (C#) UInt64 (C#) Long (Java) Operations Permutations Expansions/Contractions XOR operations Odd side consideration: we work with 4/6/28/32/48/56/64 bit values

BitSets and BitArrays Advantages: Easy to perform permutations – bit level access Can easily expand or contract length… … can express arbitrary size Has XOR operation Bit position easy to understand Disadvantages Data is not in this format natively… Must convert between bytes and BitSets/BitArrays SLOW!

Long and UInt64 Advantages Bytes convert easily to long or unsigned int64 Easy shift and XOR operations Unnecessary to convert data to a new object type THIS IS THE WAY IT SHOULD BE DONE… Disadvantages Odd size information location (e.g. where does my 56 bits start?) Permutations are no longer intuitive

What format? For better or worse, I chose long in Java to avoid and delt with the permutation issues – in retrospect that was a good choice because it made my work easier because I didn’t have to process data in two very different formats. My Advice: Chose long/UInt64 and deal with the selection problem instead.

Operations Permutations Left Circular Shifts Swaps

Operations (Single Round) XOR S-Box operations

Operations to Write Simple Ops Permutations (with expansion or contraction + odd sizes) Separate left circular shifts within a 64 bit long/UInt64 Swaps (first 32 bits for second 32 bits) XOR (Native) S-Box Operations Named Ops Data: IP and IP -1, E, S-Box, P, XOR Key: PC-1, PC-2, XOR, Left Circular shifts

Permutations Defined by Data Input Size of input data Data Output Size of output data Mapping: input  output That takes care of IP, IP -1, E, P, PC-1, PC-2 Leaving: S-Box, Left Circular Shifts, swap32 and XOR public UInt64 perm(UInt64 input, int insize, int outsize, int[] map)

Permutation Logic Move a bit from position x in the input to position y in the output.

The rest of the Ops public UInt64 leftShift(UInt64 input, int shiftAmount) public UInt64 swap32(UInt64 input) public UInt64 sBox(UInt64 input)

Putting it all together… The entire goal of this project is for you to put this together using the instructions from the Stallings textbook. Use the notes and the program skeleton provided!