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**Algorithm Efficiency in Hardware with an Emphasis on Skein**

By Phil Doughty

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**Outline Purpose of this Presentation Full Custom (ASIC) Design**

Digital Hardware Implementation Basics Gates Arithmetic Field Programmable Gate Arrays (FPGAs) Layout How FPGA’s are used Skein Hashing Algorithm

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**Purpose Touch upon basic hardware elements**

Inform future cryptographers and designers of cryptographic algorithms of the benefits and limitations of hardware Present Skein as an algorithm with pretty good hardware compatibility

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**Full Custom (ASIC) Design**

Image contributed from Dr. Shaaban, CE Dept.

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**Digital Logic Gates Basic operation block**

1 or more input voltages, and exactly 1 output voltage Voltage is either High or Low (1 or 0) TTL (Bipolar Junction Transistors) CMOS (Complementary Metal Oxide Semiconductor Field Effect Transistors)

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**Primary Gates INVERT, AND, OR NAND and NOR**

INVERT isn’t always necessary depending on underlying technology NAND and NOR NAND is an AND gate with INVERTed Output NOR is an OR gate with INVERTed Output Schematic is similar to AND and OR, but with a bubble on the output (representing inverse) Either can be solely used to build any logic

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**Inverter Schematic Truth Table Algebraic Notation Input A Output Y 1**

1 Algebraic Notation Y = A’

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**AND Gate Schematic Truth Table Algebraic Notation Input A Input B**

Output Y 1 Algebraic Notation Y = AB

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**OR Gate Schematic Truth Table Algebraic Notation Input A Input B**

Output Y 1 Algebraic Notation Y = A + B

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**XOR Gate Schematic Truth Table Algebraic Notation Input A Input B**

Output Y 1 Algebraic Notation Y = A ⊕ B

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**XOR Gate (Continued) Can be composed of INVERT, AND, & OR**

A ⊕ B = A’B + AB’ But it can be easily implemented in hardware using faster methods

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**Gate Delay Gates are not instantaneous**

There is a delay between the time an input changes to the time an output changes

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**Arithmetic Operations**

Addition/Subtraction Multiplication Division/Modulus

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**Addition and Subtraction**

Ripple-Carry Adder Easiest to analyze Faster adders are used in industry Naffziger (Intel Core 2) Carry Look-ahead Adders, etc. Uses two components, Half Adder and Full Adder Full Adder has a third input for Carry-In compared to the Half Adder Subtraction is just addition by a negative number in 2’s complement notation

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**Ripple Carry Adder Algorithm**

Similar to manual addition Least Significant Bits (A0 and B0) are added together to produce a Sum Bit and a Carry Bit (S0 and C1). The next pair of bits (A1 and B1) are added together along with the previous Carry Bit (C1) to produce a Sum Bit and a Carry Bit (S1 and C2). The process repeats

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**Ripple-Carry Adder Components**

Half Adder Full Adder 2 Gate Delays for Sum bit 3 Gate Delays for Carry bit 1 Gate Delay to change the Sum Bit if the incoming Carry bit changes 2 Gate Delays to change the Carry bit if the incoming 1 Gate Delay for both the Sum bit and the Carry bit

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Ripple-Carry Adder

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**Ripple-Carry Adder Worst Case**

Worst Case Scenario is when C0 is 0, A is all 1’s and B is all 0’s, and then C0 changes to 1 The Carry has to propagate through all of the Full Adder Blocks For an n-bit Ripple-Carry Adder 2(n-1) + 1 gate delays to change the final Sum bit 2n gate delays to change the final Carry bit

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**Multiplication Generic Multiplier Constant Coefficient Multiplier**

Any two numbers can be multiplied together A * B = Y n-bit inputs produces 2n-bit output Constant Coefficient Multiplier Multiplication by a constant A * 5 = Y Easier to implement Used in Finite Impulse Response (FIR) Filters

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**Generic Multipliers O(n2) gate delays for an n-bit Generic Multiplier**

Very slow compared to addition Uses many resources compared to addition

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**Optimized 3-bit Generic Multiplier**

At most 11 gate delays

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**Optimized 8-bit Generic Multiplier**

At most 53 Gate Delays

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**Division/Modulus More complex than Multiplication**

Can be implemented as a series of subtractions Sequential logic may be better suited Uses Registers and a Clock signal

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**Shortcuts Multiplication Division Modulus**

If multiplying by a power of 2, shift left by the power Division If dividing by a power of 2, shift right by the power Modulus If taking a modulus of a power of 2, AND the bits with the (modulus – 1)

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**Full Custom Benefits Drawbacks Best Possible Performance**

Can be specially designed for low power consumption (embedded systems) or for high speed (PC expansion card) No restrictions on logic No restrictions on routing Expensive to design Expensive to test Fabrication takes months

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**Image contributed from Dr. Shaaban, CE Dept.**

FPGA Image contributed from Dr. Shaaban, CE Dept.

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**What is an FPGA? Field Programmable Gate Array (FPGA)**

It is an array of gates that can be programmed A good compromise between General Purpose Processors and Full Custom

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**Image contributed from Dr. Łukowiak, CE Dept.**

Layout of an FPGA Input and Output (I/O) Blocks Interface with the outside world LED display Switches, buttons, etc. Logic Blocks usually take 3-4 input signals and generate the desired output signal Data can be registered Interconnects can be programmed to connect logic blocks and I/O blocks together (Logic -> Logic, I/O -> Logic, Logic -> I/O, I/O -> I/O) Usually a special Clock network to avoid Clock skew problems Image contributed from Dr. Łukowiak, CE Dept.

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**How are FPGA’s actually used?**

They use a “programming language” VHDL -> VHSIC Hardware Description Language VHSIC -> Very High Speed Integrated Circuit Verilog -> C-like Language Programs are NOT Top-Down like C, BASIC, etc. The programs describe the hardware Very parallel with some sequential parts running in parallel

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Step 1: Simulation The programs run through a simulator which applies the correct input and generates the output Once the simulator produces the desired output, THE TASK IS NOT OVER YET!

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Step 2: Synthesis The Compiler will try to Synthesize the code into the appropriate logic blocks (Previous Multiplier Schematic was Synthesized from VHDL) Not all VHDL statements are Synthesizable while loop, wait statements, etc. Many times the program has to be adjusted to use only synthesizable commands… back to Simulation

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Step 3: Place & Route The compiler now figures out where to place each logic block, and how the logic blocks are interconnected Sometimes more hardware is needed than is actually on the specific FPGA device Buy a bigger FPGA Redesign the program to reuse more hardware, or to route data differently… back to Simulation

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**Step 4: Download to FPGA Download the program onto the FPGA**

Run the program and make sure the correct results are obtained If logic is too complex, then the clock frequency may have to be scaled down Gate delay exceeds clock period If everything works, then done

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**FPGA Benefits Drawbacks**

Better performance than General Purpose Processors Even though clock frequency may be MHz Easier to design than Full Custom Easier to test than Full Custom Good for prototyping Full Custom Not a Production-Grade piece of hardware No application uses 100% of everything available on the FPGA Some FPGA’s reset on power loss, and need to be reprogrammed

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**Skein Hashing Algorithm**

Different versions depending on the internal state and output size Skein has a 512-bit internal state, and 1024 output bits Skein is the default proposal Skein will be examined in this presentation Only 256, 512, and 1024 internal states supported Any output size may be used Skein-256 and Skein-512 have 72 rounds; Skein-1024 has 80 rounds Based on the Threefish Block Cipher (introduced alongside Skein) Threefish Block Cipher has 3 components MIX Permute Add Subkey Skein wraps a 512-bit XOR around Threefish to create a UBI block, which is chained together

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**Threefish Block Cipher**

Encryption starts with 8 64-bit Subkey additions Then there are 4 rounds of MIX and Permute followed by the next Subkey addition There are a total of 72 rounds The Cipher ends with the 18th Subkey addition

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The MIX Function One 64-bit addition One 64-bit rotate One 64-bit XOR

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**MIX Function Hardware Analysis**

64-bit Addition Full Custom (ASIC) isn’t too bad FPGA’s can handle a few of these Bit Rotation Simply a wire-mapping 64-bit XOR Even easier than Addition 1 Gate Delay

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The Permute Function 64-bit words are swapped between MIX functions

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**Permute Function Hardware Analysis**

Entirely wire mappings Not an issue

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Subkey Addition

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**Subkey Hardware Analysis**

8 XOR’s chained together 8 Gate Delays Subkey Index mod 9 (and 3) Full Custom (ASIC) can be hard-coded Creative methods must be done in FPGA Two 64-bit Additions chained together Additions taken mod 264 Our only good news!

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**Subkey Hardware Analysis Continued**

Eight 64-bit Additions happen “logically” in parallel Each of those Eight is really 2 64-bit Additions chained together, as mentioned previously To actually do this in parallel is a large hardware commitment To save on hardware, each addition should happen serially using the same Logic Blocks (FPGA) This may require external memory I/O between additions to swap out the addends VERY SLOW

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UBI Blocks

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**UBI Block Hardware Analysis**

One 512-bit XOR OK for Full Custom (ASIC), but a major pain Trouble for FPGA Wire-routing nightmare Chaining is no big deal ~640-bit register (512-bit “key”, 128-bit “tweak”)

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**FPGA Stats on a Spartan 2 for Skein**

Number of Slices: out of 2352 148% (*) Number of Slice Flip Flops: 4604 out of 4704 97% Number of 4 input LUTs: 6262 out of 4704 133% (*) Number of IOs: 62 Number of bonded IOBs: 44 out of 140 31% IOB Flip Flops: 4 Number of GCLKs: 2 out of 4 50%

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**Changes Necessary to Fit**

Complete redesign of the underlying components Specifically Subkey Minimize routing More utilization of external memory module Buy a bigger FPGA Spartan 3?

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**Any Questions, Comments or Concerns?**

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**References Dr. Łukowiak, C.E. Department Dr. Shaaban, C.E. Department**

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