implementation of finite field inversion debdeep mukhopadhyay chester rebeiro dept. of computer...

IMPLEMENTATION OF

FINITE FIELD

INVERSION

Debdeep Mukhopadhyay Chester Rebeiro

Dept. of Computer Science and Engineering

Indian Institute of Technology Kharagpur

INDIA

Finite Field Inverse

23-27 May 2011 Anurag Labs, DRD0 2

Itoh-Tsujii Method for Binary Fields


The Steps


How do we do a SquaringConsider (again) the field GF(24), with

irreducible polynomial x4+x+1. What is (x3+x2+1)2 in this field ?


Squaring

Squaring can be represented in the form of a matrix multiplication T.a


Quad OperationQuad operation

can be done by two squaring operations.

Quad operation can be written in the form T2.a


Advantage of using Quad Operations

Quad circuits have better LUT utilization compared to Squarer circuits


Generalization of the Itoh-Tsujii Algorithm


Theorem 1


Theorem 2


Quad Itoh-Tsujii Inversion Algorithm


A Circuit for InversionAt every

clock cycle, either the multiplier or the quadblock is active.

The output of the multiplier is stored in mout register


Finding the Inverse


Finding the Inverse Step 2


Control Signals for the Inverse


Performance Charts


Higher Powered Itoh-Tsujii


• We seen that Quad circuits utilize LUTs in a better way compared to squarer circuits.

• Also LUT size is increasing as silicon technology reduces

• We have seen 4-LUT become 6-LUT, and now 8-LUT

• This gives us a motivation to investigate using higher powers other than quad circuits

Revisiting the Theorems


2n Itoh-Tsujii Inversion


These are the overheads

Higher Powered

Overhead in 2n Itoh-Tsujii


• Computation of .

• Using addition chain for , can be computed in clock cycles, where is the length of addition chain for .

• Computation of , for

• Using addition chain for , that contains , can be

computed during computation, because .

2n Itoh-Tsujii Design


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Configurable Parameters

• Addition chain.

• Power circuit used in power block.

• Number of cascaded power

circuits in the power block.

• These have an effect on – Number of clock cycles.

– Critical path delay.

Building the Optimal Design

For a given field and a given FPGA how do decide the optimal

design ?

23-27 May 2011 Anurag Labs, DRD0

Estimating AREA required on an FPGA


• A k input LUT (k-LUT) can implement any functionality of maximum k input variables.

• Total number of k-LUTs to implement a function with variables can be expressed as

Estimating Delay of a Design in an FPGA


• Delay in FPGAs comprise of LUT delay and routing delay..

• For this ITA architecture, we have experimentally found, total delay is proportional to number of LUTs in critical path.

• We denote number of LUTs in a delay path as maxlutpath.

• In k-LUT, maxlutpath of an variable function is

Recap : Karatsuba Multiplier


Hybrid Karatsuba Multiplier for GF(2233)Note that the school book multiplier

has replaced the general Karatsuba Multiplier


School Book Multiplier

29

• The field multiplier is a hybrid Karatsuba multiplier.

• A bit hybrid Karatsuba multiplier consists of two bit and one bit multipliers. This happens in recursive manner.

• In threshold ( ) level, School-Book multiplier is invoked.

• Total area of bit hybrid Karatsuba multiplier is given by

• Total area for the School-Book multiplier is

Estimating LUT Requirement for Hybrid Karatsuba Multiplier


Estimating Delay of Hybrid Karatsuba Multiplier


• The hybrid Karatsuba multiplier is distributed in smaller multipliers like a tree. Height of the tree is

• Each level of the Simple Karatsuba tree introduces one LUT delay.

• In threshold ( ) level, School-Book multiplier delay is added.

• Delay of School-Book multiplier is

• Delay of the entire multiplier in LUTs is given by

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• For fields generated by trinomials, area of modular reduction

is almost equal to field size and delay is one LUT considering LUT size .

• For fields generated by pentanomials, – and 2 LUT for .

– and 2 LUT for .

Estimating Area & Delay for Modular Reduction


Area & Delay Estimates for 2n Circuit


• The output of a 2n circuit, which raises an input can be expressed as , where is binary field matrix

and ,

• LUT requirement per output bit is

• Total LUT requirement for the 2n circuit is

• LUT delay per output bit is

• Since all bits are in parallel, delay of 2n circuit is

Area & Delay Estimates for Multiplexer


• For a 2s : 1 MUX, there are s selection lines and thus the output is a function of 2s + s variables.

• For a MUX in , each of the 2s input lines is of width m bits.

• Total LUT requirement is

• Total LUT delay of the MUX is

• When number of inputs to MUX , the above gives a close upper bound

Area & Delay of PowerBlock


• Let the Powerblock contains us number of cascaded 2n circuits.

• The has selection lines, where

• LUT requirement for is

• Total LUT requirement for Powerblock is

• Delay of is

• Total LUT delay of Powerblock in

Area & Delay for the Entire Architecture


• LUT estimate for the entire architecture is

• There are two parallel delay paths.– LUT delay of first path is

– LUT delay of second path is

– LUT delay of entire architecture is

Optimal Number of Cascades


• For a given field and based FPGA, Powerblock can be configured with different power circuits and cascades .

• Increase in reduces clock cycles, but increases delay of Powerblock.

• is fixed, but depends on and .

• is minimum when

• Minimum delay of the ITA architecture is thus

Power Circuit Selection to achieve Minimum Clock Cycles


• Number of clock cycles for the inversion can be approximated as

• Number of clock cycles for increases linearly with .

• The term reduces with increase in .

• When is small, the reduction in is significant for increase in .

• But, for large values of n, the increase in dominates over the decrease in

• So, increases with increase in for large values of .

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• The performance metric is

• Minimization of without increasing gives best performance. Area remains almost same.

• The following steps are performed to achieve optimal performance

• The optimal architecture is given by

Tuning Design for Optimality


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• Our estimation model uses maxlutpath to find LUT delay.

• Routing delay is difficult to model in FPGAs.

• To get overall delay, we have used experimental results for a reference ITA architecture.

• Total delay of reference architecture is the

• Let LUT delay of reference architecture is

• Total delay of any other ITA architecture in the same field is approximately

• Here is a constant and depends on FPGA technology.

• In 4-LUT based and 6-LUT based

Xilinx FPGAs, has values 0.2 and 0.1 respectively.

Validation of Theoretical Estimates


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Validation on 4-input LUT FPGAs


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Validation on 6-input LUT FPGAs


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Experimental Results


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Comparison Charts


implementation of finite field inversion debdeep mukhopadhyay chester rebeiro dept. of computer...

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