Add BigInt utility functions for number theory operations

[p-leriche]

Description

This PR adds utility functions for working with BigInt values in number theory operations. These utilities provide a foundation for upcoming cryptographic and mathematical operations.

Changes

• Add src/core/lib/BigIntUtils.mjs with:
  • parseBigInt() - Parse decimal or hexadecimal (0x...) strings to BigInt
  • egcd() - Extended Euclidean Algorithm (iterative implementation, no recursion limits)

Features

• Supports decimal and hexadecimal input (0x prefix)
• Iterative algorithm handles arbitrarily large integers without stack overflow
• Returns GCD and Bézout coefficients for use in modular arithmetic

Testing

• Tested with decimal inputs
• Tested with hexadecimal inputs
• Tested with crypto-grade large integers (2048+ bit)
• No recursion depth issues with iterative implementation
• Built successfully with npm run build

Related

This is a dependency for PRs:

• Modular Exponentiation Add Modular Exponentiation operation #2149
• Extended GCD operation Add Extended GCD operation #2206
• Modular Inverse operation Add Modular Inverse operation #2207

Related PRs:

• Random Prime Generation Add Generate Prime Number operation #2212
• String/Integer conversion Add Text/Integer Converter operation #2213

This collection of 5 new operations together with this libray of supporting functions has been created as a teaching resource for Diffie-Hellman and RSA, hopefully as an extension activity for CyberFirst Girls' competitors. (Unfortunately, my last years groups have all dispersed - but there's always another year.) After a frantic burst of activity, no more related PRs are currently envisaged.

A build containing all 5 new operations is online at https://blueskylark.org/CyberFirst/CyberChef

[GCHQDeveloper581]

I've added some very basic tests (since this represented a new "type" of testing and I wanted to establish the file structure).

Please could you add to these to more completely test the desired functionality.

[GCHQDeveloper581]

I appreciate that there are tests for the Operations you've added in separate PRs and these will to some extent overlap. However I think it best practice to have unit tests to test these library functions, and then test the higher level functionality of the Operation(s) in their tests.

(this may mean moving some of the detailed testing from the Operations into the unit tests).