feat: add ElGamalCipher with Safe Prime generation and stateless design

[singhc7]

Fixes #6934

Overview

This PR implements the ElGamal Encryption System, an asymmetric key cryptography algorithm based on the Diffie–Hellman key exchange and the difficulty of computing discrete logarithms.

This implementation focuses on cryptographic security and clean code principles, addressing common pitfalls like weak generator selection and stateful design.

Key Features

• Safe Prime Generation: explicitly generates Safe Primes ($p = 2q + 1$) to ensure large group order and prevent Pohlig-Hellman attacks.
• Rigorous Validation: Verifies that the generator $g$ is a primitive root modulo $p$ and enforces strict bounds for private keys ($1 < x < p-1$).
• Stateless Design: Uses Java record types (KeyPair, CipherText) to maintain a purely functional and thread-safe architecture.
• Semantic Security: Uses SecureRandom for ephemeral keys to ensure that encrypting the same message twice yields different ciphertexts (Indistinguishability).

Testing Strategy

Comprehensive JUnit 5 tests are included (ElGamalCipherTest.java) covering:

• Correctness: Basic encryption/decryption flow.
• Homomorphism: Verifies the Multiplicative Homomorphic property ($E(m_1) * E(m_2) = E(m_1 * m_2)$).
• Security: Verifies probabilistic encryption (same message = different ciphertexts).
• Edge Cases: Validates handling of message boundaries (0, 1, $p-1$) and invalid inputs.

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new algorithms include a corresponding test class that validates their functionality.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

❌ Patch coverage is 57.89474% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.32%. Comparing base (8e30bcb) to head (bca1864).

Files with missing lines	Patch %	Lines
.../java/com/thealgorithms/ciphers/ElGamalCipher.java	57.89%	6 Missing and 10 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7257      +/-   ##
============================================
- Coverage     79.36%   79.32%   -0.04%     
- Complexity     6981     6990       +9     
============================================
  Files           780      781       +1     
  Lines         22917    22955      +38     
  Branches       4501     4509       +8     
============================================
+ Hits          18187    18209      +22     
- Misses         4013     4019       +6     
- Partials        717      727      +10     

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