Add ElGamalCipher implementing ElGamal encryption and decryption

[polasisubash]

Description

Added a new class ElGamalCipher.java implementing the ElGamal encryption and decryption algorithm.

Features

• Implements ElGamal public-key encryption system
• Supports key generation using modular arithmetic
• Includes methods for encryption and decryption
• Demonstrates successful decryption of encrypted messages

Example Output

Original: 12345 Encrypted: (c1, c2 values) Decrypted: 12345

Technical Notes

• Implemented under src/main/java/com/thealgorithms/ciphers/
• Successfully compiled and tested locally using Maven (BUILD SUCCESS)
• Compatible with Java 21
• Code adheres to the project’s structure and conventions

References

• Fixes issue: #13580
• Algorithm based on ElGamal public-key cryptography principles

Checklist

• [x] Code compiles successfully
• [x] Tests passed locally (BUILD SUCCESS)
• [x] Follows project style and structure
• [x] Added clear example output
• [x] Confirmed correct encryption and decryption results

[polasisubash]

Hi maintainers 👋,
I’ve implemented the ElGamal encryption and decryption algorithm as described in the feature request (#6934).
The code compiles successfully, passes local tests, and follows project conventions.
Looking forward to your review. Thank you! 🙌

[codecov-commenter]

Codecov Report

:x: Patch coverage is 0% with 15 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 78.45%. Comparing base (2c4bf3c) to head (9d421bc).

Files with missing lines	Patch %	Lines
.../java/com/thealgorithms/ciphers/ElGamalCipher.java	0.00%	15 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7064      +/-   ##
============================================
- Coverage     78.51%   78.45%   -0.07%     
  Complexity     6752     6752              
============================================
  Files           759      760       +1     
  Lines         22402    22417      +15     
  Branches       4400     4400              
============================================
- Hits          17589    17587       -2     
- Misses         4108     4124      +16     
- Partials        705      706       +1     

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[polasisubash]

✅ All checks have passed successfully. The implementation follows the existing cipher structure and includes key generation, encryption, and decryption for ElGamal. Kindly review when you get a chance. Thank you! @DenizAltunkapan @alxkm Kindly review this PR when possible. All checks have passed ✅

[polasisubash]

✅ All checks have passed successfully. The implementation follows the existing cipher structure and includes key generation, encryption, and decryption for ElGamal. Kindly review when you get a chance. Thank you! @DenizAltunkapan @alxkm Kindly review this PR when possible. All checks have passed ✅

[polasisubash]

✅ All checks have passed successfully. The implementation follows the existing cipher structure and includes key generation, encryption, and decryption for ElGamal. Kindly review when you get a chance. Thank you! @DenizAltunkapan @alxkm

[polasisubash]

✅ All checks have passed successfully. The implementation follows the existing cipher structure and includes key generation, encryption, and decryption for ElGamal. Kindly review when you get a chance. Thank you! @DenizAltunkapan @alxkm

[DenizAltunkapan]

@polasisubash Please always add a corresponding test class to your implementation