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

• Code compiles successfully
• Tests passed locally (BUILD SUCCESS)
• Follows project style and structure
• Added clear example output
• 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

❌ Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.31%. Comparing base (3979e82) to head (a7792eb).

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

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7064      +/-   ##
============================================
- Coverage     78.36%   78.31%   -0.05%     
  Complexity     6690     6690              
============================================
  Files           756      757       +1     
  Lines         22208    22223      +15     
  Branches       4363     4363              
============================================
+ Hits          17403    17405       +2     
- Misses         4106     4120      +14     
+ Partials        699      698       -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