Feature/elgamal cipher

[harsha08-2k6]

This PR implements the ElGamal Encryption Algorithm as requested in issue #6934. ElGamal is a public-key cryptosystem based on the Discrete Logarithm Problem (DLP) that provides semantic security through randomization.
Fixes #6934
🎯 What Has Been Done

✅ Implemented ElGamal key generation with configurable bit length (512, 1024, 2048 bits)
✅ Implemented encryption method using random ephemeral key for semantic security
✅ Implemented decryption method using modular multiplicative inverse
✅ Added comprehensive documentation explaining the mathematical operations and security properties
✅ Created demonstration examples in main() method showing:

Integer encryption/decryption
Semantic security demonstration (same plaintext → different ciphertexts)
String encryption/decryption with utility methods

✅ Included proper error handling for invalid inputs
✅ Used SecureRandom for cryptographically secure random number generation

📂 File Details

Location: src/main/java/com/thealgorithms/ciphers/ElGamalCipher.java
Package: com.thealgorithms.ciphers
Classes:

ElGamalCipher (main utility class)
KeyPair (inner class for key storage)
Ciphertext (inner class for ciphertext representation)

🔐 Algorithm Overview
Key Generation:

Generate large prime p
Find generator g of multiplicative group mod p
Choose random private key x ∈ [2, p-2]
Compute public key y = g^x mod p

Encryption (message m):

Choose random k ∈ [2, p-2]
Compute c1 = g^k mod p
Compute c2 = m * y^k mod p
Return ciphertext (c1, c2)

Decryption (ciphertext (c1, c2)):

Compute s = c1^x mod p
Compute s^-1 mod p (modular inverse)
Recover m = c2 * s^-1 mod p

🧪 Testing
The implementation includes test cases in the main() method:

Basic encryption/decryption of integer 12345
Semantic security demonstration - encrypting the same message twice produces different ciphertexts
String encryption - demonstrates conversion utilities for text encryption

[codecov-commenter]

Codecov Report

❌ Patch coverage is 43.10345% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.39%. Comparing base (fa8ea8e) to head (1153425).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
.../java/com/thealgorithms/ciphers/ElGamalCipher.java	0.00%	31 Missing ⚠️
...a/com/thealgorithms/backtracking/SudokuSolver.java	92.59%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7100      +/-   ##
============================================
- Coverage     78.50%   78.39%   -0.11%     
+ Complexity     6752     6747       -5     
============================================
  Files           759      760       +1     
  Lines         22402    22428      +26     
  Branches       4400     4399       -1     
============================================
- Hits          17587    17583       -4     
- Misses         4109     4139      +30     
  Partials        706      706              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.