Created DiffieHellman.java and MonoAlphabetic.java

[riti2601]

Description

This PR implements the Diffie-Hellman Key Exchange and MonoAlphabetic Cipher algorithms.

Key Changes:

1. Diffie-Hellman Key Exchange:

   • Added DiffieHellman.java under the ciphers directory.
   • The implementation allows two users (Person A and Person B) to securely generate a shared secret key over an insecure channel using large prime numbers and modular arithmetic.
   • The code prompts the users to input their secret numbers and performs the necessary calculations to exchange and compute the shared secret keys.

2. MonoAlphabetic Cipher:

   • Added MonoAlphabetic.java under the ciphers directory.
   • The implementation allows the user to either encrypt or decrypt data using a predefined substitution cipher, where each letter of the alphabet is replaced with a corresponding letter.
   • The user is prompted to enter the data, and the cipher is applied to return either the encrypted or decrypted result.

Both algorithms are utility classes, with private constructors to prevent instantiation and static methods to handle their respective functionalities.

• 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 code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 65.51724% with 10 lines in your changes missing coverage. Please review.

Project coverage is 66.97%. Comparing base (5a1f681) to head (338859c).

Files with missing lines	Patch %	Lines
...java/com/thealgorithms/ciphers/MonoAlphabetic.java	66.66%	4 Missing and 2 partials ⚠️
.../java/com/thealgorithms/ciphers/DiffieHellman.java	63.63%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5508      +/-   ##
============================================
- Coverage     66.98%   66.97%   -0.01%     
- Complexity     4523     4530       +7     
============================================
  Files           613      615       +2     
  Lines         16990    17019      +29     
  Branches       3282     3286       +4     
============================================
+ Hits          11380    11398      +18     
- Misses         5160     5166       +6     
- Partials        450      455       +5     

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[vil02]

Please remove the main methods and add proper junit tests. Prefer ParameterizedTests.

[riti2601]

Hi @vil02,
I have made the necessary changes according to your suggestions.
Thank you for your valuable feedback!

[siriak]

@vil02 please review

[riti2601]

Hi @vil02, kindly review the changed code.

[riti2601]

Hello @vil02 @siriak @alxkm @BamaCharanChhandogi @yanglbme
I am participating in Hacktoberfest'25 and request that you kindly check the PR for further merging.
Thank you

Because he is not responding