feat(ciphers): add Merkle signature scheme

[dilaraacetin]

Fixes #7628

Description

Adds an educational implementation of the Merkle Signature Scheme (MSS) to the ciphers package. MSS turns a one-time signature scheme into a many-time scheme. It authenticates 2^h Winternitz one-time key pairs under a single public key, which is the root of a binary hash tree.

This builds directly on the recently merged WinternitzSignature (#) and continues the hash-based signature series: Lamport → WOTS → MSS. The same construction forms the core of XMSS (RFC 8391).

How it works

• Key generation: 2^h WinternitzSignature instances are created. Each leaf is SHA-256 of the concatenated WOTS public key, each parent is SHA-256(left || right), and the root is the MSS public key.
• Signing: The next unused WOTS key signs the message. The signature also contains the leaf index, the WOTS public key and the authentication path (one sibling hash per level). An IllegalStateException is thrown once all 2^h keys are used.
• Verification: The WOTS signature is verified first. Then the leaf is recomputed and hashed up the tree using the auth path, where the leaf index bit at each level decides left/right order. The result is compared with the public key using MessageDigest.isEqual.

Changes

• src/main/java/com/thealgorithms/ciphers/MerkleSignatureScheme.java: implementation with a nested immutable MerkleSignature class
• src/test/java/com/thealgorithms/ciphers/MerkleSignatureSchemeTest.java: JUnit 5 tests

No other files are modified.

Design notes

• Reuses the existing WinternitzSignature class, so no hashing or chain logic is duplicated.
• Tree height is limited to 2 ≤ h ≤ 10, since key generation creates 2^h WOTS key pairs up front.
• The WOTS public key is included in the signature instead of being recomputed from it. This is a deliberate simplification for readability and is documented in the Javadoc.
• All returned arrays and signature fields are deep-copied to keep the internal state immutable.
• Educational implementation only; it is not a production-grade XMSS (no stateful key management, no bitmasks or tweakable hashes).

Tests

• All 2^h signatures from one key pair are signed and verified individually.
• Signing beyond capacity throws IllegalStateException, and remainingSignatures() decreases correctly.
• Verification fails for a tampered message, WOTS signature, auth path element or leaf index, and for a different instance's public key.
• Parameterized over h ∈ {2, 4, 6} and w ∈ {4, 16}.
• Invalid constructor arguments, null inputs and immutability of getters.

mvn clean verify passes locally, and all files are formatted with clang-format -i --style=file.

References

• https://en.wikipedia.org/wiki/Merkle_signature_scheme
• RFC 8391 (XMSS): https://datatracker.ietf.org/doc/html/rfc8391

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 97.46835% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.69%. Comparing base (2105b56) to head (d7e0cd5).

Files with missing lines	Patch %	Lines
...m/thealgorithms/ciphers/MerkleSignatureScheme.java	97.46%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7629      +/-   ##
============================================
+ Coverage     81.64%   81.69%   +0.04%     
- Complexity     8167     8194      +27     
============================================
  Files           837      838       +1     
  Lines         25655    25734      +79     
  Branches       5009     5027      +18     
============================================
+ Hits          20947    21024      +77     
- Misses         3922     3923       +1     
- Partials        786      787       +1     

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[alxkm]

Looks good. Thank you for the contribution.