[FEATURE REQUEST] Implement ChaCha20 stream cipher #7753

What would you like to Propose?

I would like to propose adding ChaCha20, a modern symmetric-key stream cipher, to the ciphers package.

ChaCha20 is a stream cipher that generates a pseudorandom keystream using a 256-bit key, a 96-bit nonce, and a 32-bit block counter. It uses ARX (Addition, Rotation, and XOR) operations to generate 512-bit keystream blocks, which are XORed with plaintext to perform encryption and decryption.

Currently, the repository contains several classical and modern cryptographic algorithms, but does not appear to include a standalone ChaCha20 implementation.

Issue details

Algorithm Name

ChaCha20 Stream Cipher

Problem Statement

Implement the ChaCha20 stream cipher in Java following the specification defined in [RFC 8439](https://www.rfc-editor.org/rfc/rfc8439).

The implementation should generate a 512-bit keystream block from a 256-bit secret key, a 96-bit nonce, and a 32-bit block counter. The generated keystream should then be XORed with the input data to support encryption and decryption.

Proposed Implementation

The implementation should include the following components:

1. State Initialization: Initialize the 4 × 4 state matrix using the predefined constants, secret key, counter, and nonce.
2. Quarter Round: Implement the core ARX operations (addition, rotation, and XOR) with the specified rotation constants.
3. Round Operations: Implement the 20-round transformation consisting of column rounds and diagonal rounds.
4. Block Generation: Generate 64-byte keystream blocks by applying the ChaCha20 transformation and adding the original state.
5. Encryption: XOR plaintext bytes with the generated keystream.
6. Decryption: Reuse the same keystream generation process to recover the original plaintext.

Expected Behavior

• Accept a 256-bit key and a 96-bit nonce.
• Support arbitrary-length byte-array input.
• Correctly process inputs spanning multiple 64-byte blocks.
• Validate key and nonce lengths.
• Handle empty input.
• Ensure correct counter progression between blocks.
• Prevent counter overflow and invalid counter usage.

Testing Requirements

Add JUnit tests covering:

• RFC 8439 known test vectors.
• Quarter-round correctness.
• Block-function correctness.
• Encryption and decryption round trips.
• Empty input.
• Inputs smaller than, equal to, and larger than 64 bytes.
• Invalid key and nonce lengths.
• Multiple-block encryption and decryption.
• Counter boundary conditions.

References

• [RFC 8439 – ChaCha20 and Poly1305 for IETF Protocols](https://www.rfc-editor.org/rfc/rfc8439)
• [ChaCha20 Original Specification](https://cr.yp.to/chacha.html)

Additional Information

No response