[accless] Implement CP-ABE Hybrid Scheme

[csegarragonz]

Closes #44

[gemini-code-assist]

Summary of Changes

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This pull request introduces a hybrid Ciphertext-Policy Attribute-Based Encryption (CP-ABE) scheme, enhancing the existing ABE library by combining its key encapsulation capabilities with efficient symmetric encryption. This allows for the secure encryption of arbitrary-sized data under CP-ABE policies, leveraging AES-GCM-128 for data confidentiality and integrity, while using HKDF-SHA256 for robust key derivation. The changes include core Rust implementations, C++ Foreign Function Interface (FFI) bindings, and thorough test coverage to ensure correctness and security.

Highlights

• Hybrid CP-ABE Scheme Implementation: Introduced a new hybrid Ciphertext-Policy Attribute-Based Encryption (CP-ABE) scheme, combining the existing CP-ABE Key Encapsulation Mechanism (KEM) with AES-GCM-128 symmetric encryption for efficient data handling.
• Key Derivation: Implemented robust key derivation using HKDF-SHA256 to generate AES128 keys from the group elements (Gt) produced by the CP-ABE KEM.
• C++ FFI Bindings: Added C++ Foreign Function Interface (FFI) wrappers for the new hybrid encryption and decryption functions, allowing seamless integration with C++ applications.
• Comprehensive Testing: Included extensive test cases in both Rust and C++ to validate the hybrid scheme's functionality, including successful round-trip encryption/decryption, handling of unauthorized access, and integrity checks for Associated Authenticated Data (AAD).
• Dependency Updates: Updated and added necessary cryptographic dependencies such as aes-gcm, hkdf, and zeroize across Cargo.lock, Cargo.toml, and accless/libs/abe4/Cargo.toml to support the new hybrid scheme.

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Code Review

This pull request introduces a hybrid CP-ABE scheme, combining the existing ABE key encapsulation mechanism with AES-GCM for symmetric encryption. The changes span across the Rust core library, C++ bindings, and associated tests. The implementation of the hybrid encryption itself appears sound. My review provides feedback on several points to enhance code quality: improving error handling in the Rust library by replacing expect calls with Result propagation, adding crucial # Safety documentation to the new unsafe FFI functions, reducing boilerplate code in the FFI layer for better maintainability, ensuring naming consistency between the Rust and C++ code, and refactoring duplicated code in the tests.