Bouncy Castle is an open-source project that provides a collection of cryptographic APIs for both Java and C# (through its port, Bouncy Castle C#). It offers a wide range of cryptographic functionalities, including support for encryption, decryption, key generation, and secure hashing. Bouncy Castle is widely used for implementing security protocols and is known for its comprehensive library and active community support. Its website, http://bouncycastle.org, serves as the central hub for documentation, downloads, and the latest updates on the project.