Add support for ssh-ed25519 (ED25519) to DataPower SSH / SFTP client

SSH-ED25519, also known as ED25519, is a widely used public-key cryptographic algorithm for securing SSH (Secure Shell) connections. It offers several advantages over older cryptographic algorithms like RSA or DSA:

Strong Security: ED25519 provides strong security with relatively short key lengths. A 256-bit key in ED25519 offers security comparable to a 3072-bit RSA key, making it resistant to current and foreseeable cryptographic attacks.

Speed: ED25519 is highly efficient and performs faster than many other cryptographic algorithms, including RSA and DSA. This efficiency is especially important in resource-constrained environments and for high-performance SSH connections.

Small Key Sizes: As mentioned earlier, ED25519 keys are relatively short compared to equivalent-strength RSA keys. This means that key generation and exchange are faster and use less bandwidth.

Reduced Vulnerability to Side-Channel Attacks: ED25519 is designed with security in mind, including protection against side-channel attacks, which are attacks that exploit information leaked during cryptographic operations, such as timing or power consumption.

Simplicity: The algorithm is straightforward and easier to implement correctly than some other cryptographic systems. This reduces the risk of implementation errors that could lead to vulnerabilities.

Resistance to Quantum Attacks: While quantum computers could potentially break traditional asymmetric encryption methods like RSA and ECC (Elliptic Curve Cryptography), ED25519 is believed to be quantum-resistant, making it a more future-proof choice.

Open Standard: ED25519 is a widely accepted and standardized algorithm, ensuring compatibility across various SSH implementations and software.

Wide Adoption: Over the years, ED25519 has gained widespread adoption in the security community and is supported by many SSH clients and servers, making it a practical choice for securing SSH connections.

Forward Secrecy: ED25519 keys can be used with SSH key exchange protocols that provide forward secrecy, ensuring that even if an attacker obtains the private key in the future, they cannot decrypt past SSH sessions.

In summary, ED25519 offers a combination of strong security, efficiency, simplicity, and resistance to emerging cryptographic threats, making it an excellent choice for securing SSH connections and other cryptographic applications. It has become the recommended default for SSH key generation in many contexts due to its advantages.