Serpent Cipher

Serpent is a strong 256-bit block cipher (a finalist in the AES competition), but it is not included in OpenSSH by default.
Most SSH servers use AES (default) or ChaCha20-Poly1305 for performance and security.
Performance concerns – Serpent is slower than AES, Serpent is slower than AES due to its complex design. especially in software implementations without hardware acceleration.

Serpent Pros
256-bit Block Cipher Serpent uses 256-bit blocks, making it more resistant to certain attacks. → Unlike AES, which operates in 128-bit blocks, Serpent’s 256-bit blocks make it theoretically more resistant to attacks like meet-in-the-middle.
Stronger Against Side-Channel Attacks → AES implementations (especially hardware-accelerated ones) have been vulnerable to cache-timing attacks.
Serpent’s S-box design makes it more resistant to these threats.
No Known Practical Attacks → Unlike AES, where related-key attacks and potential NSA backdoors are rumored, Serpent remains untouched by practical cryptanalysis.
High-Round Security → Serpent uses 32 rounds of encryption Serpent uses 32 encryption rounds, making it highly secure. (AES uses only 10, 12, or 14).
This makes brute-force or differential cryptanalysis significantly more difficult.

Enabling Serpent Cipher Steps:
1. Recompile OpenSSH with a custom OpenSSL build that includes Serpent support.
This requires configuring OpenSSL with Serpent-enabled ciphers and linking OpenSSH to the modified library.
2. Patch OpenSSH source code to manually add Serpent as an accepted cipher.
Modify the cipher list in cipher.c and myproposal.h, then recompile OpenSSH.