SSH Client

What is it?

The SSH Client is the planned ssh.exe: an SSH2 client with an interactive remote shell in the terminal, host-key trust on first use, a Windows-style configuration file, scp file transfer and a stretch sftp command. The wire protocol is specified by the networking roadmap; this roadmap adds the user-facing program around it. Nothing is built yet, but every cryptographic primitive it needs already ships in the kernel.

How does it work?

Today. No SSH code exists, and the network stack has no TCP yet. The crypto it will use is in the tree:

• Monocypher (vendored, monocypher.h): crypto_x25519() and crypto_x25519_public_key() for the key exchange, ChaCha20 and Poly1305 for packet encryption, and BLAKE2b.
• Ed25519 (monocypher-ed25519.h): crypto_ed25519_key_pair(secret_key, public_key, seed), crypto_ed25519_sign() and crypto_ed25519_check(), the SHA-512 form that the ssh-ed25519 key type uses.
• SHA-256 (sha256.h) for the exchange hash and key derivation, and the kernel CSPRNG's csprng_fill() (csprng.h) for ephemeral keys and cookies.
• The terminal (terminal.h): terminal_puts() and terminal_trygetchar() are what the remote shell relay reads and writes.

Planned design.

sequenceDiagram
    participant C as ssh.exe
    participant S as Server
    C->>S: Version banner, KEXINIT
    S->>C: KEXINIT
    C->>S: ECDH init (Curve25519 public key)
    S->>C: ECDH reply (host key, public key, signature)
    Note over C: Verify host key (trust on first use)
    C->>S: NEWKEYS, then encrypted user auth
    C->>S: Session channel, PTY request, shell

1. Transport. Version exchange, curve25519-sha256 key exchange, an ssh-ed25519 host key and chacha20-poly1305@openssh.com packets. Session keys come from the RFC 4253 section 7.2 hash construction over SHA-256, which is what OpenSSH servers expect.
2. Authentication. Password with three attempts, then public key with an Ed25519 key pair created on first use under C:\Users\{name}\AppData\.
3. Interactive channel. A session channel with a PTY request, relaying bytes between the socket and the terminal, with window adjustment.
4. Host trust. Trust on first use: the first connection records the server's key, and a later mismatch shows a warning and refuses.
5. scp upload and download in 64 KiB chunks with a progress bar, and sftp as a stretch.
6. Configuration: an INI-style file with Host stanzas so ssh alias resolves to a host, user and port.

What are its interfaces?

How do I use it?

ssh cannot be run yet. The primitives it depends on already run: the kernel unit tests exercise X25519 and Ed25519 (test_klibs.c), and code integrity checks signatures with crypto_ed25519_check() (ci_crypto.c). See Kernel Embedded Libraries for the vendored crypto.

Which roadmap owns which half?

The networking roadmap SSH, FTP and SMB Clients owns the protocol: transport, authentication, channels, ssh-keygen and the agent. This roadmap owns the program the user runs: host trust, the configuration file, scp and sftp sessions. Three details still disagree between the two files and are filed in section 4 for a decision: where known host keys live (a known_hosts file there, a Registry key here), the case of the AppData folder name, and the fingerprint format.

What is not implemented yet?

Nothing in this roadmap has started:

• SSH2 Transport Layer, which needs TCP sockets
• SSH Authentication
• Interactive Channel and PTY Relay
• Shell Integration and Known Hosts
• SCP File Transfer and the SSH Config File
• sftp, a stretch goal

How does it compare with Windows 11 and Linux?

Windows 11 ships OpenSSH's ssh.exe, scp.exe and sftp.exe, keeping known_hosts and config in %USERPROFILE%\.ssh. Linux ships the same OpenSSH tools with files in ~/.ssh. The Impossible OS plan uses the kernel's own vendored crypto instead of a ported OpenSSH, relays the shell through its own terminal rather than a POSIX pty or ConPTY, and keeps settings in Windows-style locations. It does not exist yet.

See also

• SSH Client roadmap
• SSH, FTP and SMB Clients
• Terminal
• DNS and Sockets
• Kernel Embedded Libraries
• CNG Crypto and Certificate Store