Algorithms & Limitations¶

SSHClient implements a deliberately small set of modern-enough algorithms, negotiated per RFC 4253 §7.1: for each slot the client proposes a short preference list and the first algorithm the server also offers is selected, independently for each direction. The server must support at least one algorithm per slot — OpenSSH does, in its default configuration.

Negotiated algorithms¶

Preference lists, most preferred first:

Role	Algorithms
Key exchange	`diffie-hellman-group14-sha256` (RFC 3526 2048-bit MODP)
Server host key	`rsa-sha2-256` — the server must have an RSA host key
Cipher (per direction)	`aes128-ctr`, `aes256-ctr`
MAC (per direction)	`hmac-sha2-256`, `hmac-sha2-512`
Compression	none

Crypto primitives are provided by the nettle/GMP library inside Conga's SSL plugin, called via ⎕NA; the SSH protocol logic is APL.

Authentication¶

Supported	Not supported
password	keyboard-interactive
RSA public key (`rsa-sha2-256`), unencrypted `openssh-key-v1` files	encrypted (passphrase) key files
	ed25519 / ECDSA / DSA keys
	agent forwarding, certificates

Unsupported key files are rejected with a specific message, e.g. Unsupported key type: ssh-ed25519.

Features and limits¶

Feature	Status
Remote command execution (`Exec`)	✓ stdout, stderr, exit status, exit signal
SFTP v3 upload / download / list / stat	✓ binary-exact, UTF-8 names
Multiple sequential channels per connection	✓
Reconnect after close or failure	✓
Interactive shell / PTY	✗
Port forwarding / tunnels	✗
SCP	✗ (use SFTP)
Concurrent channels / pipelined SFTP	✗ (operations are sequential)
Rekeying	✗
Hashed known_hosts entries	✓ matched (HMAC-SHA-1), written on accept-new when the file uses them

Interoperability¶

Developed and continuously tested against OpenSSH (8.x–10.x, internal-sftp), including a server restricted to Ciphers aes256-ctr and MACs hmac-sha2-512 to prove the fallback path. The client selects algorithms from the server's KEXINIT before proceeding, so a slot with no mutual algorithm fails fast with a message naming the gap and listing what the server offers, e.g.:

Algorithm negotiation failed: server does not offer
diffie-hellman-group14-sha256 (key exchange); server offers:
mlkem768x25519-sha256 sntrup761x25519-sha512 curve25519-sha256 ...

OpenSSH 10 disables modp Diffie-Hellman by default

From OpenSSH 10.0 (shipped in Debian 13 "trixie" and derivatives), finite-field Diffie-Hellman key exchange is disabled by default. To accept connections from this client, the server needs it re-enabled in sshd_config (+ appends to the defaults):

KexAlgorithms +diffie-hellman-group14-sha256

The test suite's setup_sshd.sh does this for its throwaway servers, along with Ciphers +aes128-ctr,aes256-ctr and MACs +hmac-sha2-256,hmac-sha2-512 for servers whose defaults have been narrowed.