Supported Cryptographic Algorithms

Net::SSH supports a wide range of modern and legacy cryptographic algorithms. As of version 6.0, weak algorithms are disabled by default to promote better security. You can, however, re-enable them for backward compatibility.

Customizing Algorithms

You can specify your preferred algorithms when starting a connection. The value can be a single string or an array of strings. The order determines preference.

Net::SSH.start(host, user,
  kex: 'diffie-hellman-group-exchange-sha256',
  encryption: %w[aes256-ctr aes128-ctr],
  hmac: 'hmac-sha2-512',
  host_key: 'ssh-ed25519'
)

Modifying the Default List

You can also add to or remove from the default algorithm list using + and - prefixes. This is useful for disabling a specific weak algorithm without replacing the entire list.

# Remove all CBC-mode ciphers from the default list
Net::SSH.start(host, user, encryption: '-*-cbc')

# Add a legacy HMAC algorithm for an old server
Net::SSH.start(host, user, hmac: '+hmac-md5')

Re-enabling All Supported Algorithms

For compatibility with very old SSH servers, you can revert to the pre-6.0 behavior of enabling all supported algorithms, including weak ones, by using the :append_all_supported_algorithms option.

Net::SSH.start(host, user, append_all_supported_algorithms: true)

Algorithm Support Details

The following tables detail the algorithms supported by Net::SSH. Algorithms marked as "Deprecated" are disabled by default.

Host Keys

Name	Default Support	Notes
`ssh-ed25519`	Enabled	Requires the `ed25519` gem.
`ecdsa-sha2-nistp521`	Enabled	Uses potentially weak elliptic curves.
`ecdsa-sha2-nistp384`	Enabled	Uses potentially weak elliptic curves.
`ecdsa-sha2-nistp256`	Enabled	Uses potentially weak elliptic curves.
`rsa-sha2-512`	Enabled	RSA with SHA-2 512-bit signature.
`rsa-sha2-256`	Enabled	RSA with SHA-2 256-bit signature.
`ssh-rsa`	Enabled	Standard RSA with SHA-1 signature.
`ssh-dss`	Deprecated in v6.0	Insecure, will be removed in v8.0.

Key Exchange (KEX)

Name	Default Support	Notes
`curve25519-sha256`	Enabled	Requires the `x25519` gem.
`ecdh-sha2-nistp521`	Enabled	Uses potentially weak elliptic curves.
`ecdh-sha2-nistp384`	Enabled	Uses potentially weak elliptic curves.
`ecdh-sha2-nistp256`	Enabled	Uses potentially weak elliptic curves.
`diffie-hellman-group-exchange-sha256`	Enabled
`diffie-hellman-group14-sha256`	Enabled
`diffie-hellman-group14-sha1`	Enabled
`diffie-hellman-group1-sha1`	Deprecated in v6.0	Insecure, will be removed in v8.0.
`diffie-hellman-group-exchange-sha1`	Deprecated in v6.0	Insecure, will be removed in v8.0.

Encryption Algorithms (Ciphers)

Name	Default Support	Notes
`chacha20-poly1305@openssh.com`	Enabled	Requires the `rbnacl` gem.
`aes256-ctr` / `aes192-ctr` / `aes128-ctr`	Enabled
`aes256-gcm@openssh.com`	Enabled	AEAD Cipher.
`aes128-gcm@openssh.com`	Enabled	AEAD Cipher.
`aes256-cbc` / `aes192-cbc` / `aes128-cbc`	Deprecated in v6.0	Insecure, will be removed in v8.0.
`rijndael-cbc@lysator.liu.se`	Deprecated in v6.0	Insecure, will be removed in v8.0.
`blowfish-ctr`, `blowfish-cbc`	Deprecated in v6.0	Insecure, will be removed in v8.0.
`cast128-ctr`, `cast128-cbc`	Deprecated in v6.0	Insecure, will be removed in v8.0.
`3des-ctr`, `3des-cbc`	Deprecated in v6.0	Insecure, will be removed in v8.0.
`idea-cbc`	Deprecated in v6.0	Insecure, will be removed in v8.0.
`none`	Deprecated in v6.0	Unencrypted, will be removed in v8.0.

Message Authentication Code (MAC) Algorithms

Name	Default Support	Notes
`hmac-sha2-512-etm`	Enabled	Encrypt-then-MAC mode.
`hmac-sha2-256-etm`	Enabled	Encrypt-then-MAC mode.
`hmac-sha2-512`	Enabled
`hmac-sha2-256`	Enabled
`hmac-sha1`	Enabled	For backward compatibility.
`hmac-sha2-512-96`	Deprecated in v6.0	Removed from spec, will be removed in v8.0.
`hmac-sha2-256-96`	Deprecated in v6.0	Removed from spec, will be removed in v8.0.
`hmac-sha1-96`	Deprecated in v6.0	Insecure, will be removed in v8.0.
`hmac-ripemd160`	Deprecated in v6.0	Insecure, will be removed in v8.0.
`hmac-md5`	Deprecated in v6.0	Insecure, will be removed in v8.0.
`hmac-md5-96`	Deprecated in v6.0	Insecure, will be removed in v8.0.
`none`	Deprecated in v6.0	No integrity checking, will be removed in v8.0.