Legacy SSH Algorithms

Connect to older network equipment using legacy SSH algorithms

Netcatty supports legacy SSH algorithms for compatibility with older network equipment like switches, routers, and embedded systems that don’t support modern cryptographic standards.

Overview

Modern SSH implementations disable weak algorithms for security. However, many network devices (especially older Cisco, HP, Juniper equipment) only support these legacy algorithms. Netcatty provides a per-host toggle to enable these algorithms when needed.

Enabling Legacy Algorithms

Per-Host Configuration

In the host details panel:

1. Open the host you want to configure
2. Scroll to Advanced Settings
3. Enable Legacy SSH Algorithms
4. Save the host configuration

This setting is stored in the Host model:

// domain/models.ts:110
legacyAlgorithms?: boolean;

Supported Algorithms

When legacy algorithms are enabled, Netcatty adds the following to the SSH handshake:

Key Exchange Algorithms

Standard (always enabled):

• curve25519-sha256, [e-mail]
• ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521
• diffie-hellman-group14-sha256
• diffie-hellman-group16-sha512, diffie-hellman-group18-sha512
• diffie-hellman-group-exchange-sha256

Legacy (when enabled):

• diffie-hellman-group14-sha1 - Widely supported on older equipment
• diffie-hellman-group1-sha1 - Very old equipment (weak, use only if necessary)

Cipher Algorithms

Standard (always enabled):

• [e-mail], [e-mail]
• aes128-ctr, aes192-ctr, aes256-ctr

Legacy (when enabled):

• aes128-cbc - Common on older Cisco devices
• aes256-cbc - Legacy encryption
• 3des-cbc - Very old equipment (slow, use only if necessary)

Host Key Algorithms

Legacy (when enabled):

• ssh-rsa - RSA with SHA-1 (deprecated but widely supported)
• ssh-dss - DSA keys (very weak, use only if no alternative)

Standard modern algorithms (always enabled):

• ssh-ed25519, ecdsa-sha2-nistp256/384/521
• rsa-sha2-512, rsa-sha2-256

Implementation

The legacy algorithm support is implemented in electron/bridges/sshBridge.cjs:

// electron/bridges/sshBridge.cjs:324
algorithms: buildAlgorithms(options.legacyAlgorithms)

The buildAlgorithms() function:

function buildAlgorithms(legacyEnabled) {
  const algorithms = {
    cipher: [
      '[e-mail]', '[e-mail]',
      'aes128-ctr', 'aes192-ctr', 'aes256-ctr',
    ],
    kex: [
      'curve25519-sha256', '[e-mail]',
      'ecdh-sha2-nistp256', 'ecdh-sha2-nistp384', 'ecdh-sha2-nistp521',
      'diffie-hellman-group14-sha256',
      'diffie-hellman-group16-sha512', 'diffie-hellman-group18-sha512',
      'diffie-hellman-group-exchange-sha256',
    ],
    compress: ['none'],
  };

  if (legacyEnabled) {
    algorithms.kex.push(
      'diffie-hellman-group14-sha1',
      'diffie-hellman-group1-sha1',
    );
    algorithms.cipher.push(
      'aes128-cbc', 'aes256-cbc', '3des-cbc',
    );
    algorithms.serverHostKey = [
      'ssh-ed25519', 'ecdsa-sha2-nistp256', ...,
      'ssh-rsa', 'ssh-dss',
    ];
  }

  return algorithms;
}

When to Enable

Network Equipment

Common scenarios:

• Older Cisco switches and routers (IOS versions before 15.x)
• HP ProCurve switches
• Legacy Juniper devices
• Dell PowerConnect switches
• Older F5 load balancers

Error messages:

• “no matching key exchange method found”
• “no matching cipher found”
• “no matching host key type found”

Embedded Systems

Common scenarios:

• Older Linux-based appliances
• Serial console servers
• KVM-over-IP devices
• PDU (Power Distribution Units)
• Environmental monitoring systems

Indicators:

• Devices running BusyBox with dropbear SSH
• Firmware that hasn’t been updated in years
• Documentation mentioning “SSH v2 only” without algorithm details

Industrial Equipment

Common scenarios:

• Industrial PLCs with SSH
• Building automation systems
• SCADA systems
• Legacy server management cards (iLO, DRAC, iDRAC)

Security Considerations

Legacy algorithms are disabled by default for security reasons. Only enable them for specific hosts that require them.

Risks

• Weak encryption: CBC mode ciphers are vulnerable to certain attacks
• SHA-1 weaknesses: Key exchange using SHA-1 is considered weak
• DSS keys: DSA is cryptographically weak compared to modern algorithms
• 3DES performance: Very slow and provides limited security

Mitigation Strategies

1. Use per-host configuration: Only enable legacy algorithms for specific hosts, not globally
2. Network isolation: Keep legacy equipment on isolated management networks
3. Upgrade when possible: Update device firmware to support modern algorithms
4. VPN tunneling: Use a VPN to add an extra encryption layer
5. Limit exposure: Don’t expose legacy SSH services to the internet
6. Monitor connections: Keep connection logs for security audits

Troubleshooting

Connection Fails Even with Legacy Algorithms

If you still can’t connect after enabling legacy algorithms:

1. Check the error message carefully:

   • “no matching MAC found” - MAC algorithm issue (rare)
   • “connection reset” - May be authentication, not algorithm issue
   • “timeout” - Network/firewall problem, not algorithm issue

2. Verify device SSH configuration:

   # On the device, check SSH config
   show ssh server  # Cisco
   show ip ssh      # Some switches
   

3. Test with OpenSSH manually:

   # Test connection with verbose output
   ssh -vvv -o KexAlgorithms=+diffie-hellman-group1-sha1 \
       -o HostKeyAlgorithms=+ssh-rsa \
       -o Ciphers=+aes128-cbc \
       user@device
   

4. Check known_hosts conflicts:

   • Clear old host keys if device was reinstalled
   • See Security Best Practices

Performance Issues

Legacy algorithms can be slower:

• 3DES is particularly slow (avoid if possible)
• Use AES-CBC instead if the device supports it
• DH-group1 is faster than group14 but weaker (use group14 if possible)

Common Device-Specific Settings

Cisco IOS (older versions)

Required: legacyAlgorithms = true
Typical working algorithms:
- KEX: diffie-hellman-group14-sha1
- Cipher: aes128-cbc
- Host key: ssh-rsa

HP ProCurve (older firmware)

Required: legacyAlgorithms = true
Typical working algorithms:
- KEX: diffie-hellman-group14-sha1
- Cipher: aes128-cbc or 3des-cbc
- Host key: ssh-rsa

Dropbear SSH (embedded Linux)

May require: legacyAlgorithms = true
Depends on version:
- Older versions (< 2016): likely needs legacy
- Newer versions: often support modern algorithms

Upgrading Devices

When possible, upgrade device firmware to support modern algorithms:

Cisco IOS

! Generate new RSA keys (2048-bit minimum)
crypto key generate rsa modulus 2048

! Enable SSH version 2 only
ip ssh version 2

! Set minimum encryption standard
ip ssh server algorithm encryption aes128-ctr aes192-ctr aes256-ctr

Linux (OpenSSH Server)

# Edit /etc/ssh/sshd_config
KexAlgorithms curve25519-sha256,[e-mail],diffie-hellman-group14-sha256
Ciphers [e-mail],[e-mail],aes128-ctr,aes256-ctr
HostKeyAlgorithms ssh-ed25519,rsa-sha2-512,rsa-sha2-256

# Restart SSH
sudo systemctl restart sshd

Further Reading

• Mozilla SSH Guidelines - Modern SSH hardening
• NIST SP 800-52 - Guidelines for TLS/SSH
• Cisco SSH Configuration Guide