What are the types and working principles of SSH key exchange algorithms?

SSH Key Exchange is one of the most critical security mechanisms in the SSH protocol. It ensures that clients and servers can securely negotiate session keys over insecure networks without exposing key information.

Purpose of Key Exchange

1. Secure Session Key Negotiation: Securely establish shared keys over insecure networks
2. Prevent Man-in-the-Middle Attacks: Ensure identity verification of both parties
3. Forward Secrecy: Past sessions remain secure even if long-term keys are compromised
4. Key Independence: Each session uses different keys

Key Exchange Algorithms

Diffie-Hellman (DH) Algorithm

Traditional key exchange algorithm based on the mathematical problem of discrete logarithms.

bash
# Common DH groups
diffie-hellman-group1-sha1      # 1024-bit (deprecated)
diffie-hellman-group14-sha1     # 2048-bit (deprecated)
diffie-hellman-group16-sha512   # 4096-bit
diffie-hellman-group18-sha512   # 8192-bit

How it works:

1. Both parties agree on public parameters (p, g)
2. Each generates private keys (a, b)
3. Calculate public values (A = g^a mod p, B = g^b mod p)
4. Exchange public values
5. Calculate shared secret (s = B^a mod p = A^b mod p)

Elliptic Curve Diffie-Hellman (ECDH)

Based on elliptic curve discrete logarithm problem, providing smaller key sizes for the same security level.

bash
# Common ECDH algorithms
curve25519-sha256@libssh.org      # Recommended
ecdh-sha2-nistp256                # NIST P-256
ecdh-sha2-nistp384                # NIST P-384
ecdh-sha2-nistp521                # NIST P-521

Advantages:

• Faster computation speed
• Smaller key sizes
• Better security
• Suitable for mobile devices

SSH Key Exchange Process

1. Algorithm Negotiation

shell
Client -> Server: KEXINIT (list of supported algorithms)
Server -> Client: KEXINIT (list of supported algorithms)

Both parties select the first mutually supported algorithm.

2. Key Exchange

shell
Client -> Server: DH public value + signature
Server -> Client: DH public value + signature + host key

3. Key Calculation

Both parties use DH public values to calculate shared secret, then derive:

• Encryption keys (for data encryption)
• MAC keys (for data integrity)
• IV (Initialization Vector)

4. Authentication

shell
Client -> Server: NEWKEYS
Server -> Client: NEWKEYS

Confirm key exchange completion, start using new keys.

Security Features

Perfect Forward Secrecy

Even if the server's private key is compromised, attackers cannot decrypt past sessions.

bash
# Configuration to enable forward secrecy
KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256

Key Reuse Protection

Prevents key replay attacks by generating new keys for each session.

Strong Random Number Generation

Uses cryptographically secure random number generators to ensure key unpredictability.

Configuration Optimization

Recommended Key Exchange Algorithms

bash
# /etc/ssh/sshd_config

# Prioritize Curve25519
KexAlgorithms curve25519-sha256@libssh.org

# Fallback algorithms
KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256

Disable Insecure Algorithms

bash
# Disable SHA-1 algorithms
KexAlgorithms -diffie-hellman-group1-sha1,-diffie-hellman-group14-sha1

# Disable weak DH groups
KexAlgorithms -diffie-hellman-group1-sha1

Performance Optimization

bash
# Use faster algorithms
KexAlgorithms curve25519-sha256@libssh.org

# Increase DH group size
ModulusSize 4096

Security Best Practices

1. Use Modern Algorithms: Prioritize Curve25519
2. Regular Updates: Keep SSH software up to date
3. Disable Weak Algorithms: Remove insecure key exchange algorithms
4. Monitor Logs: Check for key exchange related errors
5. Test Configuration: Use ssh -Q kex to view supported algorithms

Common Questions

Q: Why do we need key exchange?

A: Key exchange allows both parties to securely negotiate keys over insecure networks without pre-shared keys.

Q: How is Curve25519 better than RSA?

A: Curve25519 provides better performance, smaller key sizes, and stronger security.

Q: How to check key exchange algorithms supported by server?

A: Use nmap --script ssh2-enum-algos -p 22 hostname or ssh -Q kex.

SSH key exchange is the core technology that ensures SSH security, and understanding its working principle is crucial for system security.