What are the types and performance characteristics of SSH encryption algorithms?

SSH encryption algorithms are core technologies that ensure SSH security, protecting the confidentiality and integrity of data transmission. SSH uses multiple encryption algorithms to meet different security requirements.

Encryption Algorithm Types

Symmetric Encryption Algorithms

Symmetric encryption uses the same key for encryption and decryption, suitable for encrypting large amounts of data transmission.

AES (Advanced Encryption Standard)

bash
# AES-GCM mode (recommended)
aes256-gcm@openssh.com
aes128-gcm@openssh.com

# AES-CTR mode
aes256-ctr
aes192-ctr
aes128-ctr

Features:

• High performance with hardware acceleration support
• 128/256-bit key length
• GCM mode provides authenticated encryption
• Widely adopted and verified

ChaCha20-Poly1305

bash
chacha20-poly1305@openssh.com

Features:

• Excellent performance on devices without hardware acceleration
• 256-bit key
• Built-in authenticated encryption
• Suitable for mobile devices and ARM architecture

3DES (Triple DES)

bash
3des-cbc  # Deprecated, not recommended

Features:

• Slower, lower security
• Only for backward compatibility
• Not recommended for production use

Asymmetric Encryption Algorithms

Asymmetric encryption uses public and private key pairs for authentication and key exchange.

RSA

bash
ssh-rsa
rsa-sha2-256
rsa-sha2-512

Features:

• Widely supported
• Key length: 2048/4096 bits
• Slower computation speed
• Suitable for signatures and key exchange

ECDSA (Elliptic Curve DSA)

bash
ecdsa-sha2-nistp256
ecdsa-sha2-nistp384
ecdsa-sha2-nistp521

Features:

• Smaller key sizes
• Faster computation speed
• Based on elliptic curves
• NIST standard curves

Ed25519

bash
ssh-ed25519

Features:

• Latest signature algorithm
• 256-bit key
• Extremely fast speed
• High security
• Recommended for use

Encryption Modes

CBC (Cipher Block Chaining)

bash
aes256-cbc
aes128-cbc

Features:

• Traditional encryption mode
• Requires padding
• Vulnerable to padding attacks
• Not recommended for new systems

CTR (Counter Mode)

bash
aes256-ctr
aes128-ctr

Features:

• Stream cipher mode
• No padding required
• Parallel encryption
• Better performance

GCM (Galois/Counter Mode)

bash
aes256-gcm@openssh.com
aes128-gcm@openssh.com

Features:

• Authenticated encryption mode
• Provides both encryption and integrity
• High performance
• Recommended for use

Configuring Encryption Algorithms

Server Configuration

bash
# /etc/ssh/sshd_config

# Recommended encryption algorithms
Ciphers aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes256-ctr

# Disable insecure algorithms
Ciphers -3des-cbc,-aes128-cbc,-aes192-cbc,-aes256-cbc

Client Configuration

bash
# ~/.ssh/config

# Specify encryption algorithms
Ciphers aes256-gcm@openssh.com,chacha20-poly1305@openssh.com

# Test connection
ssh -c aes256-gcm@openssh.com user@hostname

Performance Comparison

Algorithm	Encryption Speed	Decryption Speed	Security	Hardware Acceleration
AES-256-GCM	Fast	Fast	High	Yes
ChaCha20-Poly1305	Fast	Fast	High	No
AES-256-CTR	Fast	Fast	Medium	Yes
AES-256-CBC	Medium	Medium	Medium	Yes
3DES	Slow	Slow	Low	Yes

Security Best Practices

1. Use Modern Algorithms

bash
# Prioritize GCM mode
Ciphers aes256-gcm@openssh.com,chacha20-poly1305@openssh.com

2. Disable Weak Algorithms

bash
# Disable CBC mode and 3DES
Ciphers -aes128-cbc,-aes192-cbc,-aes256-cbc,-3des-cbc

3. Regular Updates

bash
# Check supported algorithms
ssh -Q cipher

# Test algorithm performance
ssh -c aes256-gcm@openssh.com -o "Compression yes" user@hostname

4. Monitoring and Auditing

bash
# View encryption algorithm used for connection
ssh -v user@hostname 2>&1 | grep "cipher"

# Check server algorithms using nmap
nmap --script ssh2-enum-algos -p 22 hostname

Common Questions

Q: How to choose between AES-GCM and ChaCha20-Poly1305?

A: AES-GCM performs better on devices with hardware acceleration, while ChaCha20-Poly1305 is superior on devices without hardware acceleration.

Q: Why disable CBC mode?

A: CBC mode is vulnerable to padding oracle attacks.

Q: How to check encryption algorithms supported by server?

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

Q: How is Ed25519 better than RSA?

A: Ed25519 provides better performance, smaller key sizes, and stronger security.

The choice of SSH encryption algorithms directly affects system security and performance, and proper configuration is an important part of ensuring SSH security.