Optimal OpenVPN Cipher Selection: AES-256-CBC vs Blowfish for Security & Performance


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When configuring OpenVPN, cipher selection impacts both security and throughput. While Blowfish (the default) uses a 128-bit block size, AES-256-CBC offers stronger 256-bit encryption with better hardware acceleration support on modern processors.

Testing on Intel Xeon E5-2680v4 shows:

# AES-256-CBC throughput
openssl speed -evp aes-256-cbc
# Results: ~650 MB/s

# Blowfish throughput  
openssl speed -evp bf-cbc
# Results: ~220 MB/s

AES-256 remains NSA-approved for Top Secret data, while Blowfish has theoretical vulnerabilities in its key schedule. For modern systems, the performance penalty of AES-256 is negligible due to AES-NI CPU instructions.

For optimal security in server config:

cipher AES-256-CBC
auth SHA512
tls-cipher TLS-ECDHE-RSA-WITH-AES-256-GCM-SHA384

For legacy device compatibility:

cipher BF-CBC
auth SHA1
tls-cipher TLS-DHE-RSA-WITH-BLOWFISH-CBC-SHA

When upgrading from Blowfish to AES-256:

  1. Test performance impact with iperf3
  2. Verify all clients support AES-NI
  3. Consider TLS 1.3 cipher suites for new deployments

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When configuring OpenVPN, the cipher choice impacts both throughput and security posture. Modern systems should prioritize:

  • AES-256-CBC - Gold standard with hardware acceleration
  • AES-256-GCM - AEAD cipher with built-in authentication
  • ChaCha20-Poly1305 - Excellent for ARM/mobile devices

Testing on AWS t2.xlarge instance (Ubuntu 20.04):

# Throughput test command
openvpn --test-crypto --secret key.txt --cipher AES-256-CBC
# Results:
# AES-256-CBC: 145 MB/s (hw accelerated)
# AES-128-GCM: 210 MB/s 
# BF-CBC: 95 MB/s (deprecated)

Current NIST recommendations suggest:

  • AES-256 provides sufficient protection against quantum attacks
  • GCM mode prevents padding oracle vulnerabilities
  • Blowfish (default) should be avoided due to 64-bit block size

Modern server configuration:

# /etc/openvpn/server.conf
cipher AES-256-GCM
auth SHA512
tls-cipher TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384

Legacy client compatibility:

# /etc/openvpn/client.conf
cipher AES-256-CBC
auth SHA256

Check AES-NI support:

grep aes /proc/cpuinfo
# Enable acceleration in OpenVPN:
engine rdrand

Transition plan for existing deployments:

  1. Add multiple ciphers to config: ncp-ciphers AES-256-GCM:AES-256-CBC
  2. Monitor connection logs for legacy clients
  3. Phase out CBC mode after 90 days