How Can I Configure An OpenVPN Server To Use Elliptic Curve Cryptography (ECC) With Curve25519 And ChaCha20-Poly1305 Cipher Suite, While Also Implementing A Certificate Transparency (CT) Log To Monitor And Audit TLS Certificate Issuance And Revocation, In A Multi-domain Environment With A Mix Of Windows, Linux, And MacOS Clients?

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To set up an OpenVPN server with ECC using Curve25519 and ChaCha20-Poly1305, and implement Certificate Transparency in a multi-domain environment, follow these steps:

1. Set Up the Certificate Authority (CA)

Install OpenSSL: Ensure OpenSSL is installed and supports Curve25519.

Generate ECC CA Certificate:

openssl ecparam -out ca.key -name Curve25519 -genkey
openssl req -x509 -new -nodes -key ca.key -sha256 -days 3650 -out ca.crt -extensions EXT -config <(cat /usr/lib/ssl/openssl.cnf <(printf "[EXT]\nextended_key_usage = serverAuth,clientAuth\nkeyUsage = digitalSignature\nsubjectAltName = @alt_names\n[alt_names]\nDNS.1 = example.com\nDNS.2 = *.example.com"))

2. Generate Server Certificate

Generate Server Key and Certificate:

openssl ecparam -out server.key -name Curve25519 -genkey
openssl req -new -key server.key -out server.csr
openssl x509 -req -in server.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out server.crt -days 365 -extensions EXT -config <(cat /usr/lib/ssl/openssl.cnf <(printf "[EXT]\nextended_key_usage = serverAuth\nkeyUsage = digitalSignature\nsubjectAltName = @alt_names\n[alt_names]\nDNS.1 = vpn.example.com"))

3. Set Up OpenVPN Server

Install OpenVPN: Install on your chosen OS (e.g., Ubuntu/Debian: sudo apt-get install openvpn).

Configure Server: Edit or create /etc/openvpn/server.conf:

port 1194
proto udp
dev tun
ca ca.crt
cert server.crt
key server.key
cipher CHACHA20-POLY1305
ecdh-curve Curve25519
dh none
topology subnet
server 10.8.0.0 255.255.255.0

Start and Enable Service:

sudo systemctl start openvpn-server
sudo systemctl enable openvpn-server

4. Implement Certificate Transparency

Use Step-CA: Install Step-CA for automated CT logging.

curl -s -L https://step.sh/ | sh

Configure Step-CA: Modify step-ca/config/ca.json to include CT logging.

Generate Certificates via Step-CA: Use step-ca commands to issue server and client certificates, which are automatically logged in CT.

5. Client Configuration

Distribute Certificates: Ensure each client has ca.crt, their certificate, and key.

Client Configuration File: Create client.ovpn:

client
dev tun
proto udp
remote vpn.example.com 1194
resolv-retry infinite
cipher CHACHA20-POLY1305
ecdh-curve Curve25519
ca ca.crt
cert client.crt
key client.key

Multi-Domain Support: Use SAN certificates or multiple certificates for different domains.

6. Multi-Platform Support

  • Windows: Use OpenVPN GUI, ensuring compatibility with ECC.
  • macOS: Use Tunnelblick, which supports ECC and the specified cipher.
  • Linux: Use OpenVPN client with the provided configuration.

7. Verify CT Logs

Use crt.sh to check if certificates are logged:

curl https://crt.sh/?q=vpn.example.com

Conclusion

By following these steps, you set up an OpenVPN server with ECC, ChaCha20-Poly1305, and CT logging, supporting multiple domains and various clients. Ensure all components are compatible and test thoroughly.