Policies

A security policy is a named algorithm suite: which asymmetric cipher for the OPN key exchange, which symmetric cipher for messages, which hash and signing function for everything else. The SecurityPolicy enum has ten cases — six RSA-based, four ECC-based — plus None, which leaves everything unprotected.

A policy is meaningless on its own. It pairs with a SecurityMode:

• None — no signing, no encryption.
• Sign — every message is signed (integrity), nothing is encrypted (confidentiality off).
• SignAndEncrypt — every message is signed and encrypted.

A server advertises one endpoint per (policy, mode) combination it accepts. The client picks one before opening the channel.

RSA policies

^† The OPC Foundation marks Basic128Rsa15 and Basic256 as deprecated. SHA-1 is broken against collision attacks; RSA-1.5 has padding-oracle history. They exist in this library for talking to servers that have not yet upgraded.

ECC policies

Picking a pair

A decision tree, in order:

1. 01

   Untrusted network?

   If anything between you and the server crosses hardware you do not control: SignAndEncrypt. No exceptions, no debate.

2. 02

   Server requires a specific policy?

   Some PLC vendors lock to a single policy/mode pair regardless of what the spec allows. Honour that — getEndpoints() tells you what the server actually accepts.

3. 03

   Performance budget tight?

   AES-128 (used by Basic128Rsa15, Aes128Sha256RsaOaep) costs less than AES-256 on small embedded servers. The cryptographic margin is still large; the choice is bandwidth × CPU vs theoretical strength.

4. 04

   Compliance requirements?

   Brainpool curves are mandated by some European regulatory frameworks (BSI TR-02102 for German federal use). NIST curves are mandated by US federal standards. If neither applies, pick by ecosystem familiarity — most teams find NIST tooling more available.

5. 05

   Otherwise — default:

   Basic256Sha256 + SignAndEncrypt. Universally supported, no deprecated primitives, no surprises.

Configuring the pair

use PhpOpcua\Client\Security\SecurityPolicy;
use PhpOpcua\Client\Security\SecurityMode;

$client = ClientBuilder::create()
    ->setSecurityPolicy(SecurityPolicy::Basic256Sha256)
    ->setSecurityMode(SecurityMode::SignAndEncrypt)
    ->setClientCertificate('/etc/opcua/client.pem', '/etc/opcua/client.key')
    ->connect('opc.tcp://plc.local:4840');

When the policy is anything other than None, the client needs a client certificate. setClientCertificate() accepts paths to PEM- encoded cert and key files (and optionally a CA bundle). If you skip this step but configure a non-None policy, the builder generates a self-signed RSA certificate on first connect — fine for dev, not for production. See Certificates.

Inspecting the agreed policy

After connect(), the server's chosen EndpointDescription is the source of truth for the negotiated parameters. Recover them via getEndpoints() (filtered by endpointUrl + securityPolicyUri) or listen for the SecureChannelOpened event, which carries the channel id, policy URI, and mode.

When the negotiation fails

Most cases trace back to certificate setup. See Trust store and Certificates.