What causes "unknown public key length: %d" in slackhq/nebula?

Configure the HSM to emit uncompressed EC points (disable point compression).

unknown public key length: %d

Error message

unknown public key length: %d

What it means

GetPubKey accepts EC points of 65 bytes (raw uncompressed 0x04 || X || Y for P-256) or 67 bytes (same point with a 2-byte DER header, which it strips). Any other length means the token returned an unrecognized EC point encoding, so this error is thrown with the actual length.

Source

Thrown at pkclient/pkclient_cgo.go:228

	}
	c.pubKeyObj, err = c.findDeriveKey(c.id, c.label, false)
	if err != nil {
		return nil, fmt.Errorf("pkcs11 module gave us a nil CKA_PUBLIC_KEY_INFO, and looking up the public key also failed: %w", err)
	}
	d, err = c.pubKeyObj.Attribute(pkcs11.CKA_EC_POINT)
	if err != nil {
		return nil, fmt.Errorf("pkcs11 module gave us a nil CKA_PUBLIC_KEY_INFO, and reading CKA_EC_POINT also failed: %w", err)
	}
	if d == nil || len(d) < 1 {
		return nil, fmt.Errorf("pkcs11 module gave us a nil or empty CKA_EC_POINT")
	}
	switch len(d) {
	case 65: //length of 0x04 + len(X) + len(Y)
		return d, nil
	case 67: //as above, DER-encoded IIRC?
		return d[2:], nil
	default:
		return nil, fmt.Errorf("unknown public key length: %d", len(d))
	}
}

View on GitHub (pinned to dd8f660c0a)

Solutions

Configure the HSM to emit uncompressed EC points (disable point compression).
Ensure the certificate/key uses the P-256 curve — other curves yield different point lengths.
Inspect the returned bytes and, if the module adds a fixed header, adjust provisioning rather than the client.
Regenerate the keypair with standard uncompressed encoding.

Defensive patterns

Strategy: validation

Validate before calling

// Only expect P-256 uncompressed points (65) or DER-wrapped (67)
func expectP256Point(d []byte) error {
  switch len(d) {
  case 65, 67: return nil
  default: return fmt.Errorf("token returned %d-byte EC point; configure uncompressed P-256", len(d))
  }
}

Try / catch

pub, err := client.GetPubKey()
if err != nil {
  var lenErr interface{ Error() string }
  if errors.As(err, &lenErr) && strings.Contains(err.Error(), "unknown public key length") {
    return fmt.Errorf("HSM point compression or non-P256 curve in use; fix token config: %w", err)
  }
  return err
}

Prevention

Disable EC point compression on the HSM.
Use only P-256 keys with this client (other curves produce different point lengths).
Provision keys with uncompressed public point encoding.
Catch this in staging by calling GetPubKey and validating len==65 before rollout.

When it happens

Trigger: Calling GetPubKey when CKA_EC_POINT has a length other than 65 or 67 — e.g. a compressed point (33 bytes), a point with a DER BIT STRING wrapper of a different length, or trailing garbage added by the module.

Common situations: HSM configured for point compression; curves other than P-256 (e.g. P-384 producing 97-byte points) used with this client; vendor module wrapping the point in an unexpected encoding; corruption between token and client.

Related errors

AI-assisted analysis of slackhq/nebula@dd8f660c0a (2026-09-03). Data as JSON: /api/errors/0d799b1610ecff4d. Report an issue: GitHub.