Errata exists for this version of the document.

The EncryptedSecret uses an extensible format which has the TypeId of a DataType Node as a prefix as defined for the ExtensionObject encoding in OPC 10000-6. The general layout of the EncryptedSecret is shown in Figure 39.

Figure 39 – EncryptedSecret Layout

The TypeId specifies how the EncryptedSecret is serialized and secured. For example, RsaEncryptedSecrets require that the policy header be encrypted with the public key associated with the EncryptingCertificate before it is serialized. Other TypeIds may or may not require the policy header to be encrypted.

The SecurityPolicyUri is used to determine what algorithms were used to encrypt and sign the data.

The payload is always encrypted using the symmetric encryption algorithm specified by the SecurityPolicyUri. The EncryptingKey and InitializationVector are used to initialize this algorithm. The mechanisms used to initialize the symmetric encryption algorithm depend on the TypeId. The lengths of the fields are specified by the SecurityPolicyUri.

The Signature using the symmetric signature algorithm specified by the SecurityPolicyUri. The SymmetricKey and InitializationVector are used to initialize this algorithm. The mechanisms used to initialize the symmetric signature algorithm depend on the TypeId. The length of the SymmetricKey is specified by the SecurityPolicyUri.

The EncryptedSecret is secured and serialized as follows:

• Serialize the common header;
• Serialize the policy header;
• Encrypt the policy header and append the result to the common header;
• Update the PolicyHeaderLength with the length of the encrypted header;
• Append the Nonce and the Secret to the encrypted policy header;
• Calculate padding required on the payload and append after the Secret;
• Encrypt the payload;
• Calculate a Signature on {common header | encrypted policy header | encrypted payload};
• Append the Signature.

Individual fields are serialized using the UA Binary encoding (see OPC 10000-6) for the DataType specified in Table 182. The Padding is used to ensure there is enough data to fill an integer multiple of encryption blocks. The size of the encryption block depends on the encryption algorithm. Two separate padding operations are needed because two different encryption algorithms may be used. The total length of the Padding, not including the PaddingSize, is encoded as a UInt16. The individual bytes of the Padding are set to the the least significant byte of the PaddingSize.

The EncryptedSecret is deserilized and validated as follows:

• Deserialize the common header;
• Decrypt the policy header using the private key associated with the EncryptingCertificate;
• Verify the padding in the policy header;
• Verify the Signature with the SigningKey;
• Decrypt the payload;
• Verify the padding on the payload;
• Extract the Secret;

If the TypeId does not require the policy header to be encrypted then the padding on the policy header is omitted and the PolicyHeaderLength specifies the length of the unencrypted data.

The fields in the EncryptedSecret are described in Table 182. The first three fields TypeId, EncodingMask and Length belong to the ExtensionObject encoding defined in OPC 10000-6.

Table 182 – EncryptedSecret Layout

Name	Type	Description
TypeId	NodeId	The NodeId of the DataType Node.
EncodingMask	Byte	This value is always 1.
Length	Int32	The length of the data that follows including the Signature.
SecurityPolicyUri	String	The URI for the SecurityPolicy used to apply security.
EncryptingCertificate	ByteString	The SHA1 thumbprint of the DER form of the encrypting Certificate.
SigningTime	DateTime	When the Signature was created.
PolicyHeaderLength	UInt16	The length of the policy header that follows If the policy header is encrypted this is the length of the encrypted data; Otherwise, it is the length of the unencrypted data;
SigningKey	ByteString	The key data used to create the Signature. The TypeId specifies how this data is used.
EncryptingKey	ByteString	The key data used to encrypt the payload. The TypeId specifies how this data is used.
InitializationVector	ByteString	The data used to initialize the algorithm used to encrypt the payload. The TypeId specifies how this data is used.
Nonce	ByteString	This is the last serverNonce returned in the CreateSession or ActivateSession Response when a UserIdentityToken is passed with the ActivateSession Request. If used outside of an ActivateSession call, the source of the Nonce is defined by the context in which this EncryptedSecret is used. The length of the Nonce shall equal the SecureChannelNonceLength specified by the SecurityPolicy.
Secret	ByteString	The tokenData that depends on the IssuedIdentityToken. If the tokenData is a String is it encoded using UTF-8 first.
PayloadPadding	Byte[*]	Additional padding added to ensure the size of the encrypted payload is an integer multiple of the input block size for the symmetric encryption algorithm specified by the SecurityPolicyUri. The value of each byte is the least significant byte of the PayloadPaddingSize.
PayloadPaddingSize	UInt16	The size of the padding added to the payload.
Signature	Byte[*]	The signature calculated using the symmetric signing algorithm specified by the SecurityPolicyUri. The length of the signature is specified by the SecurityPolicyUri.

The currently available EncryptedSecret DataTypes are defined in Table 183.

Table 183 – EncryptedSecret DataTypes

Type Name	When to Use
RsaEncryptedSecret	Used when the SecurityPolicy requires the use of RSA based asymmetric encryption. It is described in 7.36.2.4.