The Clientshall always prove possession of a UserIdentityToken when it passes it to the Server. Some tokens include a secret such as a password which the Serverwill accept as proof. In order to protect these secrets, the Tokenmay be encrypted before it is passed to the Server. Other types of tokens allow the Clientto create a signature with the secret associated with the Token. In these cases, the Clientproves possession of a UserIdentityToken by creating a signature with the secret and passing it to the Server.

Each UserIdentityTokenallowed by an Endpointshall have a UserTokenPolicyspecified in the EndpointDescription. The UserTokenPolicyspecifies what SecurityPolicyto use when encrypting or signing. If this SecurityPolicyis null or empty then the Clientuses the SecurityPolicyin the EndpointDescription. If the matching SecurityPolicyis set to Nonethen no encryption or signature is required. The possible SecurityPoliciesare defined in OPC 10000-7.

It is recommended that applications never set the SecurityPolicyto Nonefor UserIdentityTokensthat include a secret because these secrets could be used by an attacker to gain access to the system.

Clientsshall validate the Server Certificateand ensure it is trusted before sending a UserIdentityTokenencrypted with the Certificate.

The encrypted secret and Signatureare embedded in a ByteStringwhich is part of the UserIdentityToken. The format of this ByteStringdepends on the type of UserIdentityTokenand the SecurityPolicy. Clientsshall validate the Server Certificateand ensure it is trusted before sending a UserIdentityTokenencrypted with the Certificate.

The legacy token secret format defined in 7.41.2.2is not extensible and provides only encryption but the encrypted data is not signed. It is used together with the USERNAME UserIdentityToken. The password secret exchanged with this format shall not exceed 64 bytes.

The EncryptedSecretformat defined in 7.41.2.3provides an extensible secret format together with the definition how the secret is signed and encrypted. It allows for the layout to be updated as new token types are defined or new SecurityPoliciesare added.

The UserIdentityTokentypes and the token formats supported by the Endpointare identified by the UserTokenPolicydefined in 7.42.

To prevent the leakage of information useful to attackers, Serversshall ensure that the process of validating UserIdentityTokenscompletes in a fixed interval independent of whether an error occurs or not. The process of validation includes decrypting, check for padding and checking for a valid nonce. If any errors occur the return code is Bad_IdentityTokenInvalid.

Serversshall log details of any failure to validate a UserIdentityToken and shall lock out Clientapplications after five failures.

When encrypting a UserIdentityToken, the Clientappends the last ServerNonceto the secret. The data is then encrypted with the public key from the Server’s Certificate.

A Clientshould not add any padding after the secret. If a Clientadds padding then all bytes shall be zero. A Servershall check for padding added by Clientsand ensure that all padding bytes are zeros. Serversshall reject UserIdentityTokens with invalid padding. Administrators shall be able to configure Serversto accept UserIdentityTokens with invalid padding.

If no encryption is applied, the structure is not used and only the secret without any Nonceis passed to the Server.

Table 186describes how to serialize UserIdentityTokensbefore applying encryption.

Table 186– Legacy UserIdentityToken Encrypted Token Secret Format

Name	Type	Description
Length	Byte [4]	The length of the data to be encrypted including the ServerNoncebut excluding the lengthfield. This field is a 4-byte unsigned integer encoded with the least significant bytes appearing first.
tokenData	Byte [*]	The token data.
serverNonce	Byte [*]	The last ServerNoncereturned by the Serverin the CreateSessionor ActivateSessionresponse.

The EncryptedSecretuses an extensible format which has the TypeIdof a DataType Nodeas a prefix as defined for the ExtensionObjectencoding in OPC 10000-6. The general layout of the EncryptedSecretis shown in Figure 39.

Figure 39– EncryptedSecret layout

The TypeIdspecifies how the EncryptedSecretis serialized and secured. For example, the RsaEncryptedSecretrequires that the KeyDatabe encrypted with the public key associated with the EncryptingCertificatebefore it is serialized.

The SecurityPolicyUriis used to determine what algorithms were used to encrypt and sign the data. Valid SecurityPolicyUris are defined in OPC 10000-7.

The payload is always encrypted using the symmetric encryption algorithm specified by the SecurityPolicyUri. The KeyDatais used to create the keys used needed for the symmetric encryption. The structure of the KeyDatadepends on the EncryptedSecret DataType.

The EncryptedSecretis secured and serialized as follows:

• Serialize the common header;
• Serialize the KeyData;
• If required, encrypt the KeyData and append the result to the common header;
• Update the KeyDataLength with the length of the encrypted KeyData;
• Append the Nonce and the Secret to the encrypted KeyData;
• Calculate padding required on the payload and append after the Secret;
• Encrypt the payload;
• Calculate a Signature;
• Append the Signature.

Individual fields are serialized using the UA Binary encoding (see OPC 10000-6) for the DataTypespecified in Table 187. The Paddingis 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. The total length of the Padding, not including the PaddingSize, is encoded as a UInt16. The individual bytes of the Paddingare set to the the least significant byte of the PaddingSize.

The EncryptedSecret is deserilized and validated as follows:

• Deserialize the common header;
• Verify the Signature if the KeyData is not encrypted;
• Decrypt the KeyData and verify the Signature if the KeyData is encrypted;
• Decrypt the payload;
• Verify the padding on the payload;
• Extract the Secret;

The fields in the EncryptedSecretare described in Table 187. The first three fields TypeId, EncodingMaskand Lengthbelong to the ExtensionObjectencoding defined in OPC 10000-6.

Table 187– EncryptedSecret layout

Name	Type	Description
TypeId	NodeId	The NodeIdof 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 SecurityPolicyused to apply security.
Certificate	ByteString	The signing and/or encrypting Certificate.
SigningTime	DateTime	When the Signaturewas created.
KeyDataLength	UInt16	The length, in bytes, of the KeyDatathat follows If the KeyDatais encrypted this is the length of the encrypted data; Otherwise, it is the length of the unencrypted data.
KeyData	Byte [*]	The key data used to create the keys needed for decrypting and verifying the payload. Each EncryptedSecret DataTypedescribes how the key data is structured for different SecurityPolicies.
Nonce	ByteString	This is the last serverNoncereturned in the CreateSessionor ActivateSession Responsewhen a UserIdentityTokenis passed with the ActivateSession Request. If used outside of an ActivateSession call, the Nonceis created by the sender and is a function of the SecureChannelNonceLength.
Secret	ByteString	The secret to protect. The passwordwhen used with UserNameIdentityTokens. The tokenDatawhen used with IssuedIdentityTokens. If the Secretis a Stringis 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 Signaturecalculated after all encryption is applied. Each EncryptedSecret DataTypedescribes how the Signatureis calculated for different SecurityPolicies.

The PayloadPaddingSize adjusted with the following formula:

If (Secret.Length + PayloadPaddingSize < InputBlockSize) Then

PayloadPaddingSize = PayloadPaddingSize + InputBlockSize

Where the InputBlockSize is specified by the SymmetricEncryptionAlgorithm.

The currently available EncryptedSecret DataTypesare defined in Table 188.

Table 188– EncryptedSecret DataTypes

Type Name	When to Use
RsaEncryptedSecret	Used when the SecurityPolicyrequires the use of RSA cryptography. It is described in 7.41.2.4.
EccEncryptedSecret	Used when the SecurityPolicyrequires the use of ECC cryptography.It is described in .

The RsaEncryptedSecretuses RSA based Asymmetric Cryptography.

Additional semantics for the fields in the EncryptedSecretlayout for the RsaEncryptedSecret Structureare described in Table 189.

Table 189– RsaEncryptedSecret structure

Name	Type	Description
TypeId	NodeId	The NodeIdof the RsaEncryptedSecret DataType Node.
EncodingMask	Byte	See Table 187.
Length	UInt32	See Table 187.
SecurityPolicyUri	String	See Table 187.
Certificate	ByteString	The SHA1 hash of the DER form of the Certificate used to encrypt the KeyData.
SigningTime	DateTime	See Table 187.
KeyDataLength	UInt16	The length, in bytes, of the encrypted KeyData.
KeyData		The KeyDatais encrypted with the PublicKeyassociated with the Certificate.
SigningKey	ByteString	The key used to compute the Signature.
EncryptingKey	ByteString	The key used to encrypt payload.
InitializationVector	ByteString	The initialization vector used with the EncryptingKey.
Nonce	ByteString	A Nonce. This is the last ServerNoncereturned in the CreateSessionor ActivateSession Responsewhen proving a UserIdentityTokenpassed in the ActivateSession Request. In other contexts, this is a Noncecreated by the sender with a length equal to the SecureChannelNonceLength.
Secret	ByteString	See Table 187.
PayloadPadding	Byte[*]	See Table 187.
PayloadPaddingSize	UInt16	See Table 187.
Signature	Byte[*]	The Signaturecalculated with the SigningKey. The Signaturecalculated is calculated after encrypting the KeyDataand the payload.

The EccEncryptedSecretuses ECC based Asymmetric Cryptography.

Additional semantics for the fields in the EncryptedSecretlayout for the EccEncryptedSecretStructure are described in Table 190.

The EccEncryptedSecretuses ECC EphemeralKeysto create the symmetric key used to encrypt the Secret. The handshake required to create and use the EphemeralKeysis described in OPC 10000-6.

Table 190– EccEncryptedSecret Layout

Name	Type	Description
TypeId	NodeId	The NodeIdof the EccEncryptedSecret DataType Node.
EncodingMask	Byte	See Table 187
Length	UInt32	See Table 187
SecurityPolicyUri	String	See Table 187
Certificate	ByteString	The signing Certificateencoded in DER form. The value shall include the entire chain. This value may be null or empty if the SigningCertificateis known to the receiver. This is true if the structure is used to provide a UserIdentityTokento a Serverover a SecureChanneland the SigningCertificateis the Client ApplicationInstance Certificate.
SigningTime	DateTime	See Table 187
KeyDataLength	UInt16	The length of the KeyDatawithout encryption.
KeyData		The KeyDatais not encrypted.
SenderPublicKey	ByteString	The Public Keyfor the EphemeralKeycreated by the sender.
ReceiverPublicKey	ByteString	The Public Keyfor the EphemeralKeycreated by the receiver.
Nonce	ByteString	A Nonce. This is the last ServerNoncereturned in the CreateSessionor ActivateSession Responsewhen proving a UserIdentityTokenpassed in the ActivateSession Request. In other contexts, this is a Noncecreated by the sender with a length equal to the ½ of the SecureChannelNonceLength.
Secret	ByteString	See Table 187
PayloadPadding	Byte [*]	See Table 187
PayloadPaddingSize	UInt16	See Table 187
Signature	Byte [*]	The Signaturecalculated with the PrivateKeyof the signing Certificate. The Signaturecalculated is calculated after encrypting the payload.