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

Each UserIdentityToken allowed by an Endpoint shall have a UserTokenPolicy specified in the EndpointDescription. The UserTokenPolicy specifies what SecurityPolicy to use when encrypting or signing. If this SecurityPolicy is null or empty then the Client uses the SecurityPolicy in the EndpointDescription. If the matching SecurityPolicy is set to None then no encryption or signature is required. The possible SecurityPolicies are defined in OPC 10000-7.

It is recommended that applications never set the SecurityPolicy to None for UserIdentityTokens that include a secret because these secrets could be used by an attacker to gain access to the system.

Clients shall validate the Server Certificate and ensure it is trusted before sending a UserIdentityToken encrypted with the Certificate.

The encrypted secret and Signature are embedded in a ByteString which is part of the UserIdentityToken. The format of this ByteString depends on the type of UserIdentityToken and the SecurityPolicy. Clients shall validate the Server Certificate and ensure it is trusted before sending a UserIdentityToken encrypted with the Certificate.

The legacy token secret format defined in 7.41.2.2 is 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. If the password exceeds 64 bytes, the EncryptedSecret format shall be used or the clear text password is sent over a SecureChannel that is encrypted.

The EncryptedSecret format defined in 7.41.2.3 provides 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 SecurityPolicies are added.

The EncryptedSecret format starts with a TypeId, EncodingMask and Length. These values allow a Server to determine how to handle the secret. If these fields are not valid and a USERNAME UserIdentityToken has been provided then the Server may attempt to handle the secret using the legacy token secret format. If these fields are valid but the TypeId is not recognized or not valid for the SecurityPolicy then the Server rejects the UserIdentityToken.

The UserIdentityToken types and the token formats supported by the Endpoint are identified by the UserTokenPolicy defined in 7.42.

To prevent the leakage of information useful to attackers, Servers shall ensure that the process of validating UserIdentityTokens completes 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.

Servers shall log details of any failure to validate a UserIdentityToken and shall lock out Client applications after five failures.

When encrypting a UserIdentityToken, the Client appends the last ServerNonce to the secret. The data is then encrypted with the public key from the Server’s Certificate.

A Client should not add any padding after the secret. If a Client adds padding then all bytes shall be zero. A Server shall check for padding added by Clients and ensure that all padding bytes are zeros. Servers shall reject UserIdentityTokens with invalid padding. Administrators shall be able to configure Servers to accept UserIdentityTokens with invalid padding.

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

Table 186 describes how to serialize UserIdentityTokens before 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 ServerNonce but excluding the length field. 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 ServerNonce returned by the Server in the CreateSession or ActivateSession response.

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, the RsaEncryptedSecret requires that the KeyData be encrypted with the public key associated with the EncryptingCertificate before it is serialized.

The SecurityPolicyUri is 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 KeyData is used to create the keys used needed for the symmetric encryption. The structure of the KeyData depends on the EncryptedSecret DataType.

The EncryptedSecret is 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 DataType specified in Table 187. 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. 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 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 EncryptedSecret are described in Table 187. The first three fields TypeId, EncodingMask and Length belong to the ExtensionObject encoding defined in OPC 10000-6.

Table 187 – 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.
Certificate	ByteString	The signing and/or encrypting Certificate.
SigningTime	DateTime	When the Signature was created.
KeyDataLength	UInt16	The length, in bytes, of the KeyData that follows If the KeyData is 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 DataType describes how the key data is structured for different SecurityPolicies.
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 Nonce is created by the sender and is a function of the SecureChannelNonceLength.
Secret	ByteString	The secret to protect. The password when used with UserNameIdentityTokens. The tokenData when used with IssuedIdentityTokens. If the Secret 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 InitializationVectorLength specified by the SecurityPolicyUri. If the InitializationVectorLength is less than 16 bytes then 16 bytes are used instead. 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 after all encryption is applied. Each EncryptedSecret DataType describes how the Signature is calculated for different SecurityPolicies.

The PayloadPaddingSize adjusted with the following formula:

If (Secret.Length + PayloadPaddingSize < InitializationVectorLength) Then

PayloadPaddingSize = PayloadPaddingSize + InitializationVectorLength

Where the InitializationVectorLength is specified by the SymmetricEncryptionAlgorithm.

The currently available EncryptedSecret DataTypes are defined in Table 188.

Table 188 – EncryptedSecret DataTypes

Type Name	When to Use
RsaEncryptedSecret	Used when the SecurityPolicy requires the use of RSA cryptography. It is described in 7.41.2.4.
EccEncryptedSecret	Used when the SecurityPolicy requires the use of ECC cryptography.It is described in 7.41.2.5.

The RsaEncryptedSecret uses RSA based Asymmetric Cryptography.

Additional semantics for the fields in the EncryptedSecret layout for the RsaEncryptedSecret Structure are described in Table 189.

Table 189 – RsaEncryptedSecret structure

Name	Type	Description
TypeId	NodeId	The NodeId of 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 KeyData is encrypted with the PublicKey associated 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 ServerNonce returned in the CreateSession or ActivateSession Response when proving a UserIdentityToken passed in the ActivateSession Request. In other contexts, this is a Nonce created 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 Signature calculated with the SigningKey. The Signature calculated is calculated after encrypting the KeyData and the payload.

The EccEncryptedSecret uses ECC based Asymmetric Cryptography.

Additional semantics for the fields in the EncryptedSecret layout for the EccEncryptedSecret Structure are described in Table 190.

The EccEncryptedSecret uses ECC EphemeralKeys to create the symmetric key used to encrypt the Secret. The handshake required to create and use the EphemeralKeys is described in OPC 10000-6.

Table 190 – EccEncryptedSecret Layout

Name	Type	Description
TypeId	NodeId	The NodeId of the EccEncryptedSecret DataType Node.
EncodingMask	Byte	See Table 187
Length	UInt32	See Table 187
SecurityPolicyUri	String	See Table 187
Certificate	ByteString	The signing Certificate encoded in DER form. The value shall include the entire chain. This value may be null or empty if the SigningCertificate is known to the receiver. This is true if the structure is used to provide a UserIdentityToken to a Server over a SecureChannel and the SigningCertificate is the Client ApplicationInstance Certificate.
SigningTime	DateTime	See Table 187
KeyDataLength	UInt16	The length of the KeyData without encryption.
KeyData		The KeyData is not encrypted.
SenderPublicKey	ByteString	The Public Key for the EphemeralKey created by the sender.
ReceiverPublicKey	ByteString	The Public Key for the EphemeralKey created by the receiver.
Nonce	ByteString	A Nonce. This is the last ServerNonce returned in the CreateSession or ActivateSession Response when proving a UserIdentityToken passed in the ActivateSession Request. In other contexts, this is a Nonce created 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 [*]	When using AuthenticatedEncryption the Signature has two parts: the Signature produced when the secret is encrypted using the SymmetricEncryptionAlgorithm and the Signature calculated using the Certificate and the AsymmetricSignatureAlgorithm. Both Signatures are calculated from the start of the packet. The AsymmetricSignatureAlgorithm Signature includes the SymmetricEncryptionAlgorithm Signature. When using UnauthenticatedEncryption the Signature is only calculated using the Certificate and the AsymmetricSignatureAlgorithm.