The structure of the Messagedepends on whether the SecurityPolicyrequires an algorithm that combines encryption and authentication (e.g. Authenticated Encryptionalgorithms) used or if it requires separate algorithms for each operation (Unauthenticated Encryption algorithms).

Figure 11shows the structure of a MessageChunkand how security is applied to the Messagewhen using Unauthenticated Encryptionalgorithms. For these SecurityPoliciesany padding is appended to the message before appending the Signature.

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Figure 11– MessageChunk for Unauthenticated Encryption Algorithms

Figure 12shows the structure of a MessageChunkand how security is applied to the Messagewhen using Authenticated Encryptionalgorithms. For these SecurityPoliciesthe Signatureis calculated during encryption and appended after the encrypted data. Paddingis not needed.

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Figure 12– MessageChunk for Authenticated Encryption Algorithms

Every MessageChunkhas a Messageheader with the fields defined in Table 50.

Table 50– OPC UA Secure ConversationMessage Header

Name

Data Type

Description

MessageType

Byte [3]

A three byte ASCII code that identifies the Messagetype.

The following values are defined at this time:

MSGA Messagesecured with the keys associated with a channel.

OPN OpenSecureChannel Message.

CLO CloseSecureChannel Message.

IsFinal

Byte

A one byte ASCII code that indicates whether the MessageChunkis the final chunk in a Message.

The following values are defined at this time:

C An intermediate chunk.

F The final chunk.

A The final chunk (used when an error occurred and the Messageis aborted).

This field is only meaningful for MessageType of ‘MSG’

This field is always ‘F’ for other MessageTypes.

MessageSize

UInt32

The length of the MessageChunk, in bytes.

The length starts from the beginning of the MessageType field.

SecureChannelId

UInt32

A unique identifier for the SecureChannelassigned by the Server.

If a Serverreceives a SecureChannelId which it does not recognize it shall return an appropriate transport layer error.

When a Serverstarts the first SecureChannelIdused should be a value that is likely to be unique after each restart. This ensures that a Serverrestart does not cause previously connected Clientsto accidently ‘reuse’ SecureChannelsthat did not belong to them.

The Messageheader is followed by a security header which specifies what cryptography operations have been applied to the Message. There are two versions of the security header which depend on the type of security applied to the Message. The security header used for asymmetric algorithms is defined in Table 51. Asymmetric algorithms are used to secure the OpenSecureChannel Messages. PKCS #1defines a set of asymmetric algorithms that may be used by UASC implementations. The AsymmetricKeyWrapAlgorithmelement of the SecurityPolicystructure defined in Table 42is not used by UASC implementations.

Table 51– Asymmetric algorithm Security header

Name

Data Type

Description

SecurityPolicyUriLength

Int32

The length of the SecurityPolicyUriin bytes.

This value shall not exceed 255 bytes.

If a URI is not specified this value may be 0 or -1.

Other negative values are invalid.

SecurityPolicyUri

Byte []

The URI of the Security Policyused to secure the Message.

This field is encoded as a UTF-8string without a null terminator.

SenderCertificateLength

Int32

The length of the SenderCertificatein bytes.

This value shall not exceed MaxSenderCertificateSizebytes.

If a certificate is not specified this value may be 0 or -1.

Other negative values are invalid.

SenderCertificate

Byte []

The X.509 v3 Certificateassigned to the sending applicationInstance.

This is a DER encoded blob.

The structure of an X.509 v3 Certificateis defined in X.509 v3.

The DER format for a Certificateis defined in X690

This indicates what Private Keywas used to sign the MessageChunk.

The Stackshall close the channel and report an error to the application if the SenderCertificateis too large for the buffer size supported by the transport layer.

This field shall be null if the Messageis not signed.

If the Certificateis signed by a CA, the DER encoded CA Certificatemay be appended after the Certificate in the byte array. If the CA Certificateis also signed by another CA this process is repeated until the entire Certificate chain is in the buffer or if MaxSenderCertificateSizelimit is reached (the process stops after the last whole Certificatethat can be added without exceeding the MaxSenderCertificateSizelimit).

Receivers can extract the Certificatesfrom the byte array by using the Certificatesize contained in DER header (see X.509 v3).

Receivers that do not handle Certificatechains shall ignore the extra bytes.

ReceiverCertificateThumbprintLength

Int32

The length of the ReceiverCertificateThumbprintin bytes.

If encrypted, the value of this field is 20 bytes.

If not encrypted the value may be 0 or -1.

Other negative values are invalid.

ReceiverCertificateThumbprint

Byte []

The thumbprint of the X.509 v3 Certificateassigned to the receiving applicationInstance.

The thumbprint is the CertificateDigestof the DER encoded form of the Certificate.

This indicates what public key was used to encrypt the MessageChunk.

This field shall be null if the Messageis not encrypted.

The receiver shall close the communication channel if any of the fields in the security header have invalid lengths.

The SenderCertificate, includingany chains,shall be small enough to fit into a single MessageChunkand leave room for at least one byte of body information. The maximum size for the SenderCertificatecan be calculated with this formula:

MaxSenderCertificateSize =

MessageChunkSize –

12 - // Header size

4 - // SecurityPolicyUriLength

SecurityPolicyUri -// UTF-8 encoded string

4 - // SenderCertificateLength

4 - // ReceiverCertificateThumbprintLength

20 - // ReceiverCertificateThumbprint

8 - // SequenceHeader size

1 - // Minimum body size

1 - // PaddingSize if present

Padding - // Padding if present

ExtraPadding - // ExtraPadding if present

AsymmetricSignatureSize// If present

The MessageChunkSizedepends on the transport protocol but shall be at least 8 192 bytes. The AsymmetricSignatureSizedepends on the number of bits in the public key for the SenderCertificate. The Int32FieldLengthis the length of an encoded Int32 value and it is always 4 bytes.

The security header used for symmetric algorithms defined in Table 52. Symmetric algorithms are used to secure all Messagesother than the OpenSecureChannel Messages. FIPS 197define symmetric encryption algorithms that UASC implementations may use. FIPS 180-and HMACdefine some symmetric signature algorithms.

Table 52– Symmetric algorithm Security header

Name

Data Type

Description

TokenId

UInt32

A unique identifier for the SecureChannel SecurityTokenused to secure the Message.

This identifier is returned by the Serverin an OpenSecureChannelresponse Message. If a Serverreceives a TokenId which it does not recognize it shall return an appropriate transport layer error.

The security header is always followed by the sequence header which is defined in Table 53. The sequence header ensures that the first encrypted block of every Messagesent over a channel will start with different data.

Table 53– Sequence header

Name

Data Type

Description

SequenceNumber

UInt32

A monotonically increasing sequence number assigned by the sender to each MessageChunksent over the SecureChannel.

RequestId

UInt32

An identifier assigned by the Clientto OPC UA request Message. All MessageChunksfor the request and the associated response use the same identifier.

ASequenceNumber may not be reused for any TokenId. The SecurityTokenlifetime shall be short enough to ensure that this never happens; however, if it does the receiver shall treat it as a transport error and force a reconnect. The SequenceNumber does not reset when a new TokenId is issued and it shall be incremented by exactly one for each MessageChunk sent.

SecurityPolicies with LegacySequenceNumbersset to TRUE, the SequenceNumber shall monotonically increase for all Messages and shall not wrap around until it is greater than 4 294 966 271 (UInt32.MaxValue – 1 024). The first number after the wrap around shall be less than 1 024.

SecurityPolicies with LegacySequenceNumbersset to FALSE, the SequenceNumber shall start at 0 and monotonically increase for all Messages and shall not wrap around until it is equal to 4 294 967 295 (UInt32.MaxValue). The first number after the wrap around shall be 0.

Some applications will find it takes time to validate the OpenSecureChannel Requestsand Responsesused to renew a TokenId. In these situations, the receiver may assume the SequenceNumberis correct which allows it to process subsequent messages secured with the existing TokenIdbefore the OpenSecureChannel Messageis validated. When processing of the OpenSecureChannel Message completes, the receiver checks the SequenceNumberand closes the SecureChannelif it is incorrect.

The sequence header is followed by the Messagebody which is encoded with the OPC UA Binary encoding as described in 5.2.9. The body may be split across multiple MessageChunks.

Each MessageChunkwhen using SecurityPolicieswith an Unauthenticated Encryptionalgorithms have a footer with the fields defined in Table 54.

Table 54Message Footer for Unauthenticated Encryption Algorithms

Name

Data Type

Description

PaddingSize

Byte

The number of padding bytes (not including the byte for the PaddingSize).

Padding

OctetString

Padding added to the end of the Messageto ensure length of the data to encrypt is an integer multiple of the encryption block size.

The value of each byte of the padding is equal to PaddingSize.

ExtraPaddingSize

Byte

The most significant byte of a two-byte integer used to specify the padding size when the key used to encrypt the message chunk is larger than 2 048 bits. This field is omitted if the key length is less than or equal to 2 048 bits.

Signature

OctetString

The signature for the MessageChunk.

The signature includes the headers, all Messagedata, the PaddingSizeand the Padding.

The signature is encoded as sequence of Bytes with a length specified by theSecurityPolicy.

The formula to calculate the amount of padding depends on the amount of data that needs to be sent (called BytesToWrite). The sender shall first calculate the maximum amount of space available in the MessageChunk(called MaxBodySize) using the following formula:

MaxBodySize = PlainTextBlockSize * Floor ((MessageChunkSize – MessageHeaderSize - SecurityHeaderSize – SignatureSize - 1)/CipherTextBlockSize) – SequenceHeaderSize;

The MessageHeaderSizeis 12 bytes and the SecurityHeadersize depends on whether symmetric or asymmetric cryptography is used. The SequenceHeaderSizeis always 8 bytes.

During encryption a block with a size equal to PlainTextBlockSizeis processed to produce a block with size equal to CipherTextBlockSize. These values depend on the encryption algorithm and may be the same.

The OPC UA Messagecan fit into a single chunk if BytesToWriteis less than or equal to the MaxBodySize. In this case the PaddingSizeis calculated with this formula:

PaddingSize = PlainTextBlockSize –

((BytesToWrite + SignatureSize + 1) % PlainTextBlockSize);

If the BytesToWriteis greater than MaxBodySizethe sender shall write MaxBodySizebytes with a PaddingSize of 0. The remaining BytesToWriteMaxBodySizebytes shall be sent in subsequent MessageChunks.

The PaddingSizeand Paddingfields are not present if the MessageChunkis not encrypted.

The Signature field is not present if the MessageChunk is not signed.

Each MessageChunkwhen using SecurityPolicieswith an Authenticated Encryptionalgorithms have a footer with the fields defined in Table 55.

Table 55Message Footer for Authenticated Encryption Algorithms

Name

Data Type

Description

Signature

OctetString

The signature for the MessageChunk.

The signature includes the headers and all Messagedata.

The signature is encoded as sequence of Bytes with a length specified by theSecurityPolicy.