Version 1: Release Note 28 November 2014


Message Construction – MAC Header



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Message Construction – MAC Header


The required components of the MAC Header shall be populated with the values as per Table 7.2.5.17

MAC Header

No

xDLMS Message Elements

Contents

Length (octets)

Note




General-Ciphering

0xDD

1

xDLMS APDU tag for General-Ciphering (221 in decimal)




transaction-id

0x00

1

A value for this element is not needed so the length field is 0x00




originator-system-title

0x00

1

A value for this element is not needed so the length field is 0x00




recipient-system-title

0x00

1

A value for this element is not needed so the length field is 0x00




date-time

0x00

1

A value for this element is not needed so the length field is 0x00




other-information

0x00

1

A value for this element is not needed so the length field is 0x00




key-info

0x00

1

Key-info values are not present so encoded as 0x00




ciphered-service













Length

Encoding(X)

Len(Encoding(X))

X shall be the length in octets of the subsequent parts of the Message after this Length value. This includes the security header, the DLMS APDU being protected and the MAC




security header













security control byte (SC)

0x11

1

Bits 3..0 are security suite which is 0b0001 since Security Suite 1 is required

Bit 4 is set to 0b1 since Authentication of the APDU is required.

Bit 5 is set to 0b0 since the whole of an APDU is never encrypted

Bit 6 is set to 0b0 since messages with MACs are unicast

Bit 7 is set to 0b0 as per the Green Book





invocation counter (IC)

0x00000000

4

IC is always zero as specified in Section 8.4

Table 7.2.5: Required components of MAC Header
      1. Additional Authenticated Data (AAD) for the MAC calculation – informative


Terms in italics in this Section 7.2.6 shall have the meanings as specified in Green Book.

The Green Book requires that the AAD used as input to the MAC calculation is the concatenation of:

SC II AK II transaction-id II originator-system-title II recipient-system-title II date-time II other-information II information to be protected

The Green Book also requires that, for the elements contributing to AAD, only the values of the octet strings are included. The Green Book defines octet strings within the general-ciphering service in ASN.1 as:



General-Ciphering ::= SEQUENCE

{

transaction-id OCTET STRING,

originator-system-title OCTET STRING,

recipient-system-title OCTET STRING,

date-time OCTET STRING,

other-information OCTET STRING,

key-info OPTIONAL,

ciphered-service OCTET STRING

}

As stated in Table 7.2.5, in GBCS-compliant APDUs:



  • SC takes the value 0x11; and

  • the following octet strings in the general-ciphering service shall have zero length and so have no value:

        • transaction-id,

        • originator-system-title,

        • recipient-system-title,

        • date-time,

        • other-information.

As required by Section 4.3.3.4, AK is always absent.

Thus, the AAD to be used in MAC calculations that protect APDUs is the concatenation:

0x11 II information to be protected


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