Using RFID and DigSigs for EVI

A typical EVI data structure

A DigSig is a complex binary data structure that is self identifying, and may contain complex multi tier data structures and field of several data types.

For our example we will design a DigSig data structure using the ISO/IEC 20248 DigSig Definition Descriptor (DDD) language. This language not only describes the fields found in a DigSig, but it also describes the fields that was signed into the signature but whos values were not included in the DigSig itself.

Using the DDD language fields can also be described in such a way so that they are distributed over several data banks of a selected medium.

Create DDD field descriptors for each identified field

The data fields for a typical EVI solution can be described using the ISO/IEC 20248 DDD language. In this section we will show how much data storage we will need for each field by creating an appropriate data descriptor for each of the fields and calculating the data requirements for each such field.

• daid – The DAID is a code that uniquely identifies the domain authority. DigSig Certificates required for decoding should be obtained from the domain authority.
• cid – The DigSig Certificate Id which is unique per domain authority
• signature – The Digital signature data field that is used to cryptographically verify the integrity of the data fields and to authenticate its contents.

Tag programming profile

By using the “readmethod” pragma feature as described in the ISO/IEC 20248 specification the DigSig will be automatically broken up so that it can be written into the various selected databanks of the chosen media.

For our example we will break up the DigSig into 2 snips, a short snip to be written into the shorter but more accessible UII/EPC memory bank and then another but longer snip intended for the UserMem bank.

Storage required for UII/EPC: 40 + 16 + 4 + 3 + 32 = 95 bits
Storage required for User Memory: 256 + 32 + 54 + 125 = 467 bits

We shall use the DDD “struct” mechanism to group all of the fields intended for the user memory bank together into one data structure. We will then use the readmethod pragma feature to instruct a verifier to fetch the contents of this struct from a separate data bank.

{
  "fieldid": "usermem",
  "type": "struct",
  "pragma": {
    "readmethod": {
      "methodname": "ISO/IEC 18000-63",
      "methoddesc": {
        "en": "User memory bank"
      },
      "methodmemory": [
        3
      ],
      "methodoffset": 0,
      "methodlength": 595
    }
  },
  "fields": [
      …
      …
      … FIELDS TO BE STORED ON USER MEMORY
      …
      …
   ]
}

Example of how a readmethod can be used to describe data fields stored in the rfid UserMem bank

• methodname – The type of technology to use to collect this data. In this case ISO/IEC 18000-63
• methodoffset – The data begins at the beginning of the user memory bank. Therefore a zero offset is needed
• methodlength – The length of the read method is specified to be 595 bits. This way a reader won’t read more than it has to
• methodmemory – Instructs the reader to fetch this data from memory bank address 3 which is the user memory

Including the RFID TID field

We can also craft a readmethod which describes the reading of the TID memory of the RFID tag. By including the data of this databank into the DigSig we can effectively “sign” the TID data into the produced signature thereby forever linking this tag to the signature. By doing so we can protect the tag from forgery.

{
  "fieldid": "tid",
  "fieldname": {
    "en": "Tag ID"
  },
  "type": "bstring",
  "binaryformat": "{96}",
  "pragma": {
    "readmethod": {
      "methodname": "ISO/IEC 18000-63",
      "methoddesc": {
        "en": "Plate serialisation (TagID), memory bank 2."
      },
      "methodmemory": [
        2
      ],
      "methodoffset": 0,
      "methodlength": 96
    }
  }
}

Full example – DigSig Data Descriptor (DDD)

We can also craft a readmethod which describes the reading of the TID memory of the RFID tag. By including the data of this databank into the DigSig we can effectively “sign” the TID data into the produced signature thereby forever linking this tag to the signature. By doing so we can protect the tag from forgery.

{
  "datafields": [
    {
      "fieldid": "specificationversion"
    },
    {
      "fieldid": "dauri"
    },
    {
      "fieldid": "daid"
    },
    {
      "fieldid": "cid"
    },
    {
      "enumvalues": [
        "brown",
        "blue",
        "gold",
        "gray",
        "green",
        "orange",
        "pink",
        "purple",
        "red",
        "silver",
        "white",
        "yellow"
      ],
      "fieldid": "colour",
      "type": "enum"
    },
    {
      "enumvalues": [
        "VAN",
        "COUPE",
        "PICKUP",
        "SALOON",
        "S.WAGON",
        "LORRY/TRUCK",
        "MOTOR CYCLES"
      ],
      "fieldid": "form_factor",
      "type": "enum"
    },
    {
      "fieldid": "expiry_date",
      "type": "date"
    },
    {
      "fieldid": "usermem",
      "type": "struct",
      "pragma": {
        "readmethod": {
          "methodname": "ISO/IEC 18000-63",
          "methoddesc": {
            "en": "User memory bank"
          },
          "methodmemory": [
            3
          ],
          "methodoffset": 0,
          "methodlength": 595
        }
      },
      "fields": [
        {
          "fieldid": "signature",
          "type": "bstring",
          "binaryformat": "{256}"
        },
        {
          "fieldid": "timestamp"
        },
        {
          "fieldid": "license_plate",
          "type": "string",
          "range": "[A-Z0-9]",
          "binaryformat": "{1,8}"
        },
        {
          "fieldid": "vin",
          "range": "[A-Z0-9]",
          "type": "string"
        }
      ]
    },
    {
      "fieldid": "tid",
      "fieldname": {
        "en": "Tag ID"
      },
      "type": "bstring",
      "binaryformat": "{96}",
      "pragma": {
        "readmethod": {
          "methodname": "ISO/IEC 18000-63",
          "methoddesc": {
            "en": "Plate serialisation (TagID), memory bank 2."
          },
          "methodmemory": [
            2
          ],
          "methodoffset": 0,
          "methodlength": 96
        }
      }
    }
  ],
  "digsiginfo": {
    "cid": 811,
    "daid": "QC TRUE",
    "dauri": "https://dauri1.20248.info",
    "revocationuri": "https://dauri1.20248.info/revoke",
    "specificationversion": "ISO/IEC 20248:2018",
    "verificationuri": "https://v1.20248.info"
  }
}