16. Chain of trust bindings

The device tree allows to describe the chain of trust with the help of ‘cot’ node which contain ‘manifests’ and ‘images’ as sub-nodes. ‘manifests’ and ‘images’ nodes contains number of sub-nodes (i.e. ‘certificate’ and ‘image’ nodes) mentioning properties of the certificate and image respectively.

Also, device tree describes ‘non-volatile-counters’ node which contains number of sub-nodes mentioning properties of all non-volatile-counters used in the chain of trust.

16.1. cot

This is root node which contains ‘manifests’ and ‘images’ as sub-nodes

16.2. Manifests and Certificate node bindings definition

  • Manifests node

    Description: Container of certificate nodes.

    PROPERTIES

    • compatible:

      Usage: required

      Value type: <string>

      Definition: must be “arm, cert-descs”

  • Certificate node

    Description:

    Describes certificate properties which are used during the authentication process.

    PROPERTIES

    • root-certificate

      Usage:

      Required for the certificate with no parent. In other words, certificates which are validated using root of trust public key.

      Value type: <boolean>

    • image-id

      Usage: Required for every certificate with unique id.

      Value type: <u32>

    • parent

      Usage:

      It refers to their parent image, which typically contains information to authenticate the certificate. This property is required for all non-root certificates.

      This property is not required for root-certificates as root-certificates are validated using root of trust public key provided by platform.

      Value type: <phandle>

    • signing-key

      Usage:

      This property is used to refer public key node present in parent certificate node and it is required property for all non-root certificates which are authenticated using public-key present in parent certificate.

      This property is not required for root-certificates as root-certificates are validated using root of trust public key provided by platform.

      Value type: <phandle>

    • antirollback-counter

      Usage:

      This property is used by all certificates which are protected against rollback attacks using a non-volatile counter and it is an optional property.

      This property is used to refer one of the non-volatile counter sub-node present in ‘non-volatile counters’ node.

      Value type: <phandle>

    SUBNODES
    • Description:

      Hash and public key information present in the certificate are shown by these nodes.

    • public key node

      Description: Provide public key information in the certificate.

      PROPERTIES

      • oid

        Usage:

        This property provides the Object ID of public key provided in the certificate which the help of which public key information can be extracted.

        Value type: <string>

    • hash node

      Description: Provide the hash information in the certificate.

      PROPERTIES

      • oid

        Usage:

        This property provides the Object ID of hash provided in the certificate which the help of which hash information can be extracted.

        Value type: <string>

Example:

cot {
   manifests {
      compatible = "arm, cert-descs”

      trusted-key-cert: trusted-key-cert {
         root-certificate;
         image-id = <TRUSTED_KEY_CERT_ID>;
         antirollback-counter = <&trusted_nv_counter>;

         trusted-world-pk: trusted-world-pk {
            oid = TRUSTED_WORLD_PK_OID;
         };
         non-trusted-world-pk: non-trusted-world-pk {
            oid = NON_TRUSTED_WORLD_PK_OID;
         };
      };

      scp_fw_key_cert: scp_fw_key_cert {
         image-id = <SCP_FW_KEY_CERT_ID>;
         parent = <&trusted-key-cert>;
         signing-key = <&trusted_world_pk>;
         antirollback-counter = <&trusted_nv_counter>;

         scp_fw_content_pk: scp_fw_content_pk {
            oid = SCP_FW_CONTENT_CERT_PK_OID;
         };
      };
      .
      .
      .

      next-certificate {

      };
   };
};

16.3. Images and Image node bindings definition

  • Images node

    Description: Container of image nodes

    PROPERTIES

    • compatible:

      Usage: required

      Value type: <string>

      Definition: must be “arm, img-descs”

  • Image node

    Description:

    Describes image properties which will be used during authentication process.

    PROPERTIES

    • image-id

      Usage: Required for every image with unique id.

      Value type: <u32>

    • parent

      Usage:

      Required for every image to provide a reference to its parent image, which contains the necessary information to authenticate it.

      Value type: <phandle>

    • hash

      Usage:

      Required for all images which are validated using hash method. This property is used to refer hash node present in parent certificate node.

      Value type: <phandle>

      Note:

      Currently, all images are validated using ‘hash’ method. In future, there may be multiple methods can be used to validate the image.

Example:

cot {
   images {
      compatible = "arm, img-descs";

      scp_bl2_image {
         image-id = <SCP_BL2_IMAGE_ID>;
         parent = <&scp_fw_content_cert>;
         hash = <&scp_fw_hash>;
      };

      .
      .
      .

      next-img {

      };
   };
};

16.4. non-volatile counter node binding definition

  • non-volatile counters node

    Description: Contains properties for non-volatile counters.

    PROPERTIES

    • compatible:

      Usage: required

      Value type: <string>

      Definition: must be “arm, non-volatile-counter”

    • #address-cells

      Usage: required

      Value type: <u32>

      Definition:

      Must be set according to address size of non-volatile counter register

    • #size-cells

      Usage: required

      Value type: <u32>

      Definition: must be set to 0

    SUBNODE
    • counters node

      Description: Contains various non-volatile counters present in the platform.

    PROPERTIES

    • reg

      Usage:

      Register base address of non-volatile counter and it is required property.

      Value type: <u32>

    • oid

      Usage:

      This property provides the Object ID of non-volatile counter provided in the certificate and it is required property.

      Value type: <string>

Example: Below is non-volatile counters example for ARM platform

non-volatile-counters {
     compatible = "arm, non-volatile-counter";
     #address-cells = <1>;
     #size-cells = <0>;

     counters {
         trusted-nv-counter: trusted_nv_counter {
             reg = <TFW_NVCTR_BASE>;
             oid = TRUSTED_FW_NVCOUNTER_OID;
         };
         non_trusted_nv_counter: non_trusted_nv_counter {
             reg = <NTFW_CTR_BASE>;
             oid = NON_TRUSTED_FW_NVCOUNTER_OID;

         };
     };
};

16.5. Future update to chain of trust binding

This binding document needs to be revisited to generalise some terminologies which are currently specific to X.509 certificates for e.g. Object IDs.

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