4.18.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 ofsub-nodes mentioning properties of all non-volatile-counters used in the chain of trust.

4.18.1.cot

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

4.18.2.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 usedduring the authentication process.
PROPERTIES
- root-certificate
  Usage:
  Required for the certificate with no parent.In other words, certificates which are validatedusing 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 containsinformation to authenticate the certificate.This property is required for all non-root certificates.
  This property is not required for root-certificatesas root-certificates are validated using root of trustpublic key provided by platform.
  Value type: <phandle>
- signing-key
  Usage:
  For non-root certificates, this property is used to referpublic key node present in parent certificate node and it isrequired property for all non-root certificates which areauthenticated using public-key present in parent certificate.
  This property is not required for all root-certificates. Ifomitted, the root certificate will be validated using thedefault platform ROTPK. If instead the root certificate needsvalidating using a different ROTPK, the signing-key propertyshould provide a reference to the ROTPK node to use.
  Value type: <phandle>
- antirollback-counter
  Usage:
  This property is used by all certificates which areprotected against rollback attacks using a non-volatilecounter and it is an optional property.
  This property is used to refer one of the non-volatilecounter sub-node present in ‘non-volatile counters’ node.
  Value type: <phandle>
SUBNODES
Description:
Hash and public key information present in the certificateare 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 keyprovided in the certificate with the help of whichpublic 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 inthe certificate with the help of which hash informationcan 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_ctr>;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_ctr>;scp_fw_content_pk:scp_fw_content_pk{oid=SCP_FW_CONTENT_CERT_PK_OID;};};...next-certificate{};};};

4.18.3.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 duringauthentication 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 toits parent image, which contains the necessary informationto authenticate it.
  Value type: <phandle>
- hash
  Usage:
  Required for all images which are validated usinghash method. This property is used to refer hashnode present in parent certificate node.
  Value type: <phandle>
  Note:
  Currently, all images are validated using ‘hash’method. In future, there may be multiple methods canbe 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{};};};

4.18.4.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 sizeof 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
id
Usage: Required for every nv-counter with unique id.
Value type: <u32>
reg
Usage:
Register base address of non-volatile counter and it is requiredproperty.
Value type: <u32>
oid
Usage:
This property provides the Object ID of non-volatile counterprovided in the certificate and it is required property.
Value type: <string>

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

non_volatile_counters:non_volatile_counters{compatible="arm, non-volatile-counter";#address-cells = <1>;#size-cells = <0>;trusted_nv_ctr:trusted_nv_ctr{id=<TRUSTED_NV_CTR_ID>;reg=<TFW_NVCTR_BASE>;oid=TRUSTED_FW_NVCOUNTER_OID;};non_trusted_nv_ctr:non_trusted_nv_ctr{id=<NON_TRUSTED_NV_CTR_ID>;reg=<NTFW_CTR_BASE>;oid=NON_TRUSTED_FW_NVCOUNTER_OID;};};

4.18.5.rot_keys node binding definition

rot_keys node
Description: Contains root-of-trust keys for the root certificates.
SUBNODES
Description:
Root of trust key information present in the root certificatesare shown by these nodes.
rot key node
Description: Provide ROT key information in the certificate.
PROPERTIES
oid
Usage:
This property provides the Object ID of ROT key providedin the certificate.
Value type: <string>

Example:Below is rot_keys example for CCA platform

rot_keys{swd_rot_pk:swd_rot_pk{oid=SWD_ROT_PK_OID;};prot_pk:prot_pk{oid=PROT_PK_OID;};};

4.18.6.Future update to chain of trust binding

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

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