RFC 4108
Using Cryptographic Message Syntax (CMS) to Protect Firmware Packages
2. Firmware Package Protection
The Cryptographic Message Syntax (CMS) is used to protect a firmware package, which is treated as an opaque binary object. A digital signature is used to protect the firmware package from undetected modification and to provide data origin authentication. Encryption is optionally used to protect the firmware package from disclosure, and compression is optionally used to reduce the size of the protected firmware package. The CMS ContentInfo content type MUST always be present, and it MUST encapsulate the CMS SignedData content type. If the firmware package is encrypted, then the CMS SignedData content type MUST encapsulate the CMS EncryptedData content type. If the firmware package is compressed, then either the CMS SignedData
content type (when encryption is not used) or the CMS EncryptedData content type (when encryption is used) MUST encapsulate the CMS CompressedData content type. Finally, (1) the CMS SignedData content type (when neither encryption nor compression is used), (2) the CMS EncryptedData content type (when encryption is used, but compression is not), or (3) the CMS CompressedData content type (when compression is used) MUST encapsulate the simple firmware package using the FirmwarePkgData content type defined in this specification (see Section 2.1.5). The firmware package protection is summarized as follows (see [CMS] for the full syntax): ContentInfo { contentType id-signedData, -- (1.2.840.113549.1.7.2) content SignedData } SignedData { version CMSVersion, -- always set to 3 digestAlgorithms DigestAlgorithmIdentifiers, -- Only one encapContentInfo EncapsulatedContentInfo, certificates CertificateSet, -- Signer cert. path crls CertificateRevocationLists, -- Optional signerInfos SET OF SignerInfo -- Only one } SignerInfo { version CMSVersion, -- always set to 3 sid SignerIdentifier, digestAlgorithm DigestAlgorithmIdentifier, signedAttrs SignedAttributes, -- Required signatureAlgorithm SignatureAlgorithmIdentifier, signature SignatureValue, unsignedAttrs UnsignedAttributes -- Optional }
EncapsulatedContentInfo {
eContentType id-encryptedData, -- (1.2.840.113549.1.7.6)
-- OR --
id-ct-compressedData,
-- (1.2.840.113549.1.9.16.1.9)
-- OR --
id-ct-firmwarePackage,
-- (1.2.840.113549.1.9.16.1.16)
eContent OCTET STRING
} -- Contains EncryptedData OR
-- CompressedData OR
-- FirmwarePkgData
EncryptedData {
version CMSVersion, -- Always set to 0
encryptedContentInfo EncryptedContentInfo,
unprotectedAttrs UnprotectedAttributes -- Omit
}
EncryptedContentInfo {
contentType id-ct-compressedData,
-- (1.2.840.113549.1.9.16.1.9)
-- OR --
id-ct-firmwarePackage,
-- (1.2.840.113549.1.9.16.1.16)
contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier,
encryptedContent OCTET STRING
} -- Contains CompressedData OR
-- FirmwarePkgData
CompressedData {
version CMSVersion, -- Always set to 0
compressionAlgorithm CompressionAlgorithmIdentifier,
encapContentInfo EncapsulatedContentInfo
}
EncapsulatedContentInfo {
eContentType id-ct-firmwarePackage,
-- (1.2.840.113549.1.9.16.1.16)
eContent OCTET STRING -- Contains FirmwarePkgData
}
FirmwarePkgData OCTET STRING -- Contains firmware package
2.1. Firmware Package Protection CMS Content Type Profile
This section specifies the conventions for using the CMS ContentInfo, SignedData, EncryptedData, and CompressedData content types. It also defines the FirmwarePkgData content type.2.1.1. ContentInfo
The CMS requires that the outermost encapsulation be ContentInfo [CMS]. The fields of ContentInfo are used as follows: contentType indicates the type of the associated content, and in this case, the encapsulated type is always SignedData. The id-signedData (1.2.840.113549.1.7.2) object identifier MUST be present in this field. content holds the associated content, and in this case, the content field MUST contain SignedData.2.1.2. SignedData
The SignedData content type [CMS] contains the signed firmware package (which might be compressed, encrypted, or compressed and then encrypted prior to signature), the certificates needed to validate the signature, and one digital signature value. The fields of SignedData are used as follows: version is the syntax version number, and in this case, it MUST be set to 3. digestAlgorithms is a collection of message digest algorithm identifiers, and in this case, it MUST contain a single message digest algorithm identifier. The message digest algorithm employed by the firmware package signer MUST be present. encapContentInfo contains the signed content, consisting of a content type identifier and the content itself. The use of the EncapsulatedContentInfo type is discussed further in Section 2.1.2.2. certificates is an optional collection of certificates. If the trust anchor signed the firmware package directly, then certificates SHOULD be omitted. If it did not, then certificates SHOULD include the X.509 certificate of the firmware package signer. The set of certificates SHOULD be sufficient for the bootstrap loader to construct a certification path from the trust anchor to the firmware-signer's certificate. PKCS#6 extended certificates
[PKCS#6] and attribute certificates (either version 1 or
version 2) [X.509-97, X.509-00, ACPROFILE] MUST NOT be included in
the set of certificates.
crls is an optional collection of certificate revocation lists
(CRLs), and in this case, CRLs SHOULD NOT be included by the
firmware package signer. It is anticipated that firmware packages
may be generated, signed, and made available in repositories for
downloading into hardware modules. In such contexts, it would be
difficult for the firmware package signer to include timely CRLs
in the firmware package. However, because the CRLs are not
covered by the signature, timely CRLs MAY be inserted by some
other party before the firmware package is delivered to the
hardware module.
signerInfos is a collection of per-signer information, and in this
case, the collection MUST contain exactly one SignerInfo. The use
of the SignerInfo type is discussed further in Section 2.1.2.1.
2.1.2.1. SignerInfo
The firmware package signer is represented in the SignerInfo type.
The fields of SignerInfo are used as follows:
version is the syntax version number, and it MUST be 3.
sid identifies the signer's public key. CMS supports two
alternatives: issuerAndSerialNumber and subjectKeyIdentifier.
However, the bootstrap loader MUST support the
subjectKeyIdentifier alternative, which identifies the signer's
public key directly. When this public key is contained in a
certificate, this identifier SHOULD appear in the X.509
subjectKeyIdentifier extension.
digestAlgorithm identifies the message digest algorithm, and any
associated parameters, used by the firmware package signer. It
MUST contain the message digest algorithms employed by the
firmware package signer. (Note that this message digest algorithm
identifier MUST be the same as the one carried in the
digestAlgorithms value in SignedData.)
signedAttrs is an optional collection of attributes that are signed
along with the content. The signedAttrs are optional in the CMS,
but in this specification, signedAttrs are REQUIRED for the
firmware package; however, implementations MUST ignore
unrecognized signed attributes. The SET OF attributes MUST be DER
encoded [X.509-88]. Section 2.2 of this document lists the
attributes that MUST be included in the collection; other
attributes MAY be included as well.
signatureAlgorithm identifies the signature algorithm, and any
associated parameters, used by the firmware package signer to
generate the digital signature.
signature is the digital signature value.
unsignedAttrs is an optional SET of attributes that are not signed.
As described in Section 2.3, this set can only contain a single
instance of the wrapped-firmware-decryption-key attribute and no
others.
2.1.2.2. EncapsulatedContentInfo
The EncapsulatedContentInfo content type encapsulates the firmware
package, which might be compressed, encrypted, or compressed and then
encrypted prior to signature. The firmware package, in any of these
formats, is carried within the EncapsulatedContentInfo type. The
fields of EncapsulatedContentInfo are used as follows:
eContentType is an object identifier that uniquely specifies the
content type, and in this case, the value MUST be id-encryptedData
(1.2.840.113549.1.7.6), id-ct-compressedData
(1.2.840.113549.1.9.16.1.9), or id-ct-firmwarePackage
(1.2.840.113549.1.9.16.1.16). When eContentType contains id-
encryptedData, the firmware package was encrypted prior to
signing, and may also have been compressed prior to encryption.
When it contains id-ct-compressedData, the firmware package was
compressed prior to signing, but was not encrypted. When it
contains id-ct-firmwarePackage, the firmware package was not
compressed or encrypted prior to signing.
eContent contains the signed firmware package, which might also be
encrypted, compressed, or compressed and then encrypted, prior to
signing. The content is encoded as an octet string. The eContent
octet string need not be DER encoded.
2.1.3. EncryptedData
The EncryptedData content type [CMS] contains the encrypted firmware
package (which might be compressed prior to encryption). However, if
the firmware package was not encrypted, the EncryptedData content
type is not present. The fields of EncryptedData are used as
follows:
version is the syntax version number, and in this case, version MUST
be 0.
encryptedContentInfo is the encrypted content information. The use
of the EncryptedContentInfo type is discussed further in Section
2.1.3.1.
unprotectedAttrs is an optional collection of unencrypted attributes,
and in this case, unprotectedAttrs MUST NOT be present.
2.1.3.1. EncryptedContentInfo
The encrypted firmware package, which might be compressed prior to
encryption, is encapsulated in the EncryptedContentInfo type. The
fields of EncryptedContentInfo are used as follows:
contentType indicates the type of content, and in this case, it MUST
contain either id-ct-compressedData (1.2.840.113549.1.9.16.1.9) or
id-ct-firmwarePackage (1.2.840.113549.1.9.16.1.16). When it
contains id-ct-compressedData, then the firmware package was
compressed prior to encryption. When it contains id-ct-
firmwarePackage, then the firmware package was not compressed
prior to encryption.
contentEncryptionAlgorithm identifies the firmware-encryption
algorithm, and any associated parameters, used to encrypt the
firmware package.
encryptedContent is the result of encrypting the firmware package.
The field is optional; however, in this case, it MUST be present.
2.1.4. CompressedData
The CompressedData content type [COMPRESS] contains the compressed
firmware package. If the firmware package was not compressed, then
the CompressedData content type is not present. The fields of
CompressedData are used as follows:
version is the syntax version number; in this case, it MUST be 0.
compressionAlgorithm identifies the compression algorithm, and any
associated parameters, used to compress the firmware package.
encapContentInfo is the compressed content, consisting of a content
type identifier and the content itself. The use of the
EncapsulatedContentInfo type is discussed further in Section
2.1.4.1.
2.1.4.1. EncapsulatedContentInfo
The CompressedData content type encapsulates the compressed firmware package, and it is carried within the EncapsulatedContentInfo type. The fields of EncapsulatedContentInfo are used as follows: eContentType is an object identifier that uniquely specifies the content type, and in this case, it MUST be the value of id-ct- firmwarePackage (1.2.840.113549.1.9.16.1.16). eContent is the compressed firmware package, encoded as an octet string. The eContent octet string need not be DER encoded.2.1.5. FirmwarePkgData
The FirmwarePkgData content type contains the firmware package. It is a straightforward encapsulation in an octet string, and it need not be DER encoded. The FirmwarePkgData content type is identified by the id-ct- firmwarePackage object identifier: id-ct-firmwarePackage OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 16 } The FirmwarePkgData content type is a simple octet string: FirmwarePkgData ::= OCTET STRING2.2. Signed Attributes
The firmware package signer MUST digitally sign a collection of attributes along with the firmware package. Each attribute in the collection MUST be DER encoded [X.509-88]. The syntax for attributes is defined in [CMS], but it is repeated here for convenience: Attribute ::= SEQUENCE { attrType OBJECT IDENTIFIER, attrValues SET OF AttributeValue } AttributeValue ::= ANY Each of the attributes used with this profile has a single attribute value, even though the syntax is defined as a SET OF AttributeValue. There MUST be exactly one instance of AttributeValue present.
The SignedAttributes syntax within signerInfo is defined as a SET OF Attribute. The SignedAttributes MUST include only one instance of any particular attribute. The firmware package signer MUST include the following four attributes: content-type, message-digest, firmware-package- identifier, and target-hardware-module-identifiers. If the firmware package is encrypted, then the firmware package signer MUST also include the decrypt-key-identifier attribute. If the firmware package implements cryptographic algorithms, then the firmware package signer MAY also include the implemented-crypto- algorithms attribute. Similarly, if the firmware package implements compression algorithms, then the firmware package signer MAY also include the implemented-compress-algorithms attribute. If the firmware package is intended for use only by specific communities, then the firmware package signer MUST also include the community-identifiers attribute. If the firmware package depends on the presence of one or more other firmware packages to operate properly, then the firmware package signer SHOULD also include the firmware-package-info attribute. For example, the firmware-package-info attribute dependencies field might indicate that the firmware package contains a dependency on a particular bootstrap loader or separation kernel. The firmware package signer SHOULD also include the three following attributes: firmware-package-message-digest, signing-time, and content-hints. Additionally, if the firmware package signer has a certificate (meaning that the firmware package signer is not always configured as a trust anchor), then the firmware package signer SHOULD also include the signing-certificate attribute. The firmware package signer MAY include any other attribute that it deems appropriate.2.2.1. Content Type
The firmware package signer MUST include a content-type attribute with the value of id-encryptedData (1.2.840.113549.1.7.6), id-ct- compressedData (1.2.840.113549.1.9.16.1.9), or id-ct-firmwarePackage (1.2.840.113549.1.9.16.1.16). When it contains id-encryptedData, the firmware package was encrypted prior to signing. When it contains id-ct-compressedData, the firmware package was compressed prior to signing, but was not encrypted. When it contains
id-ct-firmwarePackage, the firmware package was not compressed or encrypted prior to signing. Section 11.1 of [CMS] defines the content-type attribute.2.2.2. Message Digest
The firmware package signer MUST include a message-digest attribute, having as its value the message digest computed on the encapContentInfo eContent octet string, as defined in Section 2.1.2.2. This octet string contains the firmware package, and it MAY be compressed, encrypted, or both compressed and encrypted. Section 11.2 of [CMS] defines the message-digest attribute.2.2.3. Firmware Package Identifier
The firmware-package-identifier attribute names the protected firmware package. Two approaches to naming firmware packages are supported: legacy and preferred. The firmware package signer MUST include a firmware-package-identifier attribute using one of these name forms. A legacy firmware package name is an octet string, and no structure within the octet string is assumed. A preferred firmware package name is a combination of an object identifier and a version number. The object identifier names a collection of functions implemented by the firmware package, and the version number is a non-negative integer that identifies a particular build or release of the firmware package. If a firmware package with a disastrous flaw is released, the firmware package that repairs the previously distributed flaw MAY designate a stale firmware package version to prevent the reloading of the flawed version. The hardware module bootstrap loader SHOULD prevent subsequent rollback to the stale version or versions earlier than the stale version. When the legacy firmware package name form is used, the stale version is indicated by a stale legacy firmware package name, which is an octet string. We assume that the firmware package signer and the bootstrap loader can determine whether a given legacy firmware package name represents a version that is more recent than the stale one. When the preferred firmware package name form is used, the stale version is indicated by a stale version number, which is an integer.
The following object identifier identifies the firmware-package-
identifier attribute:
id-aa-firmwarePackageID OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) aa(2) 35 }
The firmware-package-identifier attribute values have ASN.1 type
FirmwarePackageIdentifier:
FirmwarePackageIdentifier ::= SEQUENCE {
name PreferredOrLegacyPackageIdentifier,
stale PreferredOrLegacyStalePackageIdentifier OPTIONAL }
PreferredOrLegacyPackageIdentifier ::= CHOICE {
preferred PreferredPackageIdentifier,
legacy OCTET STRING }
PreferredPackageIdentifier ::= SEQUENCE {
fwPkgID OBJECT IDENTIFIER,
verNum INTEGER (0..MAX) }
PreferredOrLegacyStalePackageIdentifier ::= CHOICE {
preferredStaleVerNum INTEGER (0..MAX),
legacyStaleVersion OCTET STRING }
2.2.4. Target Hardware Module Identifiers
The target-hardware-module-identifiers attribute names the types of
hardware modules that the firmware package supports. A unique object
identifier names each supported hardware model type and revision.
The bootstrap loader MUST reject the firmware package if its own
hardware module type identifier is not listed in the target-
hardware-module-identifiers attribute.
The following object identifier identifies the target-hardware-
module-identifiers attribute:
id-aa-targetHardwareIDs OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) aa(2) 36 }
The target-hardware-module-identifiers attribute values have ASN.1
type TargetHardwareIdentifiers:
TargetHardwareIdentifiers ::= SEQUENCE OF OBJECT IDENTIFIER
2.2.5. Decrypt Key Identifier
The decrypt-key-identifier attribute names the symmetric key needed to decrypt the encapsulated firmware package. The CMS EncryptedData content type is used when the firmware package is encrypted. The decrypt-key-identifier signed attribute is carried in the SignedData content type that encapsulates EncryptedData content type, naming the symmetric key needed to decrypt the firmware package. No particular structure is imposed on the key identifier. The means by which the firmware-decryption key is securely distributed to all modules that are authorized to use the associated firmware package is beyond the scope of this specification; however, an optional mechanism for securely distributing the firmware-decryption key with the firmware package is specified in Section 2.3.1. The following object identifier identifies the decrypt-key-identifier attribute: id-aa-decryptKeyID OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) aa(2) 37 } The decrypt-key-identifier attribute values have ASN.1 type DecryptKeyIdentifier: DecryptKeyIdentifier ::= OCTET STRING2.2.6. Implemented Crypto Algorithms
The implemented-crypto-algorithms attribute MAY be present in the SignedAttributes, and it names the cryptographic algorithms that are implemented by the firmware package and available to applications. Only those algorithms that are made available at the interface of the cryptographic module are listed. Any cryptographic algorithm that is used internally and is not accessible via the cryptographic module interface MUST NOT be listed. For example, if the firmware package implements the decryption algorithm for future firmware package installations and this algorithm is not made available for other uses, then the firmware-decryption algorithm would not be listed. The object identifier portion of AlgorithmIdentifier identifies an algorithm and its mode of use. No algorithm parameters are included. Cryptographic algorithms include traffic-encryption algorithms, key- encryption algorithms, key transport algorithms, key agreement algorithms, one-way hash algorithms, and digital signature algorithms. Cryptographic algorithms do not include compression algorithms.
The following object identifier identifies the implemented-crypto-
algorithms attribute:
id-aa-implCryptoAlgs OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) aa(2) 38 }
The implemented-crypto-algorithms attribute values have ASN.1 type
ImplementedCryptoAlgorithms:
ImplementedCryptoAlgorithms ::= SEQUENCE OF OBJECT IDENTIFIER
2.2.7. Implemented Compression Algorithms
The implemented-compress-algorithms attribute MAY be present in the
SignedAttributes, and it names the compression algorithms that are
implemented by the firmware package and available to applications.
Only those algorithms that are made available at the interface of the
hardware module are listed. Any compression algorithm that is used
internally and is not accessible via the hardware module interface
MUST NOT be listed. For example, if the firmware package implements
a decompression algorithm for future firmware package installations
and this algorithm is not made available for other uses, then the
firmware-decompression algorithm would not be listed.
The object identifier portion of AlgorithmIdentifier identifies a
compression algorithm. No algorithm parameters are included.
The following object identifier identifies the implemented-compress-
algorithms attribute:
id-aa-implCompressAlgs OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) aa(2) 43 }
The implemented-compress-algorithms attribute values have ASN.1 type
ImplementedCompressAlgorithms:
ImplementedCompressAlgorithms ::= SEQUENCE OF OBJECT IDENTIFIER
2.2.8. Community Identifiers
If present in the SignedAttributes, the community-identifiers
attribute names the communities that are permitted to execute the
firmware package. The bootstrap loader MUST reject the firmware
package if the hardware module is not a member of one of the
identified communities. The means of assigning community membership
is beyond the scope of this specification.
The community-identifiers attributes names the authorized communities by a list of community object identifiers, by a list of specific hardware modules, or by a combination of the two lists. A specific hardware module is specified by the combination of the hardware module identifier (as defined in Section 2.2.4) and a serial number. To facilitate compact representation of serial numbers, a contiguous block can be specified by the lowest authorized serial number and the highest authorized serial number. Alternatively, all of the serial numbers associated with a hardware module family identifier can be specified with the NULL value. If the bootstrap loader does not have a mechanism for obtaining a list of object identifiers that identify the communities to which the hardware module is a member, then the bootstrap loader MUST behave as though the list is empty. Similarly, if the bootstrap loader does not have access to the hardware module serial number, then the bootstrap loader MUST behave as though the hardware module is not included on the list of authorized hardware modules. The following object identifier identifies the community-identifiers attribute: id-aa-communityIdentifiers OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) aa(2) 40 } The community-identifiers attribute values have ASN.1 type CommunityIdentifiers: CommunityIdentifiers ::= SEQUENCE OF CommunityIdentifier CommunityIdentifier ::= CHOICE { communityOID OBJECT IDENTIFIER, hwModuleList HardwareModules } HardwareModules ::= SEQUENCE { hwType OBJECT IDENTIFIER, hwSerialEntries SEQUENCE OF HardwareSerialEntry } HardwareSerialEntry ::= CHOICE { all NULL, single OCTET STRING, block SEQUENCE { low OCTET STRING, high OCTET STRING } }
2.2.9. Firmware Package Information
If a hardware module supports more than one type of firmware package, then the firmware package signer SHOULD include the firmware- package-info attribute with a populated fwPkgType field to identify the firmware package type. This value can aid the bootstrap loader in the correct placement of the firmware package within the hardware module. The firmware package type is an INTEGER, and the meaning of the integer value is specific to each hardware module. For example, a hardware module could assign different integer values for a bootstrap loader, a separation kernel, and an application. Some hardware module architectures permit one firmware package to use routines provided by another. If the firmware package contains a dependency on another, then the firmware package signer SHOULD also include the firmware-package-info attribute with a populated dependencies field. If the firmware package does not depend on any other firmware packages, then the firmware package signer MUST NOT include the firmware-package-info attribute with a populated dependencies field. Firmware package dependencies are identified by the firmware package identifier or by information contained in the firmware package itself, and in either case the bootstrap loader ensures that the dependencies are met. The bootstrap loader MUST reject a firmware package load if it identifies a dependency on a firmware package that is not already loaded. Also, the bootstrap loader MUST reject a firmware package load if the action will result in a configuration where the dependencies of an already loaded firmware package will no longer be satisfied. As described in Section 2.2.3, two approaches to naming firmware packages are supported: legacy and preferred. When the legacy firmware package name form is used, the dependency is indicated by a legacy firmware package name. We assume that the firmware package signer and the bootstrap loader can determine whether a given legacy firmware package name represents the named version of an acceptable newer version. When the preferred firmware package name form is used, an object identifier and an integer are provided. The object identifier MUST exactly match the object identifier portion of a preferred firmware package name associated with a firmware package that is already loaded, and the integer MUST be less than or equal to the integer portion of the preferred firmware package name associated with the same firmware package. That is, the dependency specifies the minimum value of the version that is acceptable.
The following object identifier identifies the firmware-package-info
attribute:
id-aa-firmwarePackageInfo OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) aa(2) 42 }
The firmware-package-info attribute values have ASN.1 type
FirmwarePackageInfo:
FirmwarePackageInfo ::= SEQUENCE {
fwPkgType INTEGER OPTIONAL,
dependencies SEQUENCE OF
PreferredOrLegacyPackageIdentifier OPTIONAL }
2.2.10. Firmware Package Message Digest
The firmware package signer SHOULD include a firmware-package-
message-digest attribute, which provides the message digest algorithm
and the message digest value computed on the firmware package. The
message digest is computed on the firmware package prior to any
compression, encryption, or signature processing. The bootstrap
loader MAY use this message digest to confirm that the intended
firmware package has been recovered after all of the layers of
encapsulation are removed.
The following object identifier identifies the firmware-package-
message-digest attribute:
id-aa-fwPkgMessageDigest OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) aa(2) 41 }
The firmware-package-message-digest attribute values have ASN.1 type
FirmwarePackageMessageDigest:
FirmwarePackageMessageDigest ::= SEQUENCE {
algorithm AlgorithmIdentifier,
msgDigest OCTET STRING }
2.2.11. Signing Time
The firmware package signer SHOULD include a signing-time attribute,
specifying the time at which the signature was applied to the
firmware package. Section 11.3 of [CMS] defines the signing-time
attribute.
2.2.12. Content Hints
The firmware package signer SHOULD include a content-hints attribute, including a brief text description of the firmware package. The text is encoded in UTF-8, which supports most of the world's writing systems [UTF-8]. Section 2.9 of [ESS] defines the content-hints attribute. When multiple layers of encapsulation are employed, the content-hints attribute is included in the outermost SignedData to provide information about the innermost content. In this case, the content- hints attribute provides a brief text description of the firmware package, which can help a person select the correct firmware package when more than one is available. When the preferred firmware package name forms are used, the content-hints attribute can provide a linkage to a legacy firmware package name. This is especially helpful when an existing configuration management system is in use, but the features associated with the preferred firmware package name are deemed useful. A firmware package name associated with such a configuration management system might look something like "R1234.C0(AJ11).D62.A02.11(b)." Including these firmware package names in the text description may be helpful to developers by providing a clear linkage between the two name forms. The content-hints attribute contains two fields, and in this case, both fields MUST be present. The fields of ContentHints are used as follows: contentDescription provides a brief text description of the firmware package. contentType provides the content type of the inner most content type, and in this case, it MUST be id-ct-firmwarePackage (1.2.840.113549.1.9.16.1.16).2.2.13. Signing Certificate
When the firmware-signer's public key is contained in a certificate, the firmware package signer SHOULD include a signing-certificate attribute to identify the certificate that was employed. However, if the firmware package signature does not have a certificate (meaning that the signature will only be validated with the trust anchor public key), then the firmware package signer is unable to include a signing-certificate attribute. Section 5.4 of [ESS] defines this attribute.
The signing-certificate attribute contains two fields: certs and
policies. The certs field MUST be present, and the policies field
MAY be present. The fields of SigningCertificate are used as
follows:
certs contains a sequence of certificate identifiers. In this case,
sequence of certificate identifiers contains a single entry. The
certs field MUST contain only the certificate identifier of the
certificate that contains the public key used to verify the
firmware package signature. The certs field uses the ESSCertID
syntax specified in Section 5.4 of [ESS], and it is comprised of
the SHA-1 hash [SHA1] of the entire ASN.1 DER encoded certificate
and, optionally, the certificate issuer and the certificate serial
number. The SHA-1 hash value MUST be present. The certificate
issuer and the certificate serial number SHOULD be present.
policies is optional; when it is present, it contains a sequence of
policy information. The policies field, when present, MUST
contain only one entry, and that entry MUST match one of the
certificate policies in the certificate policies extension of the
certificate that contains the public key used to verify the
firmware package signature. The policies field uses the
PolicyInformation syntax specified in Section 4.2.1.5 of
[PROFILE], and it is comprised of the certificate policy object
identifier and, optionally, certificate policy qualifiers. The
certificate policy object identifier MUST be present. The
certificate policy qualifiers SHOULD NOT be present.
2.3. Unsigned Attributes
CMS allows a SET of unsigned attributes to be included; however, in
this specification, the set MUST be absent or include a single
instance of the wrapped-firmware-decryption-key attribute. Because
the digital signature does not cover this attribute, it can be
altered at any point in the delivery path from the firmware package
signer to the hardware module. This property can be employed to
distribute the firmware-decryption key along with an encrypted and
signed firmware package, allowing the firmware-decryption key to be
wrapped with a different key-encryption key for each link in the
distribution chain.
The syntax for attributes is defined in [CMS], and it is repeated at
the beginning of Section 2.2 of this document for convenience. Each
of the attributes used with this profile has a single attribute
value, even though the syntax is defined as a SET OF AttributeValue.
There MUST be exactly one instance of AttributeValue present.
The UnsignedAttributes syntax within signerInfo is defined as a SET OF Attribute. The UnsignedAttributes MUST include only one instance of any particular attribute.2.3.1. Wrapped Firmware Decryption Key
The firmware package signer, or any other party in the distribution chain, MAY include a wrapped-firmware-decryption-key attribute. The following object identifier identifies the wrapped-firmware- decryption-key attribute: id-aa-wrappedFirmwareKey OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) aa(2) 39 } The wrapped-firmware-decryption-key attribute values have ASN.1 type of EnvelopedData. Section 6 of [CMS] defines the EnvelopedData content type, which is used to construct the value of the attribute. EnvelopedData permits the firmware-decryption key to be protected using symmetric or asymmetric techniques. The EnvelopedData does not include an encrypted content; rather, the EnvelopedData feature of having the encrypted content in another location is employed. The encrypted content is found in the eContent field of the EncryptedData structure. The firmware-decryption key is contained in the recipientInfos field. Section 6 of [CMS] refers to this key as the content-encryption key. The EnvelopedData syntax supports many different key management algorithms. Four general techniques are supported: key transport, key agreement, symmetric key-encryption keys, and passwords. The EnvelopedData content type is profiled for the wrapped-firmware- decryption-key attribute. The EnvelopedData fields are described fully in Section 6 of [CMS]. Additional rules apply when EnvelopedData is used as a wrapped-firmware-decryption-key attribute. Within the EnvelopedData structure, the following apply: - The set of certificates included in OriginatorInfo MUST NOT include certificates with a type of extendedCertificate, v1AttrCert, or v2AttrCert [X.509-97, X.509-00, ACPROFILE]. The optional crls field MAY be present. - The optional unprotectedAttrs field MUST NOT be present.
Within the EncryptedContentInfo structure, the following apply:
- contentType MUST match the content type object identifier carried
in the contentType field within the EncryptedContentInfo structure
of EncryptedData as described in Section 2.1.3.1.
- contentEncryptionAlgorithm identifies the firmware-encryption
algorithm, and any associated parameters, used to encrypt the
firmware package carried in the encryptedContent field of the
EncryptedContentInfo structure of EncryptedData. Therefore, it
MUST exactly match the value of the EncryptedContentInfo structure
of EncryptedData as described in Section 2.1.3.1.
- encryptedContent is optional, and in this case, it MUST NOT be
present.
3. Firmware Package Load Receipt
The Cryptographic Message Syntax (CMS) is used to indicate that a
firmware package loaded successfully. Support for firmware package
load receipts is OPTIONAL. However, those hardware modules that
choose to generate such receipts MUST follow the conventions
specified in this section. Because not all hardware modules will
have private signature keys, the firmware package load receipt can be
either signed or unsigned. Use of the signed firmware package load
receipt is RECOMMENDED.
Hardware modules that support receipt generation MUST have a unique
serial number. Hardware modules that support signed receipt
generation MUST have a private signature key to sign the receipt and
the corresponding signature validation certificate or its designator.
The designator is the certificate issuer name and the certificate
serial number, or it is the public key identifier. Memory-
constrained hardware modules will generally store the public key
identifier since it requires less storage.
The unsigned firmware package load receipt is encapsulated by
ContentInfo. Alternatively, the signed firmware package load receipt
is encapsulated by SignedData, which is in turn encapsulated by
ContentInfo.
The firmware package load receipt is summarized as follows (see [CMS] for the full syntax): ContentInfo { contentType id-signedData, -- (1.2.840.113549.1.7.2) -- OR -- id-ct-firmwareLoadReceipt, -- (1.2.840.113549.1.9.16.1.17) content SignedData -- OR -- FirmwarePackageLoadReceipt } SignedData { version CMSVersion, -- always set to 3 digestAlgorithms DigestAlgorithmIdentifiers, -- Only one encapContentInfo EncapsulatedContentInfo, certificates CertificateSet, -- Optional Module certificate crls CertificateRevocationLists, -- Optional signerInfos SET OF SignerInfo -- Only one } SignerInfo { version CMSVersion, -- either set to 1 or 3 sid SignerIdentifier, digestAlgorithm DigestAlgorithmIdentifier, signedAttrs SignedAttributes, -- Required signatureAlgorithm SignatureAlgorithmIdentifier, signature SignatureValue, unsignedAttrs UnsignedAttributes -- Omit } EncapsulatedContentInfo { eContentType id-ct-firmwareLoadReceipt, -- (1.2.840.113549.1.9.16.1.17) eContent OCTET STRING -- Contains receipt } FirmwarePackageLoadReceipt { version INTEGER, -- The DEFAULT is always used hwType OBJECT IDENTIFIER, -- Hardware module type hwSerialNum OCTET STRING, -- H/W module serial number fwPkgName PreferredOrLegacyPackageIdentifier, trustAnchorKeyID OCTET STRING, -- Optional decryptKeyID OCTET STRING -- Optional }
3.1. Firmware Package Load Receipt CMS Content Type Profile
This section specifies the conventions for using the CMS ContentInfo and SignedData content types for firmware package load receipts. It also defines the firmware package load receipt content type.3.1.1. ContentInfo
The CMS requires that the outermost encapsulation be ContentInfo [CMS]. The fields of ContentInfo are used as follows: contentType indicates the type of the associated content. If the firmware package load receipt is signed, then the encapsulated type MUST be SignedData, and the id-signedData (1.2.840.113549.1.7.2) object identifier MUST be present in this field. If the receipt is not signed, then the encapsulated type MUST be FirmwarePackageLoadReceipt, and the id-ct- firmwareLoadReceipt (1.2.840.113549.1.9.16.1.17) object identifier MUST be present in this field. content holds the associated content. If the firmware package load receipt is signed, then this field MUST contain the SignedData. If the receipt is not signed, then this field MUST contain the FirmwarePackageLoadReceipt.3.1.2. SignedData
The SignedData content type contains the firmware package load receipt and one digital signature. If the hardware module locally stores its certificate, then the certificate can be included as well. The fields of SignedData are used as follows: version is the syntax version number, and in this case, it MUST be set to 3. digestAlgorithms is a collection of message digest algorithm identifiers, and in this case, it MUST contain a single message digest algorithm identifier. The message digest algorithms employed by the hardware module MUST be present. encapContentInfo is the signed content, consisting of a content type identifier and the content itself. The use of the EncapsulatedContentInfo type is discussed further in Section 3.1.2.2. certificates is an optional collection of certificates. If the hardware module locally stores its certificate, then the X.509 certificate of the hardware module SHOULD be included. If the
hardware module does not, then the certificates field is omitted.
PKCS#6 extended certificates [PKCS#6] and attribute certificates
(either version 1 or version 2) [X.509-97, X.509-00, ACPROFILE]
MUST NOT be included in the set of certificates.
crls is an optional collection of certificate revocation lists
(CRLs). CRLs MAY be included, but they will normally be omitted
since hardware modules will not generally have access to the most
recent CRL. Signed receipt recipients SHOULD be able to handle
the presence of the optional crls field.
signerInfos is a collection of per-signer information, and in this
case, the collection MUST contain exactly one SignerInfo. The use
of the SignerInfo type is discussed further in Section 3.1.2.1.
3.1.2.1. SignerInfo
The hardware module is represented in the SignerInfo type. The
fields of SignerInfo are used as follows:
version is the syntax version number, and it MUST be either 1 or 3,
depending on the method used to identify the hardware module's
public key. The use of the subjectKeyIdentifier is RECOMMENDED,
which results in the use of version 3.
sid specifies the hardware module's certificate (and thereby the
hardware module's public key). CMS supports two alternatives:
issuerAndSerialNumber and subjectKeyIdentifier. The hardware
module MUST support one or both of the alternatives for receipt
generation; however, the support of subjectKeyIdentifier is
RECOMMENDED. The issuerAndSerialNumber alternative identifies the
hardware module's certificate by the issuer's distinguished name
and the certificate serial number. The identified certificate, in
turn, contains the hardware module's public key. The
subjectKeyIdentifier alternative identifies the hardware module's
public key directly. When this public key is contained in a
certificate, this identifier SHOULD appear in the X.509
subjectKeyIdentifier extension.
digestAlgorithm identifies the message digest algorithm, and any
associated parameters, used by the hardware module. It MUST
contain the message digest algorithms employed to sign the
receipt. (Note that this message digest algorithm identifier MUST
be the same as the one carried in the digestAlgorithms value in
SignedData.)
signedAttrs is an optional collection of attributes that are signed
along with the content. The signedAttrs are optional in the CMS,
but in this specification, signedAttrs are REQUIRED for use with
the firmware package load receipt content. The SET OF attributes
MUST be DER encoded [X.509-88]. Section 3.2 of this document
lists the attributes that MUST be included in the collection.
Other attributes MAY be included, but the recipient will ignore
any unrecognized signed attributes.
signatureAlgorithm identifies the signature algorithm, and any
associated parameters, used to sign the receipt.
signature is the digital signature.
unsignedAttrs is an optional collection of attributes that are not
signed, and in this case, there MUST NOT be any unsigned
attributes present.
3.1.2.2. EncapsulatedContentInfo
The FirmwarePackageLoadReceipt is encapsulated in an OCTET STRING,
and it is carried within the EncapsulatedContentInfo type. The
fields of EncapsulatedContentInfo are used as follows:
eContentType is an object identifier that uniquely specifies the
content type, and in this case, it MUST be the value of id-ct-
firmwareLoadReceipt (1.2.840.113549.1.9.16.1.17).
eContent is the firmware package load receipt, encapsulated in an
OCTET STRING. The eContent octet string need not be DER encoded.
3.1.3. FirmwarePackageLoadReceipt
The following object identifier identifies the firmware package load
receipt content type:
id-ct-firmwareLoadReceipt OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) ct(1) 17 }
The firmware package load receipt content type has the ASN.1 type
FirmwarePackageLoadReceipt:
FirmwarePackageLoadReceipt ::= SEQUENCE {
version FWReceiptVersion DEFAULT v1,
hwType OBJECT IDENTIFIER,
hwSerialNum OCTET STRING,
fwPkgName PreferredOrLegacyPackageIdentifier,
trustAnchorKeyID OCTET STRING OPTIONAL,
decryptKeyID [1] OCTET STRING OPTIONAL }
FWReceiptVersion ::= INTEGER { v1(1) }
The fields of the FirmwarePackageLoadReceipt type have the following
meanings:
version is an integer that provides the syntax version number for
compatibility with future revisions of this specification.
Implementations that conform to this specification MUST set the
version to the default value, which is v1.
hwType is an object identifier that identifies the type of hardware
module on which the firmware package was loaded.
hwSerialNum is the serial number of the hardware module on which the
firmware package was loaded. No particular structure is imposed
on the serial number; it need not be an integer. However, the
combination of the hwType and hwSerialNum uniquely identifies the
hardware module.
fwPkgName identifies the firmware package that was loaded. As
described in Section 2.2.3, two approaches to naming firmware
packages are supported: legacy and preferred. A legacy firmware
package name is an octet string. A preferred firmware package
name is a combination of the firmware package object identifier
and an integer version number.
trustAnchorKeyID is optional, and when it is present, it identifies
the trust anchor that was used to validate the firmware package
signature.
decryptKeyID is optional, and when it is present, it identifies the
firmware-decryption key that was used to decrypt the firmware
package.
The firmware package load receipt MUST include the version, hwType,
hwSerialNum, and fwPkgName fields, and it SHOULD include the
trustAnchorKeyID field. The firmware package load receipt MUST NOT
include the decryptKeyID, unless the firmware package associated with the receipt is encrypted, the firmware-decryption key is available to the hardware module, and the firmware package was successfully decrypted.3.2. Signed Attributes
The hardware module MUST digitally sign a collection of attributes along with the firmware package load receipt. Each attribute in the collection MUST be DER encoded [X.509-88]. The syntax for attributes is defined in [CMS], and it was repeated in Section 2.2 for convenience. Each of the attributes used with this profile has a single attribute value, even though the syntax is defined as a SET OF AttributeValue. There MUST be exactly one instance of AttributeValue present. The SignedAttributes syntax within signerInfo is defined as a SET OF Attributes. The SignedAttributes MUST include only one instance of any particular attribute. The hardware module MUST include the content-type and message-digest attributes. If the hardware module includes a real-time clock, then the hardware module SHOULD also include the signing-time attribute. The hardware module MAY include any other attribute that it deems appropriate.3.2.1. Content Type
The hardware module MUST include a content-type attribute with the value of id-ct-firmwareLoadReceipt (1.2.840.113549.1.9.16.1.17). Section 11.1 of [CMS] defines the content-type attribute.3.2.2. Message Digest
The hardware module MUST include a message-digest attribute, having as its value the message digest of the FirmwarePackageLoadReceipt content. Section 11.2 of [CMS] defines the message-digest attribute.3.2.3. Signing Time
If the hardware module includes a real-time clock, then the hardware module SHOULD include a signing-time attribute, specifying the time at which the receipt was generated. Section 11.3 of [CMS] defines the signing-time attribute.