ASN.1 Specification for TPM 2.0 Key Files

Internet-Draft	TPM 2 Key Format	July 2024
Bottomley	Expires 24 January 2025	[Page]

Abstract

This specification is designed to be an extension to the ASN.1 (defined in [X.680]) specification of PKCS #1 [RFC8017] to define the file format of private keys that need to be loaded into a TPM 2 device to operate.¶

3. Key Representation

All TPM 2.0 keys consist of two binary pieces, a public part, which can be parsed according to the TPM specification for TPM2B_PUBLIC [TPM2.0] and a private part, which is cryptographically sealed in such a way as to be only readable on the TPM that created it. The purpose of this specification is to specify a format by which the public and private pieces of a TPM key can be loaded.¶

The design of the TPMkey ASN.1 format is that it should have a distinguishing OID at the beginning so the DER form of the key can be easily recognized. In PEM form, the key MUST have "-----BEGIN TSS2 PRIVATE KEY-----" and "-----END TSS2 PRIVATE KEY-----" as the PEM guards. All additional information that may be needed to load the key is specified as optional explicit elements, which can be extended by later specifications, which is why the TPMkey is not versioned.¶

3.1. TPMkey Syntax

 TPMKey ::= SEQUENCE {
    type        OBJECT IDENTIFIER,
    emptyAuth   [0] EXPLICIT BOOLEAN OPTIONAL,
    policy      [1] EXPLICIT SEQUENCE OF TPMPolicy OPTIONAL,
    secret      [2] EXPLICIT OCTET STRING OPTIONAL,
    authPolicy  [3] EXPLICIT SEQUENCE OF TPMAuthPolicy OPTIONAL,
    description [4] EXPLICIT UTF8String OPTIONAL,
    rsaParent   [5] EXPLICIT BOOLEAN OPTIONAL,
    parent      INTEGER,
    pubkey      OCTET STRING,
    privkey     OCTET STRING
  }

The fields of type TPMKey have the following meanings:¶

3.1.1. type

A unique OID specifying the key type. This standard currently defines three types of keys: a loadable key, specified by id-loadablekey, (to be loaded with TPM2_Load), an importable key, specified by id-importablekey, (to be loaded with TPM2_Import) and a sealed data key, specified by id-sealedkey, (to be extracted with TPM2_Unseal). The TCG has reserved the following OID prefix for this:¶

  id-tpmkey OBJECT IDENTIFIER ::=
    {joint-iso-itu-t(2) international-organizations(23) 133 10 1}

And the three key types are:¶

  id-loadablekey OBJECT IDENTIFIER ::=
    {id-tpmkey 3}

  id-importablekey OBJECT IDENTIFIER ::=
    {id-tpmkey 4}

  id-sealedkey OBJECT IDENTIFIER ::=
    {id-tpmkey 5}

3.1.2. emptyAuth

An implementation needs to know as it formulates the TPM2_Load/Import/Unseal command whether it must also send down an authorization, so this parameter gives that indication. emptyAuth MUST be true if authorization is NOT required and MUST be either false or absent if authorization is required. Since this element has three states (one representing true and two representing false) it is RECOMMENDED that implementations emitting TPMkey representations use absence of the tag to represent false. However, implementations reading TPMKey MUST be able to process all three possible states.¶

3.1.3. policy

This MUST be present if the TPM key has a policy hash because it describes to the implementation how to construct the policy. The forms of the policy statement are described in Section 4.¶

3.1.4. secret

This section describes the additional cryptographic secret used to specify the outer wrapping of an importable object. For keys, it MUST be present for key type id-importablekey and MUST NOT be present for key type id-loadablekey. For sealed data objects of type id-sealedkey, it MAY be present and if present indicates the object is importable.¶

Importable objects (designed to be processed by TPM2_Import) MUST have an unencrypted inner wrapper (symmetricAlg MUST be TPM_ALG_NULL and encryptionKey MUST be empty) and an outer wrapper encrypted to the parent key using inSymSeed. The secret parameter is the fully marshalled TPM2B_ENCRYPTED_SECRET form of inSymSeed.¶

3.1.5. authPolicy

This SHOULD be present if the TPMkey policy contains a TPM2_PolicyAuthorize statement because it contains signed policies that could be used to satisfy the TPM key policy. The forms of the authPolicy statement are described in Section 5. If the TPM key has no policy hash then this MUST NOT be present.¶

3.1.6. description

An optional string description for the key which is only used for display purposes, MAY be used as a user visible mnemonic for the key.¶

3.1.7. rsaParent

This MUST be present and true if the parent is a permanent handle (MSO 0x40) and RSA 2048 is used for the primary key. If the parent is not a permanent handle then this MUST NOT be present. If the parent is a permanent handle and if P-256 is used for the primary then this MUST NOT be present. Given that P-256 primary keys are easier to generate, implementations SHOULD NOT set this flag.¶

3.1.8. parent

This MUST be present for all keys and specifies the handle of the parent key. The parent key SHOULD be either a persistent handle (MSO 0x81) or a permanent handle (MSO 0x40). Since volatile handle numbering can change unexpectedly depending on key load order, the parent SHOULD NOT be a volatile handle (MSO 0x80). The parent MUST NOT have any other MSO.¶

If a permanent handle (MSO 0x40) is specified then the implementation MUST run TPM2_CreatePrimary on the handle using the TCG specified Elliptic Curve template [TCG-Provision] (section 7.5.1 for the Storage and other seeds or 7.4.1 for the endorsement seed) which refers to the TCG EK Credential Profile [TCG-EK-Profile] . Since there are several possible templates, implementations MUST always use the H template (the one with zero size unique fields). The template used MUST be H-2 (EK Credential Profile section B.4.5) for the NIST P-256 curve if rsaParent is absent or the H-1 (EK Credential Profile section B.4.4) RSA template with a key length of 2048 if rsaParent is present and true and use the primary key so generated as the parent.¶

3.1.9. pubkey

This MUST be present and MUST correspond to the fully marshalled TPM2B_PUBLIC structure of the TPM Key.¶

3.1.10. privkey

This MUST be present and MUST correspond to the fully marshalled TPM2B_PRIVATE structure of the TPM Key. For importable keys, this must be the duplicate parameter that would be input to TPM2_Import.¶

4. Key Policy Specification

Policy is constructed on a TPM by executing a sequence of policy statements. This specification currently only defines a limited subset of the allowed policy statements. The policy is specified by a hash, which the execution of the policy statements must reach in order for the policy to be validated (See [TPM2.0] Part 1 for a detailed description.¶

The TPMPolicy ASN.1 MUST be a sequence of policy statements which correspond exactly to TPM policy instructions in the order they should be executed and additionally from which the ultimate policy hash can be constructed.¶

The current policy specification is strictly for AND based policy only and may be extended at a later date with OR policy. However, the ASN.1 for policy is formulated as CONS elements, leaving the possibility of adding additional but optional elements for policy statements which are not supported by this standard (such as TPM2_PolicyAuthorize).¶

4.1. TPMPolicy Syntax

 TPMPolicy ::= SEQUENCE {
    commandCode   [0] EXPLICIT INTEGER,
    commandPolicy [1] EXPLICIT OCTET STRING
  }

The Fields of type TPMPolicy have the following meanings:¶

4.1.1. CommandCode

This is the integer representation of the TPM command code for the policy statement.¶

4.1.2. CommandPolicy

This is a binary string representing a fully marshalled, TPM ordered, command body for the TPM policy command. Therefore to send the command, the implementation simply marshals the command code and appends this octet string as the body.¶

Commands which have no body, such as TPM2_AuthVal, MUST be specified as a zero length OCTET STRING¶

Note that there are some commands for which the simple body of the TPM policy command does not provide enough information to execute the policy command. A classic example is TPM2_PolicyAuthorize, whose body consists of a key name, a policyRef nonce and a signature. However, the implementation needs to know the actual key, not just the name, to implement the policy. Commands whose CommandPolicy must be marshalled differently are noted in Section 4.2.¶

4.2. PolicyCommand Exceptions

For some TPM2_Policy... commands, the information required to formulate the policy cannot be extracted from the body of the command and so additional information must be supplied, meaning these commands deviate from the [TPM2.0] policy body specifications. The CommandPolicy format for these exceptional commands is documented below.¶

4.2.1. TPM2_PolicyAuthorize

According to [TPM2.0] the body of this command consists of TPM2B_DIGEST approvedPolicy, TPM2B_NONCE policyRef, TPM2B_NAME keySign and TPM_TK_VERIFIED checkTicket. However, implementations must construct checkTicket at the time of policy execution from the public key of keySign and the signed policy hash. Additionally, implementations constructing a signed policy need to know the sequence of policy steps they must execute to produce approvedPolicy, but do not need to know its value because the construction steps will either fail or the end policy session will have the required hash. The implementation must know this end policyHash to construct the signature over it, but this information does not need to be stored in CommandPolicy.¶

The format of CommandPolicy for TPM2_PolicyAuthorize MUST be TPM2B_PUBLIC keySign, TPM2B_DIGEST policyRef, TPMT_SIGNATURE policySignature.¶

4.2.2. TPM2_PolicySecret

According to [TPM2.0] the body of this command consists of the handle of the authorizing object, TPM2B_NONCE nonceTPM, TPM2B_DIGEST cpHashA, TPM2B_NONCE policyRef and INT32 expiration. However, the policyHash only uses policyRef and the name of the authorizing object. Since the usual authorizing object for TPM2_PolicySecret is a permanent handle or NV Index, and those are hard to find by name, the handle is also given as an optional hint in addition to the name.¶

The format of CommandPolicy for TPM2_PolicySecret MUST be TPM_HANDLE objectHandleHint, TPM2B_NAME objectName, TPM2B_DIGEST policyRef. Where objectHandleHint MAY be zero to indicate no hint.¶

4.3. Policy Implementation Considerations

The policy hash for AND based policies is constructed by extension of the prior policy hash¶

  newHash = HASH ( oldHash || policyHash )

where policyHash is usually simply the hash of the fully marshalled policy command (including the CommandCode). However, this isn't true for TPM2_PolicyCounterTimer so always consult the [TPM2.0] specifications for how to construct the policyHash.¶

The implementation should fail fast for policy problems, so if an individual policy command returns a failure (which usually indicates a particular policy requirement cannot be met), that failure should be reported in as much detail as possible and processing of the key should fail at that point.¶

If signed policies are present, the first policy statement MUST be TPM2_PolicyAuthorize giving the public key, the nonce and and having an empty signature. TPM_PolicyAuthorize() MAY NOT appear in any subsequent position.¶

4.3.1. Authorization Policy

When Authorization (Passing in a password) is required, the emptyAuth parameter MUST be absent or set to false and additionally TPM_CC_PolicyAuthValue or TPM_CC_PolicySecret MUST be specified as the command code for one entry in the TPMPolicy sequence. However, when TPM2_PolicyAuthValue is specified, the implementation MAY choose to execute either TPM2_PolicyPassword for TPM_RS_PW or TPM2_PolicyAuthValue for HMAC based authorization depending on whether the command being authorized is using sessions or not. If the policy does not require an authorization then the emptyAuth parameter MUST be set to true.¶

Implementations should bear in mind that most crypto systems with key management expect to consume only one password per key, so implementations SHOULD avoid policy chains which would require more than one password as would happen when TPM2_PolicySecret and TPM2_PolicyAuthValue both appear in the same policy chain. Implementations MAY detect and emit an error in this case.¶

5. Signed Policy Specification

One feature of TPM policy execution is the command TPM2_PolicyAuthorize does not operate by extending the current policy hash, it completely replaces it with a hash of the signing key and a nonce called the policyRef. In order for this replacement to happen, TPM2_PolicyAuthorize must be presented with a ticket confirming verification of a signature over the old policy hash and the nonce. Since anyone possessing the private key named in the policy hash can sign a new policy to present, the key can have multiple signed policies, any or all of which might succeed, so a new structure had to be introduced to store current signed policies.¶

5.1. TPMAuthPolicy Syntax

 TPMAuthPolicy ::= SEQUENCE {
    name    [0] EXPLICIT UTF8String OPTIONAL,
    policy  [1] EXPLICIT SEQUENCE OF TPMPolicy
 }

The Fields of type TPMAuthPolicy have the following meanings:¶

5.1.1. Name

An optional string name for the current policy which is only used for display purposes, MAY be used as a user visible mnemonic for the actual policy.¶

5.1.2. Policy

A sequence of TPMPolicy statements (see Section 4.1) which MUST end with a PolicyAuthorize statement whose signature is over the hash of the current policy excluding the PolicyAuthorize statement and the nonce specified at the beginning of the TPMKey policy. There MUST be no other TPM2_PolicyAuthorize statements in the intermediate policy steps. There MAY be an initial TPM2_PolicyAuthorize statement containing a different public key (because this causes complexity building signed policy chains, implementations MAY choose to allow only a single policy signing key, in which case there MAY NOT be an initial TPM2_PolicyAuthorize statement).¶

5.2. Signed Policy Implementation Considerations

The key can only be used if one of the policies in TPMKey authPolicy actually succeeds. The implementation SHOULD try each policy in authPolicy sequentially. If the policy fails to match the implementation SHOULD try the next policy. If all statement succeed, the implementation should then execute the final TPM2_PolicyAuthorize statement and then begin executing the policy statements in TPMKeyPolicy excluding the initial TPM2_PolicyAuthorize one (if there are any subsequent statements).¶

Because policies are tried sequentially, implementations SHOULD order the policies such that the most likely to succeed appears first. For instance, if the key is used to unlock a disk and is tied by policy to the hash of the OS kernel, then the policy tied to the current kernel should appear first followed by subsequent policies tied to older kernels because the most likely kernel to be booted is the current one.¶

Since the policy order will likely change when a new policy is added (in the kernel example, when a kernel is updated, the newly added kernel becomes current and the signed policy tied to this kernel should become the first one in the TPMkey), users should keep track of policies using the optional mnemonic name and not rely on position in the TPMAuthPolicy sequence.¶

5.2.1. Authorization Policy

There may now be multiple possible policies which authorize the key, some of which might contain TPM2_PolicyAuthValue requirements (passing in a password) and others of which might not, emptyAuth MUST not be set if the TPMKey contains an Authorization Secret (password), even if testing knowledge of such a secret may not be required by signed policy. Implementations MAY choose to scan the likely policies to see whether to prompt for a password or MAY choose to prompt based on emptyAuth. The latter is more likely to be successful, because most implementations will have a password prompt at key load time which may be some time before the key is actually used, by which time the list of successful policies may be different due to changes on the system.¶

The most common use for TPM2_PolicyAuthValue is as a backup policy when all other policies tied to system configuration fail. For this use case, a password prompted for is almost never used, except in the rare event the system gets into a state where none of the other policies apply. If an implementation always prompts for a password based on emptyAuth, then most of the time the prompted for password will not be used. For this use case, consumers may safely pass in an empty password when the implementation asks and only provide the real password if all other policies fail. In the kernel boot example this would mean that the entity that unlocks the disk should first try the key with an empty password, assuming one non-password based policy will succeed and only go back and ask the user for a password if the disk unlock fails.¶

5.2.2. Signed Policy Revocation

The signature in a TPM2_PolicyAuthorize statement has no expiry or revocation capabilities. Consumers should remember that simply removing a signed policy from the key does not guarantee that the removed policy will never be used. Any subsequent consumer who obtains the policy signature could potentially add the removed policy back to the key. Therefore, use cases which may need to expire or revoke the policy SHOULD build into the policy specific expiration or revocation criteria using TPM policy statements.¶

The easiest policy statement for expiration is to use TPM2_PolicyCounterTimer. Users should remember that the TPM time is guaranteed to be monotonic, but the TPM timer does not tick while the machine is powered down and any user may arbitrarily increase (but never decrease) the TPM clock count. It is recommended that policies tied to booting a system (as in the kernel boot disk unlock above) should base their expiration on reboot count rather than the TPM clock.¶

5.2.3. Policy Signing Keys and policyRef values

There is a high likelihood that a consumer would use the same policy signing key for multiple TPM objects with signed policy. This gives rise to the risk that a malicious user could extract signed policy meant for key1 and use it instead with key2 (policy swapping). To mitigate this risk, the implementation SHOULD place a high entropy nonce in policyRef to ensure that each key has a different policyRef which prevents this policy swapping.¶

The TCG specifies that policyRef could have an implementation defined meaning in which case implementations MAY use implementation meaningful values instead of nonces for policyRef but SHOULD take other measures to prevent policy swapping.¶

7. Security Considerations

The TPM 2.0 supports a variety of algorithms, the most common being SHA1 and SHA256 for hashing and RSA2048 and NIST P-256 for asymmetric keys. Implementors SHOULD NOT use deprecated algorithms, such as SHA1, for any TPM operation. In particular, the algorithm used for the policy hash SHOULD NOT be SHA1 and this means that SHA1 SHOULD NOT be used as the name algorithm hash for any TPM key.¶

TPM 2.0 supports a session mode (TPM_RS_PW) where authorizations are passed to the TPM in clear text over the TPM connection. Implementations SHOULD consider the possibility of snooping on the wire between the implementation and the TPM, such as [TPM_GENIE], and SHOULD use HMAC session authorizations as best practice for all TPM keys.¶

In addition to snooping authorizations, snooping may also occur when key material is being exchanged between the TPM and the implementation, such as wrapping of private keys and the sealing and unsealing operations for sealed keys. Implementations SHOULD always use HMAC sessions with TPMA_SESSION_DECRYPT when sensitive information is passed in to the TPM and HMAC sessions with TPMA_SESSION_ENCRYPT when sensitive information is received from the TPM.¶

The easiest way to get the TPM to wrap an external private key is to use TPM2_Import. However, since TPMA_SESSION_DECRYPT only protects the first parameter (which is encryptionKey), the duplicate should use inner symmetric encryption with a randomly generated ephemeral key, which is then presented to the TPM via the protected encryptionKey parameter.¶

The TPM has a mode where it can generate private key material internally (using TPM2_Create) such that the private part of the key can never leave the TPM. Implementations SHOULD support this mode but should be aware that while keys created like this may be more secure than wrapped keys, they can also be used only while access to the TPM that created them is available, so implementations SHOULD also support wrapping for keys that are expected to outlive the TPM that's using them. Clients can then develop best practices around TPM wrapped identity keys, possibly with TPM created sub keys, which can only be used on the device they were wrapped for.¶

Since TPM keys can only be used by the specific TPM that created them, which is usually embedded in a piece of hardware, they are secure against exfiltration attacks. However, consideration should be given to an attacker gaining access to the system containing the TPM. TPM keys are most secure when used as part of an operating system that has guaranteed trust properties, such as secure and measured boot. Implementations SHOULD assist users in constructing key policies that ensure the key can be used only when the operating system is within its trusted parameters to minimize threats from co-located attackers.¶

10. References

10.1. Normative References

[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC8017]: Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch, "PKCS #1: RSA Cryptography Specifications Version 2.2", RFC 8017, DOI 10.17487/RFC8017, November 2016, <https://www.rfc-editor.org/info/rfc8017>.
[TPM2.0]: Trusted Computing Group, "TPM 2.0 Library Specification", 15 March 2013, <https://trustedcomputinggroup.org/resource/tpm-library-specification/>.
[X.680]: International Telecommunication Union, "ITU-T Recommendation X.680, Information technology - Abstract Syntax Notation One (ASN.1): Specification of basic notation.", August 2015, <https://itu.int/rec/T-REC-X.680-201508-I/en>.
[X.690]: International Telecommunication Union, "ITU-T Recommendataion X.690, Information technology – ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", February 2021, <https://www.itu.int/rec/T-REC-X.690-202102-I>.
[TCG-Provision]: Trusted Computing Group, "TCG TPM v2.0 Provisioning Guidance", 15 March 2017, <https://trustedcomputinggroup.org/resource/tcg-tpm-v2-0-provisioning-guidance/>.
[TCG-EK-Profile]: Trusted Computing Group, "TCG EK Credential Profile For TPM Family 2.0; Level 0", 26 January 2022, <https://trustedcomputinggroup.org/resource/http-trustedcomputinggroup-org-wp-content-uploads-tcg-ek-credential-profile-v-2-5-r2_published-pdf/>.

10.2. Informative References

[TPM_GENIE]: Boone, J., "TPM Genie: Interposer Attacks Against the Trusted Platform Module Serial Bus", 9 March 2018, <https://www.nccgroup.com/globalassets/about-us/us/documents/tpm-genie.pdf>.
[OPENSSL_TPM2_ENGINE]: Open Source Project, "OpenSSL TPM2 Engine", <https://git.kernel.org/pub/scm/linux/kernel/git/jejb/openssl_tpm2_engine.git/>.