Note: this is a patch for openssl_tpm2_engine, not the kernel.
Changes for v2: Add security considerations, iana considerations and
implementation considerations sections and more detailed description
of importable keys.
This is the text of the draft RFC for comments (although patches to
the xml would be preferred):
======
Network Working Group J. Bottomley
Internet-Draft Linux Kernel
Intended status: Informational May 2021
Expires: 25 November 2021
ASN.1 Specification for TPM 2.0 Key Files
draft-bottomley-tpm-keys-00
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 2 November 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Bottomley Expires 25 November 2021 [Page 1]
Internet-Draft TPM 2 Key Format May 2021
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Key Representation . . . . . . . . . . . . . . . . . . . . . 3
3.1. TPMkey Syntax . . . . . . . . . . . . . . . . . . . . . . 3
3.1.1. type . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.2. emptyAuth . . . . . . . . . . . . . . . . . . . . . . 4
3.1.3. policy . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.4. secret . . . . . . . . . . . . . . . . . . . . . . . 4
3.1.5. parent . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.6. pubkey . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.7. privkey . . . . . . . . . . . . . . . . . . . . . . . 5
4. Key Policy Specification . . . . . . . . . . . . . . . . . . 5
4.1. TPMPolicy Syntax . . . . . . . . . . . . . . . . . . . . 6
4.1.1. CommandCode . . . . . . . . . . . . . . . . . . . . . 6
4.1.2. CommandPolicy . . . . . . . . . . . . . . . . . . . . 6
4.2. Policy Implementation Considerations . . . . . . . . . . 6
4.2.1. Authorization Policy . . . . . . . . . . . . . . . . 7
5. Implementation Considerations . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Comments on and Enhancements to this Document . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The Security of private keys has long been a concern and the ability
of ubiquitous devices like TPMs has made it useful to use them for
secure private key storage. With the advent of TPM 2.0, private key
storage inside the TPM (acting as a token which could be referred to
by PKCS #11) has been discouraged, and instead key files which are
loaded and evicted as necessary is the encouraged format. This
standard defines an interoperable ASN.1 representation for such key
files, so that a key created by one tool should be loadable by a
different one.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Bottomley Expires 25 November 2021 [Page 2]
Internet-Draft TPM 2 Key Format May 2021
2.1. Notation
ASN.1 Abstract Syntax Notation defined in [X.680]
DER Distinguished Encoding Rules. Basically a defined binary
representation for ASN.1
MSO Most Significant Octet (the highest order byte of an integer)
PEM Privacy enhanced Electronic Mail. An ASCII compatible
representation of DER
TCG Trusted Computing Group
TPM Trusted Platform Module
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
parent INTEGER
pubkey OCTET STRING
privkey OCTET STRING
}
The fields of type TPMKey have the following meanings:
Bottomley Expires 25 November 2021 [Page 3]
Internet-Draft TPM 2 Key Format May 2021
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}
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 Section 4.
3.1.4. secret
This section describes the additional cryptographic secret used to
specify the outer wrapping of an importable key. It MUST be present
for key type id-importablekey and MUST NOT be present for any other
key type.
Bottomley Expires 25 November 2021 [Page 4]
Internet-Draft TPM 2 Key Format May 2021
Importable keys (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. 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 for the NIST P-256 curve and use the primary
key so generated as the parent.
3.1.6. pubkey
This MUST be present and MUST correspond to the fully marshalled
TPM2B_PUBLIC structure of the TPM Key.
3.1.7. 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
Bottomley Expires 25 November 2021 [Page 5]
Internet-Draft TPM 2 Key Format May 2021
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 marshalls 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
4.2. 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.
Bottomley Expires 25 November 2021 [Page 6]
Internet-Draft TPM 2 Key Format May 2021
4.2.1. Authorization Policy
When Authorization (Passing in a password) is required, the emptyAuth
parameter MUST be absent or set to false and additionally the
TPM_CC_PolicyAuthValue MUST be specified as the command code for one
entry in the TPMPolicy sequence. However, 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.
5. Implementation Considerations
Implementations SHOULD support all TCG mandated algorithms, but MAY
omit those deemed insecure, such as the SHA1 hash.
TPM2_Import transforms the privKey into a TPM2B_PRIVATE which can
then be used as a source to TPM2_Load, making the loading of
importable keys is necessarily a two stage process, which can be time
consuming on some TPMs. Since the TPM2B_PRIVATE structure emitted by
TPM2_Import is fully secure, Implementations SHOULD minimize the
number of TPM2_Import operations by caching the emitted
TPM2B_PRIVATE.
6. 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.
Bottomley Expires 25 November 2021 [Page 7]
Internet-Draft TPM 2 Key Format May 2021
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.
7. IANA Considerations
None.
8. Comments on and Enhancements to this Document
Comments on this document should be addressed to the author
(James.Bottomley@xxxxxxxxxxxxxxxxxxxxx) but should also CC the email
lists of the two projects implementing this specification:
Bottomley Expires 25 November 2021 [Page 8]
Internet-Draft TPM 2 Key Format May 2021
The OpenSSL engine: openssl_tpm2_engine@xxxxxxxxx
The Linux Kernel: linux-integrity@xxxxxxxxxxxxxxx
The OpenSSL TPM2 engine [OPENSSL_TPM2_ENGINE] is currently the only
implementation of this full specification, so enhancements should be
proposed after patches implementing the enhancement have been
accepted by openssl_tpm2_engine or another full specification
implementation.
9. References
9.1. 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>.
9.2. 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/>.
Bottomley Expires 25 November 2021 [Page 9]
Internet-Draft TPM 2 Key Format May 2021
Author's Address
James E.J. Bottomley
Linux Kernel
United States of America
Email: James.Bottomley@xxxxxxxxxxxxxxxxxxxxx
Bottomley Expires 25 November 2021 [Page 10]
======
James Bottomley (1):
doc: add draft RFC for TPM Key format
Makefile.am | 2 +-
configure.ac | 4 +-
doc/Makefile.am | 15 +
doc/draft-bottomley-tpm2-keys.xml | 465 ++++++++++++++++++++++++++++++
4 files changed, 484 insertions(+), 2 deletions(-)
create mode 100644 doc/Makefile.am
create mode 100644 doc/draft-bottomley-tpm2-keys.xml
--
2.26.2
