[v2,0/1] draft RFC for TPM key format

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@HansenPartnership.com) 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@groups.io The Linux Kernel: linux-integrity@vger.kernel.org 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@HansenPartnership.com 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

Message ID	20210524222011.24313-1-James.Bottomley@HansenPartnership.com (mailing list archive)
Headers	show Return-Path: <linux-integrity-owner@kernel.org> From: James Bottomley <James.Bottomley@HansenPartnership.com> To: openssl-tpm2-engine@groups.io Cc: linux-integrity@vger.kernel.org, Mimi Zohar <zohar@linux.ibm.com>, Jarkko Sakkinen <jarkko@kernel.org>, David Woodhouse <dwmw2@infradead.org>, keyrings@vger.kernel.org, David Howells <dhowells@redhat.com> Subject: [PATCH v2 0/1] draft RFC for TPM key format Date: Mon, 24 May 2021 15:20:10 -0700 Message-Id: <20210524222011.24313-1-James.Bottomley@HansenPartnership.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk
Series	draft RFC for TPM key format \| expand [v2,0/1] draft RFC for TPM key format [v2,1/1] doc: add draft RFC for TPM Key format

[v2,0/1] draft RFC for TPM key format

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