]>

me@brandonweeks.com

ganesh.mallaya@appviewx.com

sven.rajala@keyfactor.com

Security ACME Working Group Internet-Draft This document specifies new identifiers and a challenge for the Automated Certificate Management Environment (ACME) protocol which allows validating the identity of a device using attestation.

Introduction The Automatic Certificate Management Environment (ACME) standard specifies methods for validating control over identifiers, such as domain names. It is also useful to be able to validate properties of the device requesting the certificate, such as the identity of the device /and whether the certificate key is protected by a secure cryptoprocessor. Many operating systems and device vendors offer functionality enabling a device to generate a cryptographic attestation of their identity, such as:

• Android Key Attestation
• Chrome OS Verified Access
• Trusted Platform Module
• Managed Device Attestation for Apple Devices

Using ACME and device attestation to issue client certificates for enterprise PKI is to be a common use case. The following variances to the ACME specification are described in this document:

• Addition of permanent-identifier and hardware-module identifier types.
• Addition of the device-attest-01 challenge type to prove control of the permanent-identifier and hardware-module identifier types.
• The challenge response payload contains a serialized WebAuthn attestation statement format instead of an empty JSON object ({}).
• Accounts and external account binding being used as a mechanism to pre-authenticate requests to an enterprise CA.

This document does not specify the attestation verification procedures. Section 13 of gives some guidance, however verification procedures are complex and may require changes to address future security issues. Efforts are underway within the Remote ATtestation ProcedureS (RATS) working group to define a set of standard formats and protocols for attestation. An explict aim of this document is to support vendor specific formats and protocols that are widely deployed at publication time of this specification.

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Permanent Identifier A new identifier type, "permanent-identifier" is introduced to represent the identity of a device assigned by the manufacturer, typically a serial number. The name of this identifier type was chosen to align with , it does not prescribe the lifetime of the identifier, which is at the discretion of the Assigner Authority. The identity along with the assigning organization can be included in the Subject Alternate Name Extension using the PermanentIdentifier form described in . Clients MAY include this identifier in the certificate signing request (CSR). Alternatively if the server wishes to only issue privacy-preserving certificates, it MAY reject CSRs containing a PermanentIdentifier in the subjectAltName extension.

Hardware Module A new identifier type, "hardware-module" is introduced to represent the identity of the secure cryptoprocessor that generated the certificate key. The hardware module identity can be included in the Subject Alternate Name Extension using the HardwareModuleName form described in . The HardwareModuleName is encoded as an otherName with the OID id-on-hardwareModuleName (1.3.6.1.5.5.7.8.4) and consists of:

• hwType: An OBJECT IDENTIFIER that identifies the type of hardware module
• hwSerialNum: An OCTET STRING containing the hardware module serial number

Clients MAY include this identifier in the certificate signing request (CSR). When included in a CSR, it MUST appear in an extensionRequest attribute requesting a subjectAltName extension. If the server includes HardwareModule in the subjectAltName extension the CA MUST verify that the certificate key was generated on the secure cryptoprocessor with the asserted identity and type. The key MUST NOT be able to be exported from the cryptoprocessor. If the server wishes to issue privacy-preserving certificates, it MAY omit HardwareModule from the subjectAltName extension.

Device Attestation Challenge The client can prove control over a permanent identifier of a device by providing an attestation statement containing the identifier of the device. The device-attest-01 ACME challenge object has the following format:

type (required, string):

The string "device-attest-01".

token (required, string):

A random value that uniquely identifies the challenge.

A client fulfills this challenge by constructing a key authorization () from the "token" value provided in the challenge and the client's account key. The client then generates a WebAuthn attestation object using the key authorization as the challenge. This specification borrows the WebAuthn attestation object representation as described in Section 6.5.4 of for encapsulating attestation formats, but with these modifications:

• The key authorization is used to form attToBeSigned. This replaces the concatenation of authenticatorData and clientDataHash. attToBeSigned is hashed using an algorithm specified by the attestation format.
• The authData field is unused and SHOULD be omitted.

A client responds with the response object containing the WebAuthn attestation object in the "attObj" field to acknowledge that the challenge can be validated by the server. On receiving a response, the server constructs and stores the key authorization from the challenge's "token" value and the current client account key. To validate a device attestation challenge, the server performs the following steps:

1. Perform the verification procedures described in Section 6 of .
2. Verify that key authorization conveyed by attToBeSigned matches the key authorization stored by the server.

The webauthn payload MAY contain any identifiers registered in "WebAuthn Attestation Statement Format Identifiers" and any extensions registered in "WebAuthn Extension Identifiers" , .

Operational Considerations Although this document focuses guidance on implementing new type and challenge for certificate issuance using ACME, it does not define a New Protocol, a Protocol Extension, or an architecture.

Enterprise PKI ACME was originally envisioned for issuing certificates in the Web PKI, however this extension will primarily be useful in enterprise PKI. The subsection below covers some operational considerations for an ACME-based enterprise CA.

External Account Binding An enterprise CA likely only wants to receive requests from authorized devices. It is RECOMMENDED that the server require a value for the "externalAccountBinding" field to be present in "newAccount" requests. If an enterprise CA desires to limit the number of certificates that can be requested with a given account, including limiting an account to a single certificate. After the desired number of certificates have been issued to an account, the server MAY revoke the account as described in Section 7.1.2 of .

Security Considerations Please reference for other security considerations. See Section 13 of for additional security considerations related to attestation statement formats, including certificate revocation. Key attestation statements may include a variety of information in addition to the public key being attested. While not described in this document, the server MAY use any policy when evaluating this information. This evaluation can result in rejection of a certificate request that features a verifiable key attestation for the public key contained in the request. For example, an attestation statement may indicate use of an unacceptable firmware version. The "token" value MUST have at least 128 bits of entropy. It MUST NOT contain any characters outside the base64url alphabet, including padding characters ("="). See , Appendix C.1 for additional information on randomness requirements.

IANA Considerations

ACME Identifier Types The "ACME Identifier Types" registry is to be updated to include the following entries:

Label	Reference
permanent-identifier	RFC XXXX
hardware-module	RFC XXXX

ACME Validation Method The "ACME Validation Methods" registry is to be updated to include the following entry:

Label	Identifier Type	ACME	Reference
device-attest-01	permanent-identifier	Y	RFC XXXX

New Error Types This document adds the following entries to the ACME Error Type registry:

Type	Description	Reference
badAttestationStatement	The attestation statement is unacceptable (e.g. not signed by an attestation authority trusted by the CA)	RFC XXXX

References Normative References This document describes the use of the Cryptographic Message Syntax (CMS) to protect firmware packages, which provide object code for one or more hardware module components. CMS is specified in RFC 3852. 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. A firmware package loading receipt can optionally be generated to acknowledge the successful loading of a firmware package. Similarly, a firmware package load error report can optionally be generated to convey the failure to load a firmware package. [STANDARDS-TRACK] This document defines a new form of name, called permanent identifier, that may be included in the subjectAltName extension of a public key certificate issued to an entity. The permanent identifier is an optional feature that may be used by a CA to indicate that two or more certificates relate to the same entity, even if they contain different subject name (DNs) or different names in the subjectAltName extension, or if the name or the affiliation of that entity stored in the subject or another name form in the subjectAltName extension has changed. The subject name, carried in the subject field, is only unique for each subject entity certified by the one CA as defined by the issuer name field. However, the new name form can carry a name that is unique for each subject entity certified by a CA. [STANDARDS-TRACK] Public Key Infrastructure using X.509 (PKIX) certificates are used for a number of purposes, the most significant of which is the authentication of domain names. Thus, certification authorities (CAs) in the Web PKI are trusted to verify that an applicant for a certificate legitimately represents the domain name(s) in the certificate. As of this writing, this verification is done through a collection of ad hoc mechanisms. This document describes a protocol that a CA and an applicant can use to automate the process of verification and certificate issuance. The protocol also provides facilities for other certificate management functions, such as certificate revocation. This specification defines IANA registries for W3C Web Authentication (WebAuthn) attestation statement format identifiers and extension identifiers. Independent This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705, 6066, 7627, and 8422 and obsoletes RFCs 5077, 5246, 6961, 8422, and 8446. This document also specifies new requirements for TLS 1.2 implementations. Google Mozilla Microsoft Microsoft Yubico In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This memo represents a republication of PKCS #9 v2.0 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document, except for the security considerations section, is taken directly from that specification. This memo provides information for the Internet community. Informative References n.d.

Acknowledgments Thank you for all the reivews from the work group.