



Automated Certificate Management Environment Working Group       F. Geng
Internet-Draft                                                     P. Wu
Intended status: Standards Track                                  L. Xia
Expires: 3 October 2026                              Huawei Technologies
                                                                 X. Chen
                                                               TrustAsia
                                                            1 April 2026


Automated Certificate Management Environment (ACME) Extension for Public
                             Key Challenges
                     draft-geng-acme-public-key-05

Abstract

   The current ACME protocol [RFC8555] requires applicants to submit a
   PKCS#10 Certificate Signing Request (CSR) during the finalization
   phase.  The construction, ASN.1 encoding, and transmission of the CSR
   impose additional implementation burdens on both the client
   (especially resource-constrained devices) and the server.  Moreover,
   the CSR cannot prevent a public key from being replaced by an
   intermediary at the protocol level.

   This document introduces the "pk-01" challenge extension based on the
   ACME protocol.  Its core mechanism is as follows: the applicant
   declares the public key to be authenticated during the "newOrder"
   phase and completes the Proof of Possession (PoP) by signing with the
   private key during the challenge phase.  Since the public key is
   declared when the order is created and verified during the challenge
   phase, there is no need to submit a CSR during the finalization
   phase; the ACME server can issue the certificate directly based on
   the verified public key, thereby eliminating the CSR at the protocol
   level.

   The "pk-01" challenge supports two verification modes via the
   pop_mode field:

   1.  *Asynchronous Mode (Async)*: The applicant pre-deploys a
       signature proof to a designated location (such as a DNS TXT
       record, HTTP path, or email reply).  The ACME server then
       performs the verification query asynchronously, thereby
       completing dual verification of both control over the identifier
       and proof of private key ownership.








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   2.  *Synchronous Mode (Sync)*: The ACME server issues a random number
       (nonce) in the challenge object and performs dual verification of
       identifier control and proof of private key possession through
       real-time protocol interactions with the applicant (similar to
       TLS-ALPN handshake).  The applicant must remain online during the
       verification process.

   The challenge object declares the available delivery methods via the
   supported_delivery field, and the client selects one of them to
   choose an authentication method and resource deployment.

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 3 October 2026.

Copyright Notice

   Copyright (c) 2026 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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Background and Motivation . . . . . . . . . . . . . . . .   4
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   5
   3.  Protocol Extension: newOrder Fields . . . . . . . . . . . . .   7



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     3.1.  Description of New Fields . . . . . . . . . . . . . . . .   7
     3.2.  newOrder Request Example  . . . . . . . . . . . . . . . .   8
       3.2.1.  Asynchronous Mode—DNS Identifier  . . . . . . . . . .   8
       3.2.2.  Synchronization Mode — DNS Identifier . . . . . . . .   9
       3.2.3.  Asynchronous Mode — Email Certificate Scenario  . . .   9
   4.  Extended KeyAuthorization . . . . . . . . . . . . . . . . . .   9
     4.1.  Base KeyAuthorization Value . . . . . . . . . . . . . . .   9
     4.2.  Signature Construction in Asynchronous Mode . . . . . . .  10
     4.3.  Signature Construction in Synchronous Mode  . . . . . . .  10
     4.4.  Signature Algorithm Convention  . . . . . . . . . . . . .  11
       4.4.1.  Explanation of Post-Quantum Algorithms  . . . . . . .  12
   5.  The pk-01 Challenge . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Asynchronous mode of DNS  . . . . . . . . . . . . . . . .  12
       5.1.1.  Challenge Object  . . . . . . . . . . . . . . . . . .  12
       5.1.2.  Client Preparation Steps  . . . . . . . . . . . . . .  13
       5.1.3.  Server Validation Steps . . . . . . . . . . . . . . .  14
     5.2.  Asynchronous mode of HTTP . . . . . . . . . . . . . . . .  14
       5.2.1.  Challenge Object  . . . . . . . . . . . . . . . . . .  14
       5.2.2.  Client Preparation Steps  . . . . . . . . . . . . . .  15
       5.2.3.  Server Validation Steps . . . . . . . . . . . . . . .  15
     5.3.  Email Scenario  . . . . . . . . . . . . . . . . . . . . .  16
       5.3.1.  Challenge Object  . . . . . . . . . . . . . . . . . .  16
       5.3.2.  Client Preparation Steps  . . . . . . . . . . . . . .  16
       5.3.3.  Server Validation Steps . . . . . . . . . . . . . . .  16
     5.4.  Synchronous mode of TLS-ALPN  . . . . . . . . . . . . . .  16
       5.4.1.  Challenge Object  . . . . . . . . . . . . . . . . . .  17
       5.4.2.  Client Preparation Steps  . . . . . . . . . . . . . .  17
       5.4.3.  Server Validation Steps . . . . . . . . . . . . . . .  18
     5.5.  Protocol Interaction Process  . . . . . . . . . . . . . .  18
       5.5.1.  Asynchronous mode (DNS identifier, csr_less: true)  .  18
       5.5.2.  Synchronous mode (TLS-ALPN delivery, csr_less:
               true) . . . . . . . . . . . . . . . . . . . . . . . .  19
   6.  Finalization  . . . . . . . . . . . . . . . . . . . . . . . .  20
     6.1.  csr_less: true（No CSR）  . . . . . . . . . . . . . . . . .  21
     6.2.  csr_less: false（Compatibility mode, default value）  . . .  21
     6.3.  Public Key Consistency Validation and Byte
           Normalization . . . . . . . . . . . . . . . . . . . . . .  22
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
     7.1.  Proof of Key Possession . . . . . . . . . . . . . . . . .  22
     7.2.  Unknown Key Share Attack Mitigation . . . . . . . . . . .  22
     7.3.  Replay Attack Prevention  . . . . . . . . . . . . . . . .  23
     7.4.  DNS Control Dependency  . . . . . . . . . . . . . . . . .  23
     7.5.  Cross-Protocol Attack Mitigation  . . . . . . . . . . . .  23
     7.6.  Algorithm Agility . . . . . . . . . . . . . . . . . . . .  24
     7.7.  Authorization Reuse Binding . . . . . . . . . . . . . . .  24
     7.8.  Security Notes for csr_less Mode  . . . . . . . . . . . .  24
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
     8.1.  ACME Validation Methods . . . . . . . . . . . . . . . . .  24



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     8.2.  IANA ACME Message Fields  . . . . . . . . . . . . . . . .  25
     8.3.  TLS ALPN Protocol Identifier Registration . . . . . . . .  28
   9.  Implementation Considerations . . . . . . . . . . . . . . . .  28
     9.1.  ACME Server . . . . . . . . . . . . . . . . . . . . . . .  28
     9.2.  ACME Client . . . . . . . . . . . . . . . . . . . . . . .  29
   10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     10.1.  Asynchronous Mode: pk-01 (DNS) – Complete Interaction,
            Without Extensions . . . . . . . . . . . . . . . . . . .  30
     10.2.  Asynchronous Mode: pk-01 (HTTP) – Complete Interaction,
            Without Extensions . . . . . . . . . . . . . . . . . . .  31
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

1.1.  Background and Motivation

   In the current ACME process, applicants must separately generate and
   submit a PKCS#10 CSR during the "finalize" phase, which presents the
   following issues:

   *  *Implementation Burden*: CSR requires clients to implement ASN.1
      encoding, DER format construction, and PKCS#10 encapsulation.  For
      standard Domain Validation (DV) certificates and resource-
      constrained devices, this introduces unnecessary complexity.

   *  *Semantic Redundancy*: In the ACME certificate scenario, the
      certificate’s subject information can be provided through existing
      methods; the CSR serves solely as a carrier for transmitting the
      public key, and other fields (such as the Subject DN) are
      typically ignored by the CA.

   *  *Post-Quantum Migration*: The signature sizes of post-quantum
      signature algorithms (such as ML-DSA and SLH-DSA) are
      significantly larger than those of traditional algorithms, and
      ASN.1 toolchains do not yet fully support the identifiers for
      these algorithms; as a result, CSR-based processes face higher
      compatibility risks in post-quantum scenarios.  This extension
      provides a more streamlined migration path for the introduction of
      post-quantum cryptography by moving the public key declaration to
      newOrder, transmitting it in the original SPKI format, and using
      the declared key directly to complete the PoP signature.









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   The solution proposed in this document is to move the public key
   declaration to the "newOrder" phase and perform ownership
   verification during the challenge phase using methods such as private
   key signatures.  As a result, the "finalize" phase does not require a
   CSR; instead, the ACME server issues the certificate directly using
   the verified public key and the identifiers in the order.

   This extension minimizes changes to the existing ACME infrastructure,
   adding only three top-level fields — public_key, pop_mode, and
   csr_less — to the "newOrder" request and introducing a new challenge
   type, "pk-01".  No modifications are required to the behavior during
   the finalization phase.

2.  Conventions and Definitions

   The key terms used in this document are defined as follows:

   *  *End Entity (EE, Applicant)*: The entity that initiates the ACME
      certificate request and holds the private key to be verified.

   *  *ACME Server (AS)*: A Certificate Authority (CA) or its delegated
      service that implements the ACME protocol.

   *  *Claimed Public Key*: The public key to be authenticated, declared
      by the applicant in the public_key field of the "newOrder"
      request, in Base64URL-encoded Subject Public Key Information
      (SPKI) format [RFC5480].  The claimed public key *MUST* correspond
      to the private key actually held by the applicant; the ACME server
      *MUST* perform signature verification using the claimed public key
      and *MUST* perform a byte-by-byte comparison with the public key
      in the final certificate before issuing it.

   *  *Proof of Possession (PoP)*: A mechanism whereby an applicant
      demonstrates, through cryptographic signature operations, that
      they actually possess the private key corresponding to the
      declared public key.

   *  *Extended Key Authorization Value (keyAuthorization)*: An extended
      version of the challenge-response value defined in this document.
      It builds upon the standard ACME keyAuthorization ( token + "." +
      base64url (JWK_Thumbprint (accountKey) )).  It incorporates a
      domain identifier to defend against UKS attacks and is signed with
      the declared private key to simultaneously verify both resource
      control and private key ownership (see Section 4 for details).

   *  *public_key*: A new top-level extension field added to the
      "newOrder" request, containing the declared public key (SPKI
      encoded in Base64URL).



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   *  *pop_mode*: A top-level field added to the newOrder request in
      this document, used by the client to specify the selected PoP
      validation mode (async or sync, or other extensible mode strings).

   *  *csr_less*: A new top-level boolean field added to the "newOrder"
      request, the default value is false.  Controls whether the CSR
      submission is omitted during the finalization phase: "true"
      indicates that the certificate is issued directly using the
      declared public key; "false" indicates that a standard PKCS#10 CSR
      must still be submitted during the finalize phase, and the "pk-01"
      challenge is executed as an additional public key pre-check step.

   *  *usage context*: A fixed ASCII string "ACME-pk-01" followed by a
      null byte (\x00) serves as a fixed prefix for all "pk-01" signed
      messages, providing cryptographic domain separation from the
      signing operations of other protocols.

   *  *supported_delivery*: A list (array of strings) of available
      delivery methods declared by the AS in the challenge object.  In
      asynchronous mode, this may include "dns", "http", and "email"; in
      synchronous mode, it may include "tls-alpn".

   *  *delivery*: A field included by the client in the body of the
      challenge-response POST request, specifying the selected delivery
      method.

   *  *acme-pk/1*: A TLS Application Layer Protocol Negotiation (ALPN,
      [RFC7301]) identifier defined in this document, intended
      exclusively for TLS-ALPN delivery in the "pk-01" challenge-
      synchronization mode.  In TLS sessions using this identifier, the
      requesting server returns the raw proof bytes directly as TLS
      application data after the handshake is established, without
      constructing an X.509 certificate or embedding the proof in
      certificate extension fields. "acme-pk/1" and "acme-tls/1", as
      defined in [RFC8737], are two distinct application-layer protocols
      (they have different formats and cannot be used interchangeably):
      "acme-tls/1" requires the construction of a self-signed
      certificate containing OID extensions, whereas "acme-pk/1"
      directly transmits the raw signature bytes.

   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 [RFC2119] [RFC8174] when, and only
   when, they appear in all capitals, as shown here.






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3.  Protocol Extension: newOrder Fields

   This document introduces minimal extensions to the "newOrder"
   request: the identifiers field retains its standard ACME semantics
   and carries the certificate subject information; a new top-level
   field, public_key, is added to carry the declared public key; a new
   top-level field, pop_mode, is added for the client to declare the
   selected validation mode; and a new top-level boolean field,
   csr_less, is added to control whether the CSR submission is omitted
   during the finalization phase.

   After the ACME server receives a "newOrder" request containing the
   public_key field, it will issue a "pk-01" challenge for each
   identifier.  When csr_less is set to "true", the server issues the
   certificate directly during the finalize phase based on the verified
   declared public key and the identifiers; when csr_less is set to
   "false" (default), the applicant must still submit a CSR during the
   finalize phase, and the "pk-01" challenge is executed as an
   additional public key pre-check step.

   If the public_key field does not exist, the ACME server *SHOULD*
   process the request according to the standard ACME procedure; this
   extension does not apply.

   The pk-01 authorization is bound to the public_key in the order.
   [RFC8555] permits the reuse of valid authorizations, but this
   extension imposes a restriction: if the public_key declared in the
   new order does not match byte-for-byte with the public key recorded
   in the original valid authorization, the ACME server *MUST* reject
   the reuse of that authorization and issue a new "pk-01" challenge for
   the new order.  When storing authorizations, the server *MUST* bind
   the raw bytes of the public_key field to the authorization record.

3.1.  Description of New Fields

   +============+=========+===========+================================+
   | Field name | Type    | Existence | Description                    |
   +============+=========+===========+================================+
   | public_key | String  | OPTIONAL  | Claimed public key, SPKI       |
   |            |         |           | [RFC5480] encoded in           |
   |            |         |           | Base64URL.  If present,        |
   |            |         |           | triggers the "pk-01"           |
   |            |         |           | challenge and the CSR-less     |
   |            |         |           | issuance process.              |
   +------------+---------+-----------+--------------------------------+
   | pop_mode   | String  | OPTIONAL  | PoP verification mode          |
   |            |         |           | declared by the client:        |
   |            |         |           | "async" (the applicant pre-    |



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   |            |         |           | deploys the proof, and the     |
   |            |         |           | AS verifies it                 |
   |            |         |           | independently) or "sync"       |
   |            |         |           | (requires online interaction   |
   |            |         |           | with the applicant).  This     |
   |            |         |           | can be extended to other       |
   |            |         |           | third-party modes.             |
   +------------+---------+-----------+--------------------------------+
   | csr_less   | Boolean | OPTIONAL  | Whether to skip the CSR        |
   |            |         |           | submission during the          |
   |            |         |           | finalization phase. "true":    |
   |            |         |           | Issue directly using the       |
   |            |         |           | declared public key; "false"   |
   |            |         |           | (default): A CSR must still    |
   |            |         |           | be submitted, with "pk-01"     |
   |            |         |           | serving as a pre-check step.   |
   +------------+---------+-----------+--------------------------------+

                                  Table 1

   The rules for the pop_mode field are as follows:

   *  "async" (Asynchronous mode): The applicant pre-deploys the PoP
      signature proof to a specified resource location, and the AS
      independently queries and verifies it at any time; the applicant
      does not need to be online during verification.  Supported
      delivery methods are declared by the supported_delivery field of
      the challenge target and can be "dns", "http", or "email".

   *  "sync" (Synchronous mode): AS performs authentication by
      interacting directly with the applicant's server via a real-time
      protocol; the applicant *MUST* remain online during
      authentication.  Supported delivery methods are specified by the
      supported_delivery declaration; a typical implementation is a
      simplified "tls-alpn" handshake.

   *  If pop_mode is omitted, CA *SHOULD* use the "async" by default.

   *  If the CA does not support the scheme declared by the client, it
      *SHOULD* return an error in the response.

3.2.  newOrder Request Example

3.2.1.  Asynchronous Mode—DNS Identifier







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   {
     "identifiers": [
       { "type": "dns", "value": "example.com" }
     ],
     "public_key": "<Claimed Public Key, Base64URL-encoded SPKI>",
     "pop_mode": "async",
     "csr_less": true
   }

3.2.2.  Synchronization Mode — DNS Identifier

   {
     "identifiers": [
       { "type": "dns", "value": "example.com" }
     ],
     "public_key": "<Claimed Public Key, Base64URL-encoded SPKI>",
     "pop_mode": "sync",
     "csr_less": true
   }

3.2.3.  Asynchronous Mode — Email Certificate Scenario

   {
     "identifiers": [
       { "type": "rfc822name", "value": "user@example.com" }
     ],
     "public_key": "<Claimed Public Key, Base64URL-encoded SPKI>",
     "pop_mode": "async",
     "csr_less": true
   }

4.  Extended KeyAuthorization

   The "pk-01" extends the standard ACME certificate format by
   introducing domain identifier binding and private key signature
   verification, thereby validating both control over the resource and
   ownership of the private key.

4.1.  Base KeyAuthorization Value

   The base keyAuthorization value is consistent with [RFC8555]:

keyAuthorization = token || "." || base64url(JWK_Thumbprint(accountKey))








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4.2.  Signature Construction in Asynchronous Mode

   In asynchronous mode, signed messages (to_sign) append a usage
   context prefix and an identifier string to keyAuthorization.  The
   usage context prefix (ACME-pk-01\x00) provides cryptographic domain
   separation from other protocols (see §7.5), while the identifier
   serves as a defense against UKS attacks (see §7.2):

to_sign_async = "ACME-pk-01\x00" || keyAuthorization || "." || identifier

   Proof of Possession Value:

   proof = base64url(Sign(claimedPrivateKey, to_sign_async))

   Here, identifier is the string value of the identifier associated
   with the current authorization (such as "example.com" or
   "user@example.com").  For the semantics of Sign(key, message), see
   §4.4.

   Deployment locations for proofs of each identifier type:

   +==================+==========+==========================+=========+
   | Identifier Types | Delivery | Deployment Location      | Search  |
   |                  | type     |                          | Method  |
   +==================+==========+==========================+=========+
   | dns              | dns      | DNS TXT record _acme-    | DNS     |
   |                  |          | challenge.<domain>       | Query   |
   +------------------+----------+--------------------------+---------+
   | dns              | http     | HTTP path /.well-known/  | HTTP    |
   |                  |          | acme-challenge/<token>   | Query   |
   +------------------+----------+--------------------------+---------+
   | rfc822name       | email    | Body of the S/MIME email | Email   |
   |                  |          | reply                    | Receipt |
   +------------------+----------+--------------------------+---------+

                                 Table 2

4.3.  Signature Construction in Synchronous Mode

   In synchronous mode, the token in a signed message is replaced with a
   new random number (nonce) issued by the CA to ensure the timeliness
   of the signature, and the message is similarly appended with a usage
   context prefix and identifier.

keyAuthorization_sync = nonce || "." || base64url(JWK_Thumbprint(accountKey))
to_sign_sync = "ACME-pk-01\x00" || keyAuthorization_sync || "." || identifier

   Proof of Possession Value:



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   proof = base64url(Sign(claimedPrivateKey, to_sign_sync))

   The nonce is issued by the CA in the challenge object (see §5.4.1)
   and has at least 128 bits of entropy.  The semantics of Sign(key,
   message) are described in §4.4.

   A typical implementation of the synchronous mode is the TLS-ALPN
   handshake: the CA initiates a TLS connection to the applicant’s
   domain (using the ALPN protocol identifier "acme-pk/1"), and the
   applicant’s server returns a proof during the handshake.  The CA
   simultaneously verifies both domain reachability (control) and the
   signature (proof of private key possession).  The time-sensitive
   nature of the nonce ensures that the signature cannot be precomputed.

4.4.  Signature Algorithm Convention

   In this document, Sign(key, message) denotes performing a complete
   signing operation on the raw bytes of message (the algorithm handles
   the hashing internally; the caller must not perform any additional
   hashing on message beforehand).  The specific algorithm is determined
   by the key type of the declared public key:

               +==========+===============================+
               | Key type | Signature Algorithm           |
               +==========+===============================+
               | EC P-256 | ECDSA with SHA-256            |
               +----------+-------------------------------+
               | EC P-384 | ECDSA with SHA-384            |
               +----------+-------------------------------+
               | Ed25519  | Ed25519 (PureEdDSA, RFC 8032) |
               +----------+-------------------------------+
               | RSA      | RSASSA-PSS with SHA-256       |
               +----------+-------------------------------+

                                 Table 3

   The server *SHOULD* reject signature algorithms that are not included
   in the table above or do not comply with the current security
   baseline requirements, and explicitly declare the set of supported
   algorithms in the metadata of the Directory resource.  Clients *MUST*
   use algorithms that correspond to the declared public key types;
   algorithm mismatches will result in signature verification failure.









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4.4.1.  Explanation of Post-Quantum Algorithms

   The set of algorithms currently specified in this document consists
   primarily of traditional cryptographic algorithms.  For post-quantum
   migration scenarios, implementers may refer to NIST post-quantum
   standardized algorithms (such as ML-DSA/CRYSTALS-Dilithium [FIPS-204]
   and SLH-DSA/SPHINCS+ [FIPS-205]), whose algorithm identifiers and
   SPKI formats have been defined by relevant standards.  Servers may
   negotiate the set of supported post-quantum algorithms through
   Directory metadata.

   The signature sizes of post-quantum signature algorithms are
   significantly larger than those of traditional algorithms (for
   example, an ML-DSA-44 signature is approximately 2,420 bytes, an ML-
   DSA-65 signature is approximately 3,309 bytes, and an SLH-DSA-
   SHA2-128s signature is approximately 7,856 bytes), all of which
   significantly exceed the 255-byte limit for a single DNS TXT record.
   Even when using multi-string concatenation (see §5.1.1), signature
   data as large as several kilobytes places a practical burden on DNS
   infrastructure, rendering asynchronous DNS delivery impractical in
   post-quantum scenarios.

   For post-quantum algorithms, *RECOMMENDED* using synchronous mode
   (TLS-ALPN delivery, pop_mode: "sync"): TLS 1.3 [RFC8446] provides
   native support for post-quantum key exchange and authentication
   algorithms; there are no artificial message size limits at the
   handshake layer; the nonce mechanism provides built-in timeliness
   guarantees; The TLS handshake itself serves as a post-quantum-
   friendly authentication channel, requiring no additional protocol-
   level adaptation for post-quantum algorithms.

5.  The pk-01 Challenge

   The "pk-01" challenge distinguishes between asynchronous and
   synchronous modes via the pop_mode field in "newOrder".  The
   challenge object declares the list of delivery methods supported by
   the AS via the supported_delivery field, and the client specifies the
   selected method via the delivery field in the POST response.  All
   challenge objects adhere to the structure defined in [RFC8555] §8,
   and the semantics of the standard fields (url, status, validated,
   error) remain unchanged.

5.1.  Asynchronous mode of DNS

5.1.1.  Challenge Object

   *  *type*: "pk-01"




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   *  *token*: Unpredictable random challenge token (Base64URL-encoded,
      with an entropy of at least 128 bits) [RFC4086].

   {
     "type": "pk-01",
     "url": "https://acme.example.com/acme/chall/abc123",
     "status": "pending",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA",
     "supported_delivery": ["dns", "http"]
   }

   *Note (DNS TXT Record Length)*: The maximum length of a single DNS
   TXT record is 255 bytes ([RFC1035]).  The length of the Base64URL-
   encoded signature value varies depending on the key type: EC P-256/
   Ed25519 is approximately 86 characters (well below the limit);
   RSA-2048 is approximately 342 characters, and RSA-4096 is
   approximately 683 characters (both exceed the single-string limit).
   If the signature value exceeds 255 bytes, the client *MUST* split it
   into multiple strings, each no longer than 255 bytes, and write them
   into the same TXT resource record ( [RFC4408] §3.1.3), and the server
   *MUST* concatenate the multiple strings in order to reconstruct the
   original string before performing signature verification.  Servers
   *SHOULD* declare the supported key types and corresponding signature
   lengths in the Directory metadata to guide clients in selecting the
   appropriate key type.  For post-quantum algorithms, DNS TXT delivery
   is not feasible in practice; *SHOULD* instead use the synchronous
   mode (TLS-ALPN delivery).  For details, see §4.4.

5.1.2.  Client Preparation Steps

   1) Construct the base keyAuthorization using the standard ACME
   format:

keyAuthorization = token || "." || base64url(JWK_Thumbprint(accountKey))

   2) Construct the signed message (see §4.4 for the Sign semantics):

   to_sign = "ACME-pk-01\x00" || keyAuthorization || "." || identifier

   Here, identifier is a string representing the order association
   domain (e.g., "example.com").

   3) Calculate the proof using the claimed private key:

   proof = base64url(Sign(claimedPrivateKey, to_sign))

   4) Write proof to the _acme-challenge.<domain> DNS TXT record for the
   domain:



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   _acme-challenge.example.com.  120  IN  TXT  "<proof>"

   5) Send a POST request to "url" with the payload {"delivery": "dns"}
   to notify the server that the allocation is complete and specify that
   DNS delivery is selected.

5.1.3.  Server Validation Steps

   1) Look up the _acme-challenge.<domain> TXT resource record to
   retrieve the proof.

   2) Rebuild local signature message:

keyAuthorization = token || "." || base64url(JWK_Thumbprint(accountKey))
to_sign = "ACME-pk-01\x00" || keyAuthorization || "." || identifier

   3) Verify the signature of proof using public_key (the declared
   public key) (see §4.4 for the semantics of Sign).

   4) If validation passes, set the challenge status to "valid"; if
   validation fails, set it to "invalid".

   The DNS query itself confirms the applicant's control over the
   domain's DNS zone, while the signature verification verifies
   ownership of the private key; both verifications are completed
   simultaneously in a single operation.

5.2.  Asynchronous mode of HTTP

   The asynchronous mode supports deploying PoP proofs to an HTTP path
   on a domain, which the AS retrieves independently via a GET request.
   Unlike DNS delivery, this method does not require DNS write
   permissions, but the applicant’s HTTP server *MUST* be accessible
   from the outside.  The applicant does not need to remain online while
   the AS performs verification.

5.2.1.  Challenge Object

   {
     "type": "pk-01",
     "url": "https://acme.example.com/acme/chall/def456",
     "status": "pending",
     "token": "DGyRejmCefe7v4NfDGDKfA",
     "supported_delivery": ["dns", "http"]
   }






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5.2.2.  Client Preparation Steps

   1) Construct a base keyAuthorization value using the standard ACME
   method (using token):

keyAuthorization = token || "." || base64url(JWK_Thumbprint(accountKey))

   2) Construct a signed message (same as the asynchronous DNS mode,
   using token):

keyAuthorization = token || "." || base64url(JWK_Thumbprint(accountKey))
to_sign = "ACME-pk-01\x00" || keyAuthorization || "." || identifier

   3) Calculate the proof using the claimed private key:

   proof = base64url(Sign(claimedPrivateKey, to_sign))

   4) Deploy proof to the following HTTP path (using token as part of
   the path):

   http://<domain>/.well-known/acme-challenge/<token>

   5) Send a POST request to "url" with {"delivery": "http"} in the
   request body to notify the server that it can proceed with
   verification.

5.2.3.  Server Validation Steps

   1) Send an HTTP GET request to http://<domain>/.well-known/acme-
   challenge/<token> and retrieve the response body as proof.  A
   successful HTTP GET request confirms the applicant's control over the
   domain's HTTP service.

   2) Reconstruct the signed message using the locally stored token
   (using the token, as in the asynchronous DNS verification formula):

keyAuthorization = token || "." || base64url(JWK_Thumbprint(accountKey))
to_sign = "ACME-pk-01\x00" || keyAuthorization || "." || identifier

   3) Verify the signature of proof using public_key (the claimed public
   key) (see §4.4 for the semantics of Sign).

   4) If validation passes, set the challenge status to "valid"; if
   validation fails, set it to "invalid".

   The HTTP GET request verifies domain ownership (HTTP reachability),
   and the signature verification confirms possession of the private
   key.



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5.3.  Email Scenario

   The "pk-01" email scenario is based on the "email-reply-00" challenge
   mechanism defined in [RFC8823] and is suitable for email certificate
   application scenarios such as S/MIME (using the rfc822name type
   identifier); it always uses the asynchronous mode.

5.3.1.  Challenge Object

   *  *type*: "pk-01"

   *  *token*: Unpredictable random challenge token (Base64URL-encoded,
      with an entropy of at least 128 bits) [RFC4086].

5.3.2.  Client Preparation Steps

   1) The ACME server sends a challenge email containing a token to the
   email address associated with the order.

   2) The applicant constructs a signed message in accordance with §4.2
   (including the prefix “ACME-pk-01\x00”, where identifier is a string
   representing an email address, such as "user@example.com") and
   computes proof.

   3) Send the proof as the body of a reply to the server's specified
   address in an S/MIME email.

   4) Send a POST request to "url" with the payload {“delivery”:
   “email”} to notify the server that the email has been sent.

5.3.3.  Server Validation Steps

   The server receives the applicant's reply email, extracts the proof
   from the email body, and performs signature verification using the
   same logic as in §5.1.3 (using the identifier as the email address).
   If the verification passes, the challenge status is set to "valid".

5.4.  Synchronous mode of TLS-ALPN

   In synchronous mode, the AS performs real-time verification through a
   direct TLS handshake with the applicant’s server; the applicant
   *MUST* remain online during the verification process.  This document
   defines a new ALPN protocol identifier, "acme-pk/1" (see §2 and
   §8.3), for this handshake negotiation, using a simplified
   implementation: The client uses a signature message containing a
   nonce signed with the claimed private key pair to complete the PoP
   signature, and transmits the proof to the AS via the TLS handshake
   using the ALPN protocol identifier "acme-pk/1", without embedding the



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   proof in the X.509 certificate extension fields.

   "acme-pk/1" is a different application-layer protocol from "acme-
   tls/1" as defined in [RFC8737]: "acme-tls/1" requires the
   construction of a self-signed X.509 certificate containing OID
   extensions; in the "acme-pk/1" handshake, the server directly returns
   the raw proof bytes in the TLS application data, resulting in a
   simpler implementation with no additional restrictions on post-
   quantum large-size signatures.

5.4.1.  Challenge Object

   The sync mode challenge object includes the nonce and
   supported_delivery fields:

   *  *type*: "pk-01"

   *  *nonce*: A new random number generated by CA specifically for this
      challenge (Base64URL-encoded, with an entropy of at least 128
      bits).

   *  *supported_delivery*：["tls-alpn"]

   {
     "type": "pk-01",
     "url": "https://acme.example.com/acme/chall/sync789",
     "status": "pending",
     "nonce": "Kz3mVpQeRd9fLwYbN5hXuT6oJsIc0vAg2nEp1yMrFqZ",
     "supported_delivery": ["tls-alpn"]
   }

5.4.2.  Client Preparation Steps

   1) Retrieve the nonce from the challenge source.  If it is missing,
   the client *MUST* terminate and report an error.

   2) Construct a synchronous signed message (see §4.4 for the Sign
   semantics):

to_sign = "ACME-pk-01\x00" || keyAuthorization_sync || "." || identifier

   3) Calculate the proof using the claimed private key:

   proof = base64url(Sign(claimedPrivateKey, to_sign))

   4) Configure a TLS listener on port 443 of the domain.  When an AS
   initiates a connection using the ALPN protocol identifier “acme-
   pk/1”, return proof as the handshake response data.



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   5) Send a POST request to "url" with {“delivery”: “tls-alpn”} in the
   request body to notify the server that it can proceed with
   verification.

5.4.3.  Server Validation Steps

   1) Initiate a TLS connection to <domain>:443 (using the ALPN
   identifier "acme-pk/1") and retrieve the proof returned by the
   applicant's server.  A successful TLS connection confirms the
   applicant's control over the TLS service for that domain.

   2) Reconstruct the signed message using the locally stored nonce
   (*MUST NOT* trust any nonce value provided by the client):

keyAuthorization_sync = nonce || "." || base64url(JWK_Thumbprint(accountKey))
to_sign = "ACME-pk-01\x00" || keyAuthorization_sync || "." || identifier

   3) Verify the signature of proof using public_key (the claimed public
   key) (see §4.4 for the semantics of Sign).

   4) If validation passes, set the challenge status to "valid"; if
   validation fails, set it to "invalid".

   A TLS connection verifies domain ownership (TLS reachability), and
   signature verification confirms possession of the private key.

5.5.  Protocol Interaction Process

5.5.1.  Asynchronous mode (DNS identifier, csr_less: true)






















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   EE (Applicant)              AS (ACME Server)         DNS Server
        |                            |                       |
        |-------1. newOrder--------->|                       |
        |       identifiers,         |                       |
        |       public_key: PK,      |                       |
        |       pop_mode: "async"    |                       |
        |                            |                       |
        |<------2. pk-01 (token)-----|                       |
        |                            |                       |
        |   [keyAuth, to_sign]       |                       |
        |   [proof=Sign(SK,to_sign)] |                       |
        |                            |                       |
        |-------------------3. DNS TXT record--------------->|
        |       _acme-challenge.example.com = proof          |
        |                            |                       |
        |-------4. POST {"delivery": |                       |
        |        ----   "dns"}------>|                       |
        |                            |                       |
        |              5. Look up DNS TXT records----------->|
        |                            |<------TXT: proof------|
        |                            |                       |
        |    [6. Reconstruct to_sign,|                       |
        |      verify the signature using the PK]            |
        |                            |                       |
        |<---7. Challenge valid------|                       |
        |                            |                       |
        |----8. finalizeOrder------->|                       |
        |   (without CSR)            |                       |
        |                            |                       |
        |<--9. Certificate(PK, SAN)--|                       |
        |                            |                       |

        Figure 1: Asynchronous mode (DNS identifier, csr_less: true)

5.5.2.  Synchronous mode (TLS-ALPN delivery, csr_less: true)
















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   EE (Applicant)                                   AS (ACME Server)
        |                                                    |
        |-------------------1. newOrder--------------------->|
        |                 identifiers,                       |
        |                 public_key: PK,                    |
        |                 pop_mode: "sync"                   |
        |                                                    |
        |<------------------2. pk-01 (nonce: N)--------------|
        |                                                    |
        |  [keyAuth_sync,to_sign, proof]                     |
        |  [Configure TLS Listener,                          |
        |             ALPN: "acme-pk/1"]                     |
        |                                                    |
        |-----------3. POST {"delivery": "tls-alpn"}-------->|
        |                                                    |
        |<---------------4. TLS handshake--------------------|
        |           (domain:443, ALPN: "acme-pk/1")          |
        |                                                    |
        |----------------5. return proof-------------------->|
        |                                                    |
        |   [6. Reconstruct `to_sign` using the local `N`    |
        |               verify the signature using the PK]   |
        |                                                    |
        |<---------------7. Challenge valid------------------|
        |                                                    |
        |----------------8. finalizeOrder------------------->|
        |                  (without CSR)                     |
        |                                                    |
        |<-------------9. Certificate(PK, SAN)---------------|
        |                                                    |

       Figure 2: Synchronous mode (TLS-ALPN delivery, csr_less: true)

6.  Finalization

   After the "pk-01" challenge was validated, the ACME server has
   confirmed the following:

   *  *Resource Control*: Verified via DNS TXT records (asynchronous/
      DNS), HTTP paths (asynchronous/HTTP), TLS handshakes (synchronous/
      TLS-ALPN), or email replies (email scenarios).

   *  *Proof of Private Key Ownership*: Verified via the proof
      signature.

   *  *Claimed public key*: Already claimed in the public_key field
      during the "newOrder" phase and bound to the order.




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   The behavior in the finalization phase is determined by the csr_less
   field in "newOrder":

6.1.  csr_less: true（No CSR）

   "finalize" requests do not require a CSR, and the body may be an
   empty object.  The server constructs and issues the certificate
   directly based on the following sources:

   *  *Public Key*: Taken from the public_key field in "newOrder" (the
      declared public key, verified via a challenge).

   *  *Subject Alternative Name (SAN)*: Taken from the identifiers field
      in "newOrder".

   POST /acme/order/xyz/finalize HTTP/1.1
   Host: acme.example.com
   Content-Type: application/jose+json

   {}

   The server issues a certificate using the public_key (claimed public
   key) as the public key and the domain name in identifiers as the SAN,
   and returns the certificate download URL.

6.2.  csr_less: false（Compatibility mode, default value）

   "finalize" requests *MUST* still include a standard PKCS#10 CSR; the
   process is consistent with the standard ACME [RFC8555] procedure.  At
   this point, the "pk-01" challenge is performed as an additional
   public key pre-validation step: after verifying the proof of
   ownership of the claimed public key during the challenge phase, the
   server performs an additional byte-by-byte comparison during the
   "finalize" phase to ensure that the public key in the CSR matches the
   public_key field exactly before issuing the certificate.  If the two
   do not match, the server *MUST* reject the request and return an
   error.

   POST /acme/order/xyz/finalize HTTP/1.1
   Host: acme.example.com
   Content-Type: application/jose+json

   {
     "csr": "<DER-encoded PKCS#10 CSR, Base64URL-encoded>"
   }






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6.3.  Public Key Consistency Validation and Byte Normalization

   Regardless of the value of csr_less, the server *MUST*:

   1.  Confirm that the order status is "ready" (all challenges have
       been passed).

   2.  Perform a strict byte-by-byte comparison between the public key
       to be written to the certificate and the public_key declared in
       "newOrder" to ensure they are exactly the same.

   To ensure the reliability of byte-level comparison, the server *MUST*
   treat the raw bytes of the public_key received in the "newOrder"
   request as the sole authoritative source and *MUST NOT* perform any
   form of DER normalization, re-encoding, or attribute pruning on it.
   A single cryptographic key may have multiple valid DER encodings (for
   example, the ECParameters field of an EC public key can be in OID
   format or implicit format).  If the server normalizes the data during
   storage or comparison, this can result in false negatives (valid
   requests being rejected) or false positives (keys with different
   encodings being mistakenly identified as identical).

7.  Security Considerations

7.1.  Proof of Key Possession

   "pk-01" requires applicants to possess the private key corresponding
   to the public key they declare.  The server *MUST* verify the
   signature using the public_key in "newOrder" and *MUST NOT* rely on
   indirect methods to infer ownership of the public key.  When issuing
   a certificate, the server *MUST* compare the public key bytes again
   to ensure consistency.

7.2.  Unknown Key Share Attack Mitigation

   An Unknown Key Sharing (UKS) attack is described as follows: An
   attacker claims another person’s public key and requests PoP
   verification for a domain under their control, attempting to bind the
   other person’s public key to their own domain.

   This document defends against this attack by including an identifier
   field in the signed message:

   to_sign = "ACME-pk-01\x00" || keyAuthorization || "." || identifier

   identifier explicitly binds the current authorization to a specific
   domain or email address, ensuring that signatures generated for
   different identifiers cannot be used interchangeably.



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7.3.  Replay Attack Prevention

   In asynchronous mode, the token *MUST* meet the unpredictability
   requirement (entropy of at least 128 bits) [RFC4086].  In synchronous
   mode, the nonce *MUST* also have an entropy of at least 128 bits.
   The server accepts each nonce only once; after use, it *MUST*
   immediately mark it as consumed.  Any subsequent authentication
   requests carrying the same nonce *MUST* be rejected to prevent replay
   attacks.

7.4.  DNS Control Dependency

   In asynchronous mode (DNS identifier), security relies on the
   applicant having actual control over the corresponding DNS zone.  If
   DNS control is compromised (e.g., through DNS hijacking), an attacker
   could write a forged signature into the TXT record.  Implementers
   *SHOULD* use this in conjunction with DNSSEC.

7.5.  Cross-Protocol Attack Mitigation

   When a single private key is used across multiple protocols (such as
   TLS, CMS, and IKEv2), an attacker may trick the key into signing a
   maliciously crafted ACME challenge within the context of one of these
   protocols, thereby passing PoP verification without actually
   possessing the private key.  This type of attack is known as a cross-
   protocol attack.

   This document appends a fixed usage context prefix, "ACME-pk-01\x00"
   (a fixed ASCII string followed by a NUL byte), to the beginning of
   all "pk-01" signed messages.  Its function is identical to the
   64-byte padding and context string mechanism used in the
   CertificateVerify message in TLS 1.3:

   to_sign = "ACME-pk-01\x00" || keyAuthorization || "." || identifier
   proof   = base64url(Sign(claimedPrivateKey, to_sign))

   This prefix ensures that the signature message for "pk-01" is
   structurally distinct from the signature fields of other protocols.
   Even if the same private key is used for signing by another protocol,
   the resulting signature cannot be reused in the context of "pk-01".

   Implementers *MUST* include this prefix when constructing and
   verifying signed messages; omitting the prefix is considered a
   protocol violation.







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7.6.  Algorithm Agility

   The type of the claimed public key determines the range of signature
   algorithms that can be selected.  The server *SHOULD* reject
   signature algorithms that do not meet the current security baseline
   requirements and explicitly state the supported algorithms in the
   metadata of the Directory resource.

7.7.  Authorization Reuse Binding

   RFC 8555 supports the reuse of valid, unexpired authorizations.
   However, "pk-01" authorizations are bound to specific public_key: if
   the public key associated with the reused authorization differs from
   the public key claimed in the new order, an attacker could exploit
   this to request a certificate for an unverified public key.

   The server *MUST* persistently store the original bytes of the
   public_key from the successful "pk-01" challenge in the authorization
   record, and perform a byte-by-byte comparison when reusing the
   authorization: if there is a mismatch, *MUST* reject the reuse and
   trigger a new challenge.

7.8.  Security Notes for csr_less Mode

   After enabling csr_less: true, the finalization phase no longer
   passes the public key via the CSR; the CA relies entirely on the
   public_key declared in "newOrder" and validated by the "pk-01"
   challenge.  In this scenario, the proof of private key ownership from
   the "pk-01" challenge is the sole cryptographic basis for the public
   key’s legitimacy.  The server *MUST* perform byte-level locking on
   the order’s public key after the challenge verification passes to
   prevent subsequent requests from replacing the public key.

8.  IANA Considerations

8.1.  ACME Validation Methods

   This document requests that IANA add the following entry to the ACME
   Validation Methods registry.

    +=======+============================================+===========+
    | Label | Note                                       | Reference |
    +=======+============================================+===========+
    | pk-01 | Public key challenge with async (DNS/HTTP/ | RFC XXX   |
    |       | email) and sync (TLS-ALPN) PoP modes       |           |
    +-------+--------------------------------------------+-----------+

                                 Table 4



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8.2.  IANA ACME Message Fields

   This document requests that IANA add the following entry to the ACME
   Validation Fields registry.

   newOrder request fields:

      +=============+==============================================+
      | Properties  | Value                                        |
      +=============+==============================================+
      | Field Name  | public_key                                   |
      +-------------+----------------------------------------------+
      | Message     | newOrder Request                             |
      | Type        |                                              |
      +-------------+----------------------------------------------+
      | Data Type   | String                                       |
      +-------------+----------------------------------------------+
      | Presence    | OPTIONAL（This must be included only when     |
      |             | using the public key challenge extension）    |
      +-------------+----------------------------------------------+
      | Description | A public key awaiting certification, encoded |
      |             | as a Base64URL-encoded SPKI [RFC5480].  If   |
      |             | present, triggers the pk-01 challenge and    |
      |             | the CSR-less issuance process.               |
      +-------------+----------------------------------------------+
      | Reference   | RFC XXX                                      |
      +-------------+----------------------------------------------+

                                 Table 5






















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     +=============+=================================================+
     | Properties  | Value                                           |
     +=============+=================================================+
     | Field Name  | pop_mode                                        |
     +-------------+-------------------------------------------------+
     | Message     | newOrder Request                                |
     | Type        |                                                 |
     +-------------+-------------------------------------------------+
     | Data Type   | String                                          |
     +-------------+-------------------------------------------------+
     | Presence    | OPTIONAL（Default value: "async"）                |
     +-------------+-------------------------------------------------+
     | Description | PoP verification mode declared by the client:   |
     |             | async (the applicant pre-deploys the proof, and |
     |             | the AS verifies it independently) or “sync”     |
     |             | (requires real-time interaction).  Extensible.  |
     +-------------+-------------------------------------------------+
     | Reference   | RFC XXX                                         |
     +-------------+-------------------------------------------------+

                                  Table 6

      +=============+===============================================+
      | Properties  | Value                                         |
      +=============+===============================================+
      | Field Name  | csr_less                                      |
      +-------------+-----------------------------------------------+
      | Message     | newOrder Request                              |
      | Type        |                                               |
      +-------------+-----------------------------------------------+
      | Data Type   | Boolean                                       |
      +-------------+-----------------------------------------------+
      | Presence    | OPTIONAL（Default value:false）                 |
      +-------------+-----------------------------------------------+
      | Description | Controls whether the CSR submission is        |
      |             | skipped during the finalization phase. true   |
      |             | indicates that the certificate is issued      |
      |             | directly using the declared public key; false |
      |             | indicates that a PKCS#10 CSR must still be    |
      |             | submitted, with the "pk-01" challenge serving |
      |             | as an additional pre-validation step.         |
      +-------------+-----------------------------------------------+
      | Reference   | RFC XXX                                       |
      +-------------+-----------------------------------------------+

                                  Table 7

   Challenge Target Field:



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   +=============+====================================================+
   | Properties  | Value                                              |
   +=============+====================================================+
   | Field Name  | supported_delivery                                 |
   +-------------+----------------------------------------------------+
   | Message     | pk-01 Challenge Object                             |
   | Type        |                                                    |
   +-------------+----------------------------------------------------+
   | Data Type   | Array of String                                    |
   +-------------+----------------------------------------------------+
   | Presence    | OPTIONAL                                           |
   +-------------+----------------------------------------------------+
   | Description | A list of available delivery methods declared by   |
   |             | AS.  Asynchronous modes may include "dns", "http", |
   |             | and "email"; synchronous modes may include "tls-   |
   |             | alpn".  The client selects one from this list and  |
   |             | declares it in the challenge-response.             |
   +-------------+----------------------------------------------------+
   | Reference   | RFC XXX                                            |
   +-------------+----------------------------------------------------+

                                 Table 8

      +=============+===============================================+
      | Properties  |Value                                          |
      +=============+===============================================+
      | Field Name  |delivery                                       |
      +-------------+-----------------------------------------------+
      | Message     |pk-01 Challenge Response (POST body)           |
      | Type        |                                               |
      +-------------+-----------------------------------------------+
      | Data Type   |String                                         |
      +-------------+-----------------------------------------------+
      | Presence    |REQUIRED（when supported_delivery is present）   |
      +-------------+-----------------------------------------------+
      | Description |The client must specify the selected delivery  |
      |             |method in the POST body of the challenge-      |
      |             |response request; this must be one of the      |
      |             |values in the supported_delivery list.         |
      +-------------+-----------------------------------------------+
      | Reference   |RFC XXX                                        |
      +-------------+-----------------------------------------------+

                                  Table 9







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8.3.  TLS ALPN Protocol Identifier Registration

   This document requests that IANA add the following entry to the TLS
   Application-Layer Protocol Negotiation (ALPN) Protocol IDs registry
   [RFC7301] with the IETF as the Change Controller:

         +===========+===============================+===========+
         | Protocol  | Identification Sequence       | Reference |
         +===========+===============================+===========+
         | acme-pk/1 | 0x61 0x63 0x6d 0x65 0x2d 0x70 | RFC XXX   |
         |           | 0x6b 0x2f 0x31 ("acme-pk/1")  |           |
         +-----------+-------------------------------+-----------+

                                  Table 10

   The "acme-pk/1" identifier is reserved for TLS-ALPN delivery in the
   "pk-01" challenge synchronization mode; its operational semantics
   differ from those of [RFC8737]: In TLS sessions using "acme-pk/1",
   the server returns the raw proof bytes directly in the TLS
   application data after the handshake is complete, without
   constructing an X.509 certificate or embedding any challenge data in
   the certificate extension fields.

9.  Implementation Considerations

9.1.  ACME Server

   *  When processing a "newOrder" request, the server must perform
      strict format validation on the public_key field to verify that it
      is a valid DER-encoded SPKI; for unsupported key types, it
      *SHOULD* return an error.  The server *MUST* store public_key in
      the raw bytes received and must not perform any DER normalization
      or re-encoding (see §6.3).

   *  The server must verify the uniqueness of the token in asynchronous
      mode and the nonce in synchronous mode to prevent token reuse.

   *  Before issuing the certificate, the server must verify the public
      key bytes once again to ensure they match exactly, byte-for-byte,
      with the public key declared in "newOrder".

   *  The server *MUST* mark the nonce as used immediately after it is
      consumed for the first time and reject any subsequent attempts to
      reuse it.  The server should limit the validity window of the
      nonce (*RECOMMENDED* to be no longer than the challenge token's
      validity period).





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   *  When executing an asynchronous HTTP GET request, the server *MUST
      NOT* follow HTTP redirects that change the target domain to a
      different domain (cross-domain redirects), as following such
      redirects would invalidate the domain control verification.  The
      server *SHOULD* allow same-domain redirects (such as HTTP → HTTPS
      redirects within the same host) and adhere to the standard ACME
      http-01 HTTP request specifications (timeouts, maximum number of
      redirects, etc.) to prevent server-side request forgery (SSRF).

   *  The server *SHOULD* include a supported_delivery field in the
      challenge object to declare all supported delivery methods for the
      client to choose from.

   *  After the server receives the Challenge-Response POST request, it
      must read the delivery field to determine which verification
      method to use; if the delivery field is missing or contains an
      unsupported value, the server *SHOULD* return an error.

9.2.  ACME Client

   *  The client *MUST* ensure that the private key used for signing
      strictly matches the public_key declared in "newOrder"; a mismatch
      in key types will cause the server to fail the signature
      verification.

   *  In synchronous mode, the client *MUST* send a response to the
      challenge URL only after the TLS listening configuration is
      complete, to ensure it is reachable when the server initiates the
      TLS handshake.

   *  In asynchronous mode, the client *SHOULD* verify that the resource
      (DNS TXT record or HTTP path) has taken effect before notifying
      the server, taking into account DNS propagation delays or HTTP
      service availability.

   *  When csr_less: true, the body of the "finalize" request may be an
      empty object {}, the client does not need to construct a CSR; when
      csr_less: false (default), the client *MUST* still submit a
      standard PKCS#10 CSR during the "finalize" phase, and the public
      key in the CSR *MUST* match the public_key declared in "newOrder".

   *  When sending a challenge-response POST request, the client *MUST*
      include a delivery field in the request body, and its value must
      be one of the items in the supported_delivery list of the
      challenge object.

10.  Examples




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10.1.  Asynchronous Mode: pk-01 (DNS) – Complete Interaction, Without
       Extensions

   *Prerequisites*: The applicant *MUST* have DNS control over the
   domain example.com and a P-256 key pair (private key d, public key Q,
   SPKI-encoded as pk_spki).

   *Step 1: newOrder Request*

   POST /acme/new-order HTTP/1.1
   Host: acme.example.com
   Content-Type: application/jose+json

   {
     "identifiers": [
       { "type": "dns", "value": "example.com" }
     ],
     "public_key": "pk_spki",
     "pop_mode": "async",
     "csr_less": true
   }

   *Step 2: The server returns the pk-01 challenge*

   {
     "type": "pk-01",
     "url": "https://acme.example.com/acme/chall/abc123",
     "status": "pending",
     "token": "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA"
   }

   *Step 3: The client generates a signature and writes it to the DNS
   TXT record*

keyAuthorization = "evaGxfADs6pSRb2LAv9IZf17Dt3juxGJ-PCt92wr-oA.pswg5_v_JaVFRXrHxGfJkg"
to_sign = "ACME-pk-01\x00" || keyAuthorization || "." || "example.com"
proof = base64url(Sign(d, to_sign))  →  "<proof_value>"
# The Sign() function uses ECDSA-with-SHA256 (P-256 key) and internally signs the `to_sign` value after applying SHA-256 to it.

   _acme-challenge.example.com.  120  IN  TXT  "<proof_value>"

   *Step 4: The client notifies the server, and the server queries and
   verifies*

   POST /acme/chall/abc123 HTTP/1.1
   Content-Type: application/jose+json

   {"delivery": "dns"}



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   The server queries the TXT record for _acme-challenge.example.com,
   reconstructs to_sign locally, verifies the signature using pk_spki,
   and sets the challenge status to valid upon successful verification.

   *Step 5: finalizeOrder (without CSR)*

   POST /acme/order/xyz/finalize HTTP/1.1
   Content-Type: application/jose+json

   {}

   The server issues a certificate using pk_spki as the public key and
   example.com as the subject alternative name (SAN).

10.2.  Asynchronous Mode: pk-01 (HTTP) – Complete Interaction, Without
       Extensions

   *Prerequisites*: The applicant *MUST* have HTTP control over the
   domain example.com and a P-256 key pair.

   *Step 1: newOrder Request*

   POST /acme/new-order HTTP/1.1
   Content-Type: application/jose+json

   {
     "identifiers": [
       { "type": "dns", "value": "example.com" }
     ],
     "public_key": "pk_spki",
     "pop_mode": "async",
     "csr_less": true
   }

   *Step 2: The server returns the pk-01 challenge*

   {
     "type": "pk-01",
     "url": "https://acme.example.com/acme/chall/def456",
     "status": "pending",
     "token": "DGyRejmCefe7v4NfDGDKfA",
     "supported_delivery": ["dns", "http"]
   }

   *Step 3: The client generates a signature and deploys it to the HTTP
   path*





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keyAuthorization = "DGyRejmCefe7v4NfDGDKfA.pswg5_v_JaVFRXrHxGfJkg"
to_sign = "ACME-pk-01\x00" || keyAuthorization || "." || "example.com"
proof = base64url(Sign(d, to_sign))
# The Sign() function uses ECDSA-with-SHA256 (P-256 key) and internally signs the `to_sign` value after applying SHA-256 to it.

   Deploy to: http://example.com/.well-known/acme-challenge/
   DGyRejmCefe7v4NfDGDKfA

   Content: <proof>

   *Step 4: The client notifies the server, and the server initiates an
   HTTP GET authentication request*

   POST /acme/chall/def456 HTTP/1.1
   Content-Type: application/jose+json

   {"delivery": "http"}

   The server sends an HTTP GET request to http://example.com/.well-
   known/acme-challenge/DGyRejmCefe7v4NfDGDKfA to verify domain
   ownership.  After validating the signature, it sets the challenge
   status to "valid".

   *Step 5: finalizeOrder (without CSR)*

   The server issues a certificate using pk_spki as the public key and
   example.com as the subject alternative name (SAN).

11.  Normative References

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
              <https://www.rfc-editor.org/info/rfc5480>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/info/rfc7301>.







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   [RFC8737]  Shoemaker, R.B., "Automated Certificate Management
              Environment (ACME) TLS Application-Layer Protocol
              Negotiation (ALPN) Challenge Extension", RFC 8737,
              DOI 10.17487/RFC8737, February 2020,
              <https://www.rfc-editor.org/info/rfc8737>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC4408]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1",
              RFC 4408, DOI 10.17487/RFC4408, April 2006,
              <https://www.rfc-editor.org/info/rfc4408>.

   [RFC8823]  Melnikov, A., "Extensions to Automatic Certificate
              Management Environment for End-User S/MIME Certificates",
              RFC 8823, DOI 10.17487/RFC8823, April 2021,
              <https://www.rfc-editor.org/info/rfc8823>.

Authors' Addresses

   Feng Geng
   Huawei Technologies
   Email: gengfeng@huawei.com


   Panyu Wu
   Huawei Technologies
   Email: wupanyu3@huawei.com



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   Liang Xia
   Huawei Technologies
   Email: frank.xialiang@huawei.com


   Xin Chen
   TrustAsia
   Email: palos.chen@trustasia.com











































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