Automated Certificate Management Environment (ACME) Extension for Public Key Challenges

Introduction

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.

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.

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 . 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).
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, ) 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 , 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 when, and only when, they appear in all capitals, as shown here.

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. 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.

Description of New Fields

Field name	Type	Existence	Description
`public_key`	String	OPTIONAL	Claimed public key, SPKI 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-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.

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.

newOrder Request Example

Asynchronous Mode—DNS Identifier ", "pop_mode": "async", "csr_less": true } ]]>

Synchronization Mode — DNS Identifier ", "pop_mode": "sync", "csr_less": true } ]]>

Asynchronous Mode — Email Certificate Scenario ", "pop_mode": "async", "csr_less": true } ]]>

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.

Base KeyAuthorization Value The base keyAuthorization value is consistent with :

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): Proof of Possession Value: 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 type	Deployment Location	Search Method
`dns`	`dns`	DNS TXT record `_acme-challenge.<domain>`	DNS Query
`dns`	`http`	HTTP path `/.well-known/acme-challenge/<token>`	HTTP Query
`rfc822name`	`email`	Body of the S/MIME email reply	Email Receipt

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. Proof of Possession Value: 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.

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

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.

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 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.

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 §8, and the semantics of the standard fields (url, status, validated, error) remain unchanged.

Asynchronous mode of DNS

Challenge Object

type: "pk-01"
token: Unpredictable random challenge token (Base64URL-encoded, with an entropy of at least 128 bits) .

Note (DNS TXT Record Length): The maximum length of a single DNS TXT record is 255 bytes (). 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 ( §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.

Client Preparation Steps 1) Construct the base keyAuthorization using the standard ACME format: 2) Construct the signed message (see §4.4 for the Sign semantics): Here, identifier is a string representing the order association domain (e.g., "example.com"). 3) Calculate the proof using the claimed private key: 4) Write proof to the _acme-challenge.<domain> DNS TXT record for the domain: " ]]> 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.

Server Validation Steps 1) Look up the _acme-challenge.<domain> TXT resource record to retrieve the proof. 2) Rebuild local signature message: 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.

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.

Challenge Object

Client Preparation Steps 1) Construct a base keyAuthorization value using the standard ACME method (using token): 2) Construct a signed message (same as the asynchronous DNS mode, using token): 3) Calculate the proof using the claimed private key: 4) Deploy proof to the following HTTP path (using token as part of the path): /.well-known/acme-challenge/ ]]> 5) Send a POST request to "url" with {"delivery": "http"} in the request body to notify the server that it can proceed with verification.

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): 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.

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

Challenge Object

type: "pk-01"
token: Unpredictable random challenge token (Base64URL-encoded, with an entropy of at least 128 bits) .

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.

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".

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 proof in the X.509 certificate extension fields. "acme-pk/1" is a different application-layer protocol from "acme-tls/1" as defined in : "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.

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"]

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): 3) Calculate the proof using the claimed private key: 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. 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.

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): 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.

Protocol Interaction Process

Asynchronous mode (DNS identifier, csr_less: true)

Asynchronous mode (DNS identifier, csr_less: true) | | | 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)--| | | | | ]]>

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

Synchronous mode (TLS-ALPN delivery, csr_less: true) | | 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)---------------| | | ]]>

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.

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

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".

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.

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 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. " } ]]>

Public Key Consistency Validation and Byte Normalization Regardless of the value of csr_less, the server MUST:

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

Security Considerations

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.

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

Replay Attack Prevention In asynchronous mode, the token MUST meet the unpredictability requirement (entropy of at least 128 bits) . 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.

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.

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: 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.

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.

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.

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.

IANA Considerations

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/email) and sync (TLS-ALPN) PoP modes	RFC XXX

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 Type	newOrder Request
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 . If present, triggers the pk-01 challenge and the CSR-less issuance process.
Reference	RFC XXX

Properties	Value
Field Name	`pop_mode`
Message Type	newOrder Request
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

Properties	Value
Field Name	`csr_less`
Message Type	newOrder Request
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

Challenge Target Field:

Properties	Value
Field Name	`supported_delivery`
Message Type	pk-01 Challenge Object
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

Properties	Value
Field Name	`delivery`
Message Type	pk-01 Challenge Response (POST body)
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

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 with the IETF as the Change Controller:

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

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 : 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.

Implementation Considerations

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).
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.

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.

Examples

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 Step 2: The server returns the pk-01 challenge Step 3: The client generates a signature and writes it to the DNS TXT record " # The Sign() function uses ECDSA-with-SHA256 (P-256 key) and internally signs the `to_sign` value after applying SHA-256 to it. ]]> " ]]> Step 4: The client notifies the server, and the server queries and verifies 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) The server issues a certificate using pk_spki as the public key and example.com as the subject alternative name (SAN).

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 Step 2: The server returns the pk-01 challenge Step 3: The client generates a signature and deploys it to the HTTP path 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 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).