]> Vigil Security, LLC

HerndonVA United States of America housley@vigilsec.com

SEC lamps This document specifies an attribute for a statement of possession of a private key by a certificate subject. As part of X.509 certificate enrollment, a Certification Authority (CA) typically demands proof that the subject possesses the private key that corresponds to the to-be-certified public key. In some cases, a CA might accept a signed statement from the certificate subject. For example, when a certificate subject needs separate certificates for signature and key establishment, a statement that can be validated with the previously issued signature certificate for the same subject might be adequate for subsequent issuance of the key establishment certificate.

Introduction This document specifies an attribute for a statement of possession of a private key by a certificate subject. X.509 certificate enrollment often depends on PKCS#10 or the Certificate Request Message Format (CRMF) . As part of enrollment, a Certification Authority (CA) typically demands proof that the subject possesses the private key that corresponds to the to-be-certified public key. Alternatively, a CA may accept a signed statement from the certificate subject claiming knowledge of that private key. When a certificate subject needs separate certificates for signature and key establishment, a signed statement that can be validated with the previously issued signature certificate for the same subject might be adequate for subsequent issuance of the key establishment certificate. For example, a subject may need a signature certificate that contains an ML-DSA (Module-Lattice-Based Digital Signature Algorithm) public key and a key establishment certificate that contains an ML-KEM (Module-Lattice-Based Key-Encapsulation Mechanism) public key. For another example, a subject may need a signature certificate that contains an ECDSA (Elliptic Curve Digital Signature Algorithm) public key and a key establishment certificate that contains an ECDH (Elliptic Curve Diffie-Hellman) public key. A statement of possession may be used in lieu of the usual proof-of-possession mechanisms. The statement is simply a signed assertion that the requestor of a key establishment certificate has possession of the key establishment private key and that statement is signed using a signature private key that was previously shown to be in the possession of the same certificate subject. If allowed by the Certificate Policy , the CA is permitted to accept this statement in lieu of proof that the requestor has possession of the private key, such as . Note that offers some algorithms that provide proof of possession for Diffie-Hellman private keys; however, these algorithms are not suitable for use with PKCS#10 . In addition, the algorithms in do not support key encapsulation mechanism algorithms, such as ML-KEM. The attribute specified in this document, on the other hand, is suitable for use with both PKCS#10 and the CRMF .

ASN.1 The attribute defined in this document is generated using ASN.1 , using the Distinguished Encoding Rules (DER) .

Terminology 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.

Overview When using the attribute defined in this document to make a statement about the possession of the key establishment private key, the process to obtain two certificates with PKCS#10 is as follows:

1. The subject generates the signature key pair.
2. The subject composes a PKCS#10 Certificate Signing Request (CSR) in the usual manner. It includes a signature that is produced with the private key from step 1.
3. The subject sends the CSR to the CA, and it gets back a signature certificate. The signature certificate includes a key usage of digitalSignature, nonRepudiation, or both (see ).
4. The subject generates the key establishment key pair.
5. The subject composes a PKCS#10 CSR containing the key establishment public key. The CSR attributes include the attribute specified in of this document. The subject name matches the one from step 3. The CSR includes a signature that is produced with the private key from step 1.
6. The subject sends the CSR to the CA, and it gets back a key establishment certificate. The key establishment certificate includes a key usage of keyEncipherment or keyAgreement (see ).

In general, the issuer of the key establishment certificate will be the same as the issuer of the signature certificate. If the issuers of the two certificates will be different, then the certificate policy of the issuer of the key establishment certificate MUST explain the procedure that is used to verify the subject and subject alternative names.

Attribute for Statement of Possession of a Private Key The attribute for statement of possession of a private key is included in a certificate request to make the following statement: The subject of the signature certificate that is used to validate the signature on this certificate request states, without providing proof, that it has possession of the private key that corresponds to the public key in the certificate request. The CA MUST perform certification path validation for the signature certificate as specified in . If the certification path is not valid, then the CA MUST reject the request for the key establishment certificate. The CA MUST validate the signature on the certificate request using the public key from the signature certificate. If the signature is not valid, then the CA MUST reject the certificate request. The subject in the signature certificate SHOULD be the same as the subject name in the certificate request. If they are different, the certificate policy MUST describe how the CA can determine that the two subject names identify the same entity. If the CA is unable to determine that the two subject names identify the same entity, then the CA MUST reject the certificate request. If subject alternative names are present in the certificate request, they SHOULD match subject alternative names in the signature certificate. If they are different, the certificate policy MUST describe how the CA can determine that the two subject alternative names identify the same entity. If the CA is unable to determine that each of subject alternative names identifies the same entity as is named in the signature certificate, then the CA MUST reject the certificate request. When the CA rejects a certificate request for any of the reasons listed above, the CA should provide information to the requestor about the reason for the rejection to aid with diagnostic efforts. Likewise, the CA should log the rejection events. The attribute for statement of possession of a private key has the following structure: The components of the PrivateKeyStatement SEQUENCE have the following semantics:

signer:

The issuer name and certificate serial number of the signature certificate.

cert:

The signature certificate. If the issuer of the key establishment certificate will be the same as the issuer of the signature certificate, then this component MAY be omitted. When the signature certificate is omitted, the signer is assuming that the CA has a mechanism to obtain all valid certificates that it issued.

Conventions for PKCS#10 This section specifies the conventions for using the attribute for statement of possession of a private key with PKCS#10 when requesting a key establishment certificate. The PKCS#10 CertificationRequest always has three components, as follows:

certificationRequestInfo:

The subject name SHOULD be the same as the subject name in the signature certificate, the subjectPKInfo MUST contain the public key for the key establishment algorithm, and the attributes MUST include privateKeyPossessionStatement attribute as specified in of this document.

signatureAlgorithm:

The signature algorithm MUST be one that can be validated with the public key in the signature certificate.

signature:

The signature over certificationRequestInfo MUST validate with the public key in the signature certificate, and certification path validation for the signature certificate MUST be successful as specified in .

Conventions for CRMF This section specifies the conventions for using the attribute for statement of possession of a private key with the CRMF when requesting a key establishment certificate. The following ASN.1 types are defined for use with CRMF. They have exactly the same semantics and syntax as the attribute discussed above, but they offer a similar naming convention to the Registration Controls in . The CRMF CertificationRequest always has three components, as follows:

certReq:

The certTemplate MUST include the subject and the publicKey components. The same subject name SHOULD match the subject name in the signature certificate, and publicKey MUST contain the public key for the key establishment algorithm.

popo:

The ProofOfPossession MUST use the signature CHOICE, the poposkInput MUST be present, POPOSigningKeyInput.authInfo MUST use the sender CHOICE, the sender SHOULD be set to the subject name that appears in the signature certificate, the publicKey MUST contain a copy of the public key from the certTemplate, the algorithmIdentifier MUST identify a signature algorithm that can be validated with the public key in the signature certificate, the signature over the poposkInput MUST validate with the public key in the signature certificate, and certification path validation for the signature certificate MUST be successful as specified in .

regInfo:

The attributes MUST include the privateKeyPossessionStatement attribute as specified in of this document.

Security Considerations The privateKeyPossessionStatement attribute MUST NOT be used to obtain a signature certificate. Performing proof of possession of the signature private key is easily accomplished by signing the certificate request. The subject is signing the privateKeyPossessionStatement attribute to tell the CA that it has possession of the key establishment private key. This is being done instead of providing technical proof of possession. If the subject has lost control of the signature private key, then the signed privateKeyPossessionStatement attribute could be generated by some other party. Timely revocation of the compromised signature certificate is the only protection against such loss of control. If the CA revokes a compromised signature certificate, then the CA SHOULD also revoke all key establishment certificates that were obtained with privateKeyPossessionStatement attributes signed by that compromised signature certificate. The signature key pair and the key establishment key pair are expected to have roughly the same security strength. To ensure that the signature on the statement is not the weakest part of the certificate enrollment, the signature key pair SHOULD be at least as strong as the key establishment key pair. If a CA allows a subject in the key establishment certificate to be different than the subject name in the signature certificate, then certificate policy MUST describe how to determine that the two subject names identify the same entity. Likewise, if a CA allows subject alternative names in the key establishment certificate that are not present in the signature certificate, then certificate policy MUST describe how to determine that the subject alternative names identify the same entity as is named in the signature certificate.

IANA Considerations For the ASN.1 Module in of this document, IANA has assigned an object identifier (OID) for the module identifier (118) with a Description of "id-mod-private-key-possession-stmt-2025" in the "SMI Security for PKIX Module Identifier" registry (1.3.6.1.5.5.7.0).

References Normative References ITU-T ITU-T Informative References

ASN.1 Module This ASN.1 Module uses the conventions established by and .

Example Use of the privateKeyPossessionStatement Attribute In this example, the self-signed certificate for the CA is as follows: Alice generates her ECDSA signature key pair. Then, Alice composes a PKCS#10 Certificate Signing Request (CSR) in the usual manner as specified in . The CSR includes a signature that is produced with her ECDSA private key. The CSR is as follows: The CA issues a signature certificate to Alice: Alice generates her ECDH key establishment key pair. Then, Alice composes a PKCS#10 CSR. The CSR attributes include the privateKeyPossessionStatement attribute, which points to her ECDSA signature certificate. The CSR includes her ECDH public key and a signature that is produced with her ECDSA private key. The CSR is as follows: The CSR decodes to the following: The CA issues a key establishment certificate to Alice:

Acknowledgements Thanks to , , , , , , , , , , and for their constructive comments.