Certificate signing request

In public key infrastructure (PKI) systems, a certificate signing request (CSR or certification request) is a message sent from an applicant to a certificate authority of the public key infrastructure (PKI) in order to apply for a digital identity certificate. The CSR usually contains the public key for which the certificate should be issued, identifying information (such as a domain name) and a proof of authenticity including integrity protection (e.g., a digital signature). The most common format for CSRs is the PKCS #10 specification; others include the more capable Certificate Request Message Format (CRMF) and the SPKAC (Signed Public Key and Challenge) format generated by some web browsers.
Before creating a CSR for an X.509 certificate, the applicant generates a key pair, keeping the private key of that pair secret, e.g.:
The CSR contains information identifying the applicant (such as a distinguished name), the public key chosen by the applicant, and possibly further information. When using the PKCS #10 format, the request must be self-signed using the applicant's private key, which provides proof of possession (POP) of the private key but limits the use of this format to keys that can be used for (some form of) signing.
The CSR must be accompanied by a proof of origin (i.e., proof of identity of the applicant), which is required for security reasons by the certificate authority. The certificate authority may contact the applicant for further information.
Typical information required in a CSR (sample column from sample X.509 certificate). Note that there are often alternatives for the Distinguished Names (DN), the preferred value is listed.
This OpenSSL sample command line uses the details as listed in the table above to create a CSR in PKCS #10 format:
The CSR is typically sent to a Registration Authority (RA), which checks the CSR contents and authenticates the applicant. On success the CSR is forwarded to a Certificate Authority (CA), which produces the X.509 public-key certificate, digitally signing it using the CA private key, and sends the new certificate to the applicant.
A certification request in PKCS #10 format consists of three main parts: the certification request information, a signature algorithm identifier, and a digital signature on the certification request information using the private key related to the public key being certified.
The signature constitutes a self-signature with the key pair of the applicant. Due to the self-signature requirement, this format is applicable only to types of keys that support signing. Yet there are variants of this format that do not include an actual signature, such as described in Appendix C.1 of RFC 5272 (CMS).
The first part contains as its most significant information the public key and the identity of the applicant. The self-signature by the applicant provides a proof of possession (POP). Checking the POP prevents an entity from requesting a bogus certificate of someone else's public key. Thus the private key is required to produce a PKCS #10 CSR. Yet note that the POP for the key pair by the subject entity does not provide any authentication of the subject entity. The proof of origin for the request by the applicant must therefore be provided and checked by other means. Otherwise illegitimate certificates can be produced where the subject/holder information is wrong.
CSR for personal ID certificates and signing certificates usually includes the email address of the ID holder or the name of organisation in case of business ID.
The first part, ASN.1 type CertificationRequestInfo, consists of a version number (which is 0 for all known versions, 1.0, 1.5, and 1.7 of the specifications), the subject name, the public key (algorithm identifier + bit string), and a collection of attributes providing additional information about the subject of the certificate. The attributes can contain required certificate extensions, a challenge-password to restrict revocations, as well as any additional information about the subject of the certificate, possibly including local or future types.
The PKCS#10 standard defines syntax, semantics, and binary format for CSRs for use with X.509. It is encoded in ASN.1 with DER format. Here is an example of how you can examine its ASN.1 structure using OpenSSL:
A CSR may be represented as a Base64 encoded PKCS#10; an example of which is given below:
The above certificate signing request's ASN.1 DER structure (as parsed by openssl) appears as the following, where the first number is the byte offset, d=depth, hl=header length of the current type, l=length of content:
This was generated by supplying the base64 encoding into the command openssl asn1parse -in your_request.p10 -inform PEM -i where PEM (Privacy-Enhanced Mail) is the encoding of the ASN.1 Distinguished Encoding Rules in base64.