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.

Procedure
Before creating a CSR for an X.509 certificate, the applicant first generates a key pair, keeping the private key of that pair secret, e.g.:


 * 1) https://www.openssl.org/docs/manmaster/man1/openssl-genrsa.html
 * 2) "openssl genrsa" creates an RSA private key:

$ openssl genrsa -out 2024_wikipedia.org.key

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 of the private key but limits the use of this format to keys that can be used for (some form of) signing. The CSR should be accompanied by a proof of origin (i.e., proof of identity of the applicant) that is required by the certificate authority, and 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 sample command line uses the details as listed in the table above:


 * 1) https://www.openssl.org/docs/manmaster/man1/openssl-req.html
 * 2) "openssl req" creates a signing request:

$ openssl req -sha512 -new -subj "/C=US/ST=California/L=San Francisco/O=Wikimedia Foundation, Inc./CN=*.wikipedia.org" -key 2024_wikipedia.org.key -out 2024_wikipedia.org.csr

If the request is successful, the certificate authority will send back an identity certificate that has been digitally signed using the private key of the certificate authority.

Structure of a PKCS #10 CSR
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. The first part contains the significant information, including the public key. The signature by the requester prevents an entity from requesting a bogus certificate of someone else's public key. Thus the private key is needed to produce a PKCS #10 CSR, but it is not part of, the CSR.

CSR for personal ID certificates and signing certificates must have the email address of the ID holder or 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.

Example of a PKCS #10 CSR
The PKCS#10 standard defines a binary format for encoding CSRs for use with X.509. It is expressed in ASN.1. Here is an example of how you can examine its ASN.1 structure using OpenSSL:

openssl asn1parse -i -in your_request.p10

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 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: 0:d=0 hl=4 l= 716 cons: SEQUENCE 4:d=1 hl=4 l= 436 cons:  SEQUENCE 8:d=2 hl=2 l=   1 prim:   INTEGER           :00 11:d=2 hl=3 l= 134 cons:   SEQUENCE 14:d=3 hl=2 l=  11 cons:    SET 16:d=4 hl=2 l=   9 cons:     SEQUENCE 18:d=5 hl=2 l=   3 prim:      OBJECT            :countryName 23:d=5 hl=2 l=   2 prim:      PRINTABLESTRING   :EN 27:d=3 hl=2 l=  13 cons:    SET 29:d=4 hl=2 l=  11 cons:     SEQUENCE 31:d=5 hl=2 l=   3 prim:      OBJECT            :stateOrProvinceName 36:d=5 hl=2 l=   4 prim:      UTF8STRING        :none 42:d=3 hl=2 l=  13 cons:    SET 44:d=4 hl=2 l=  11 cons:     SEQUENCE 46:d=5 hl=2 l=   3 prim:      OBJECT            :localityName 51:d=5 hl=2 l=   4 prim:      UTF8STRING        :none 57:d=3 hl=2 l=  18 cons:    SET 59:d=4 hl=2 l=  16 cons:     SEQUENCE 61:d=5 hl=2 l=   3 prim:      OBJECT            :organizationName 66:d=5 hl=2 l=   9 prim:      UTF8STRING        :Wikipedia 77:d=3 hl=2 l=  13 cons:    SET 79:d=4 hl=2 l=  11 cons:     SEQUENCE 81:d=5 hl=2 l=   3 prim:      OBJECT            :organizationalUnitName 86:d=5 hl=2 l=   4 prim:      UTF8STRING        :none 92:d=3 hl=2 l=  24 cons:    SET 94:d=4 hl=2 l=  22 cons:     SEQUENCE 96:d=5 hl=2 l=   3 prim:      OBJECT            :commonName 101:d=5 hl=2 l=  15 prim:      UTF8STRING        :*.wikipedia.org 118:d=3 hl=2 l=  28 cons:    SET 120:d=4 hl=2 l=  26 cons:     SEQUENCE 122:d=5 hl=2 l=   9 prim:      OBJECT            :emailAddress 133:d=5 hl=2 l=  13 prim:      IA5STRING         :none@none.com 148:d=2 hl=4 l= 290 cons:   SEQUENCE 152:d=3 hl=2 l=  13 cons:    SEQUENCE 154:d=4 hl=2 l=   9 prim:     OBJECT            :rsaEncryption 165:d=4 hl=2 l=   0 prim:     NULL 167:d=3 hl=4 l= 271 prim:    BIT STRING 442:d=2 hl=2 l=   0 cons:   cont [ 0 ] 444:d=1 hl=2 l=  13 cons:  SEQUENCE 446:d=2 hl=2 l=   9 prim:   OBJECT            :md5WithRSAEncryption 457:d=2 hl=2 l=   0 prim:   NULL 459:d=1 hl=4 l= 257 prim:  BIT STRING

This was generated by supplying the base64 encoding into the command  where PEM (Privacy-Enhanced Mail) is the encoding of the ASN.1 Distinguished Encoding Rules in base64.