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)^[1] 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 first generates a key pair, keeping the private key of that pair secret, e.g.:

# https://www.openssl.org/docs/manmaster/man1/openssl-genrsa.html # "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:

# https://www.openssl.org/docs/manmaster/man1/openssl-req.html # "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.

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.^[3] Thus the private key is needed to produce a PKCS #10 CSR, but it is not part of, the CSR.^[4]

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.^[3]

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:

-----BEGIN CERTIFICATE REQUEST----- MIICzDCCAbQCAQAwgYYxCzAJBgNVBAYTAkVOMQ0wCwYDVQQIDARub25lMQ0wCwYD VQQHDARub25lMRIwEAYDVQQKDAlXaWtpcGVkaWExDTALBgNVBAsMBG5vbmUxGDAW BgNVBAMMDyoud2lraXBlZGlhLm9yZzEcMBoGCSqGSIb3DQEJARYNbm9uZUBub25l LmNvbTCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAMP/U8RlcCD6E8AL PT8LLUR9ygyygPCaSmIEC8zXGJung3ykElXFRz/Jc/bu0hxCxi2YDz5IjxBBOpB/ kieG83HsSmZZtR+drZIQ6vOsr/ucvpnB9z4XzKuabNGZ5ZiTSQ9L7Mx8FzvUTq5y /ArIuM+FBeuno/IV8zvwAe/VRa8i0QjFXT9vBBp35aeatdnJ2ds50yKCsHHcjvtr 9/8zPVqqmhl2XFS3Qdqlsprzbgksom67OobJGjaV+fNHNQ0o/rzP//Pl3i7vvaEG 7Ff8tQhEwR9nJUR1T6Z7ln7S6cOr23YozgWVkEJ/dSr6LAopb+cZ88FzW5NszU6i 57HhA7ECAwEAAaAAMA0GCSqGSIb3DQEBBAUAA4IBAQBn8OCVOIx+n0AS6WbEmYDR SspR9xOCoOwYfamB+2Bpmt82R01zJ/kaqzUtZUjaGvQvAaz5lUwoMdaO0X7I5Xfl sllMFDaYoGD4Rru4s8gz2qG/QHWA8uPXzJVAj6X0olbIdLTEqTKsnBj4Zr1AJCNy /YcG4ouLJr140o26MhwBpoCRpPjAgdYMH60BYfnc4/DILxMVqR9xqK1s98d6Ob/+ 3wHFK+S7BRWrJQXcM8veAexXuk9lHQ+FgGfD0eSYGz0kyP26Qa2pLTwumjt+nBPl rfJxaLHwTQ/1988G0H35ED0f9Md5fzoKi5evU1wG5WRxdEUPyt3QUXxdQ69i0C+7 -----END CERTIFICATE REQUEST----- 

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         :[email protected]   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 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.