Certificate signing request

In public key infrastructure (PKI) systems, a certificate signing request (also CSR or certification request) is a message sent from an applicant to a certificate authority in order to apply for a digital identity certificate. The most common format for CSRs is the PKCS #10 specification and another is the Signed Public Key and Challenge Spkac format generated by some Web browsers.

Procedure

Before creating a CSR, the applicant first generates a key pair, keeping the private key secret. The CSR contains information identifying the applicant (such as a distinguished name in the case of an X.509 certificate) which must be signed using the applicant's private key. The CSR also contains the public key chosen by the applicant. The CSR may be accompanied by other credentials or proofs of identity required by the certificate authority, and the certificate authority may contact the applicant for further information.

Typical information required in a CSR::

Information	Description
Distinguished Name (DN)	This is fully qualified domain name that you wish to secure e.g. 'www.example.com’ or 'mail.example.com'. This includes the Common Name (CN) e.g. 'www' or 'mail'
Business name / Organization	Usually the legal incorporated name of a company and should include any suffixes such as Ltd., Inc., or Corp.
Department Name / Organizational Unit	e.g. HR, Finance, IT
Town/City	e.g. London, Waterford, Paris, New York, Dhaka, Kochi
Province, Region, County or State	This should not be abbreviated e.g. Sussex, Normandy, New Jersey
Country	The two-letter ISO code for the country where your organization is located e.g. GB, FR or US etc..
An email address	An email address to contact the organization. Usually the email address of the certificate administrator or IT department

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

Structure

The three main parts that a certification request consists of are 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.^[1] Thus the private key is needed to produce, but it is not part of, the CSR.^[2]

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.^[1]

Example

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

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= 413 cons: SEQUENCE          
    4:d=1  hl=4 l= 262 cons:  SEQUENCE          
    8:d=2  hl=2 l=   1 prim:   INTEGER           :00
   11:d=2  hl=2 l=  93 cons:   SEQUENCE          
   13:d=3  hl=2 l=  11 cons:    SET               
   15:d=4  hl=2 l=   9 cons:     SEQUENCE          
   17:d=5  hl=2 l=   3 prim:      OBJECT            :countryName
   22:d=5  hl=2 l=   2 prim:      PRINTABLESTRING   :SG
   26:d=3  hl=2 l=  17 cons:    SET               
   28:d=4  hl=2 l=  15 cons:     SEQUENCE          
   30:d=5  hl=2 l=   3 prim:      OBJECT            :organizationName
   35:d=5  hl=2 l=   8 prim:      PRINTABLESTRING   :M2Crypto
   45:d=3  hl=2 l=  18 cons:    SET               
   47:d=4  hl=2 l=  16 cons:     SEQUENCE          
   49:d=5  hl=2 l=   3 prim:      OBJECT            :commonName
   54:d=5  hl=2 l=   9 prim:      PRINTABLESTRING   :localhost
   65:d=3  hl=2 l=  39 cons:    SET               
   67:d=4  hl=2 l=  37 cons:     SEQUENCE          
   69:d=5  hl=2 l=   9 prim:      OBJECT            :emailAddress
   80:d=5  hl=2 l=  24 prim:      IA5STRING         :admin@server.example.dom
  106:d=2  hl=3 l= 159 cons:   SEQUENCE          
  109:d=3  hl=2 l=  13 cons:    SEQUENCE          
  111:d=4  hl=2 l=   9 prim:     OBJECT            :rsaEncryption
  122:d=4  hl=2 l=   0 prim:     NULL              
  124:d=3  hl=3 l= 141 prim:    BIT STRING        
  268:d=2  hl=2 l=   0 cons:   cont [ 0 ]        
  270:d=1  hl=2 l=  13 cons:  SEQUENCE          
  272:d=2  hl=2 l=   9 prim:   OBJECT            :md5WithRSAEncryption
  283:d=2  hl=2 l=   0 prim:   NULL              
  285:d=1  hl=3 l= 129 prim:  BIT STRING

This was generated by supplying the base64 encoding into the command openssl asn1parse -in your_request -inform PEM -i where PEM stands for Privacy-enhanced mail and describes the encoding of the ASN.1 Distinguished Encoding Rules in base64.

Tools

OpenSSL can decode a CSR locally, without transmitting sensitive information over unsecure networks.^[3]

Microsoft Windows OS versions newer than XP contain certutil.exe. Older OS versions may be able to install certutil.exe as part of another package, e.g. the Windows 2003 Server Service Pack 1 version of adminpak.

Many other programs that are capable of creating a CSR are also capable of decoding it locally into a human readable format.

Notes