How can I extract the Subject DN from an X509 certificate in Go?

Extracting Subject DN from X509 Certificates in Go

Obtaining the complete subject or issuer DN from an X509 certificate in Go as a string can be a challenge. The pkix.Name type does not provide a simple String() method for retrieving the DN.

Solution Provided:

To address this, a custom function can be implemented to convert the pkix.Name into a string representation of the DN:

<code class="go">import (
    "fmt"
    "strings"

    pkix "github.com/google/certificate-transparency-go/x509"
)

var oid = map[string]string{
    "2.5.4.3":                    "CN",
    "2.5.4.4":                    "SN",
    "2.5.4.5":                    "serialNumber",
    "2.5.4.6":                    "C",
    "2.5.4.7":                    "L",
    "2.5.4.8":                    "ST",
    "2.5.4.9":                    "streetAddress",
    "2.5.4.10":                   "O",
    "2.5.4.11":                   "OU",
    "2.5.4.12":                   "title",
    "2.5.4.17":                   "postalCode",
    "2.5.4.42":                   "GN",
    "2.5.4.43":                   "initials",
    "2.5.4.44":                   "generationQualifier",
    "2.5.4.46":                   "dnQualifier",
    "2.5.4.65":                   "pseudonym",
    "0.9.2342.19200300.100.1.25": "DC",
    "1.2.840.113549.1.9.1":       "emailAddress",
    "0.9.2342.19200300.100.1.1":  "userid",
}

func getDNFromCert(namespace pkix.Name, sep string) (string, error) {
    subject := []string{}
    for _, s := range namespace.ToRDNSequence() {
        for _, i := range s {
            if v, ok := i.Value.(string); ok {
                if name, ok := oid[i.Type.String()]; ok {
                    // <oid name>=<value>
                    subject = append(subject, fmt.Sprintf("%s=%s", name, v))
                } else {
                    // <oid>=<value> if no <oid name> is found
                    subject = append(subject, fmt.Sprintf("%s=%s", i.Type.String(), v))
                }
            } else {
                // <oid>=<value in default format> if value is not string
                subject = append(subject, fmt.Sprintf("%s=%v", i.Type.String, v))
            }
        }
    }
    return sep + strings.Join(subject, sep), nil
}</code>

This function takes a pkix.Name instance and a separator as input, and combines the individual OID values and their corresponding values into a string representation of the DN.

Usage:

To obtain the subject DN:

<code class="go">subj, err := getDNFromCert(cert.Subject, "/")
if err != nil {
    // Error handling
}</code>

To obtain the issuer DN:

<code class="go">issuer, err := getDNFromCert(cert.Issuer, "/")
if err != nil {
    // Error handling
}</code>

The output will be a string representing the complete DN.

Example:

/CN=common name/OU=unit/O=some organization/C=US

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