golang subject dn from x509 cert

Question

Is there any easy way to get the complete subject DN (or issuer DN) from an x509 certificate in go as a string?

I was not able to find any methods like ".String()" in pkix.Name

Answer 1

Solution (thanks to a colleague):

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
}

calling the function:

subj, err := getDNFromCert(x509Cert.Subject, "/")
if err != nil {
   // do error handling
}
fmt.Println(subj)

output (example):

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

this seems to be the only "easy" solution

Answer 2

In order to get complete subject DN (or issuer DN) from an x509 certificate, you may use next code:

cert, err := x509.ParseCertificate(certData)
if err != nil {
    return err
}

var subject pkix.RDNSequence
if _, err := asn1.Unmarshal(cert.RawSubject, &subject); err != nil {
    return err
}

fmt.Plrintln(subject.String()

Similarly, if you need to get only some specific object value from the subject (or issuer) you may use next approach. Example below retrieves UID from subject (which is not defined in the stdlib https://github.com/golang/go/issues/25667)

// http://www.alvestrand.no/objectid/0.9.2342.19200300.100.1.1.html
const oidUserID = "0.9.2342.19200300.100.1.1"
var UID string

cert, err := x509.ParseCertificate(certData)
if err != nil {
    return err
}

// manually parsing the Certificate subject to get the
// UID field, which is being ignored by the stdlib
// https://github.com/golang/go/issues/25667
var subject pkix.RDNSequence
if _, err := asn1.Unmarshal(cert.RawSubject, &subject); err != nil {
    return err
}

for _, s := range subject {
    for _, i := range s {
        if i.Type.String() == oidUserID {
            if v, ok := i.Value.(string); ok {
                UID = v
            }
        }
    }
}

fmt.Println(UID)

UPDATE: Simplified way to get the UID, thanks to @FiloSottile:

// http://www.alvestrand.no/objectid/0.9.2342.19200300.100.1.1.html
var oidUserID = []int{0, 9, 2342, 19200300, 100, 1, 1}
var UID string

cert, err := x509.ParseCertificate(certData)
if err != nil {
    return err
}

// reading the UID from list of unprased 
// objects from Subject
for _, n := range cert.Subject.Names {
    if n.Type.Equal(oidUserID) {
        if v, ok := n.Value.(string); ok {
            UID = v
        }
    }
}

fmt.Println(UID)