How to sign cert with an arbitrary or deprecated extension

Question

For example say I want to sign a cert with an arbitrary or deprecated extension (nsCertType for example): https://www.openssl.org/docs/manmaster/man5/x509v3_config.html

I believe I'm supposed to add the arbitrary extension as part of the certificate as per below but how / where do you discover the asn1 object identifier? I've read more documentation that I care to admit today and am still stumped.

tmpl := &x509.Certificate{ SerialNumber: big.NewInt(time.Now().Unix()*1000), Subject: pkix.Name{CommonName: "edgeproxy", Organization: []string{"edgeproxy"}}, NotBefore: now, NotAfter: now.Add(caMaxAge), ExtraExtensions: []pkix.Extension{ { Id: asn1.ObjectIdentifier{}, //what goes here Critical: false, []byte("sslCA"), }, }, ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth,x509.ExtKeyUsageClientAuth,x509.ExtKeyUsageEmailProtection, x509.ExtKeyUsageTimeStamping, x509.ExtKeyUsageMicrosoftCommercialCodeSigning, x509.ExtKeyUsageMicrosoftServerGatedCrypto, x509.ExtKeyUsageNetscapeServerGatedCrypto} , KeyUsage: x509.KeyUsageCRLSign | x509.KeyUsageCertSign, IsCA: true, BasicConstraintsValid: true, }

In python I would do this but don't know how to port this into go (which is what I'm doing at the end of the day):

 OpenSSL.crypto.X509Extension( b"nsCertType", False, b"sslCA" ),

Thanks! Already found what I needed! Want to make that an answer and I will accept it? — Jesse
– Jesse, Commented Sep 18, 2019 at 22:31

Patrick Mevzek · Accepted Answer · 2019-09-18 23:13:38Z

Go sources at https://golang.org/src/encoding/asn1/asn1.go define:

// An ObjectIdentifier represents an ASN.1 OBJECT IDENTIFIER. type ObjectIdentifier []int

So the object identifier (OID for short) is an array of integers. The asn1 module has methods to parse them, like parseObjectIdentifier.

This is the structure you need to put after the Id: attribute.

But now you need to find out the OID you want.

While difficult to read, OpenSSL source code can show you OIDs of many things in the X.400/X.500/X.509 worlds, or at least those known by OpenSSL.

If you go to https://github.com/openssl/openssl/blob/1aec7716c1c5fccf605a46252a46ea468e684454/crypto/objects/obj_dat.h

and searching on nsCertType you get:

{"nsCertType", "Netscape Cert Type", NID_netscape_cert_type, 9, &so[407]},

so is defined previously, and if you jump at its 407^th item you see:

 0x60,0x86,0x48,0x01,0x86,0xF8,0x42,0x01,0x01, /* [ 407] OBJ_netscape_cert_type */

and doing a final search on OBJ_netscape_cert_type in same file gives:

 71, /* OBJ_netscape_cert_type 2 16 840 1 113730 1 1 */

which means the corresponding OID is 2.16.840.1.113730.1.1

Or you can decode the above list of integers that describe this OID (see How does ASN.1 encode an object identifier? for details).

first 0x60 is 96₁₀ so 2*40 + 16, which means the OID starts with 2.16.
then each other one is in "base128" form: if most significant bit is 1 combine the 7 least significant bits together of all following numbers until one has 0 as most significant bit
0x86 is 10000110₂ so has to go with 0x48 aka 01001000₂ so it is in fact 00001101001000₂ or 840₁₀
0x01 is less than 128 so it is itself, 1
0x86 is still 10000110₂ but has to be paired with both 0xF8 (11111000₂) and 0x42 (01000010₂ and we stop here since first bit is 0) so 000011011110001000010₂ altogether or 113730₁₀
and the two last 0x01 are themselves, 1.

so we do get again 2.16.840.1.113730.1.1

You can double check it at some online OID browser like here: http://oid-info.com/cgi-bin/display?oid=2.16.840.1.113730.1.1&action=display that gives the following description for it:

Netscape certificate type (a Rec. ITU-T X.509 v3 certificate extension used to identify whether the certificate subject is a Secure Sockets Layer (SSL) client, an SSL server or a Certificate Authority (CA))

You can then even browse various arcs, like the netscape one, or others, to find out other OIDs.

You also get the full ASN.1 notation:

{joint-iso-itu-t(2) country(16) us(840) organization(1) netscape(113730) cert-ext(1) cert-type(1)}

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