Problem providing ExtendedKeyUsage information to CSR during generation in golang

I've stumbled upon a strange problem. I'm writing a small golang tool, which generates a CSR based on some user-provided input. I'm mostly successful at achieving my goal, but have a problem with ExtendedKeyUsage. Simply put it does not work.
A bit of code with asn1 marshalling of x509 fields:

    var oidEmailAddress = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 9, 1}
    var OidExtensionKeyUsage = asn1.ObjectIdentifier{2, 5, 29, 15}
    var OidExtensionExtendedKeyUsage = asn1.ObjectIdentifier{2, 5, 29, 37}

    asn1KeyUsageDigSig, err := asn1.Marshal(asn1.BitString{
        Bytes:     []byte{byte(x509.KeyUsageDigitalSignature)},
        BitLength: 8,
    })
    asn1KeyUsageDatEnc, err := asn1.Marshal(asn1.BitString{
        Bytes:     []byte{byte(x509.KeyUsageDataEncipherment)},
        BitLength: 8,
    })
    asn1KeyUsageCAuth, err := asn1.Marshal(asn1.BitString{
        Bytes:     []byte{byte(x509.ExtKeyUsageClientAuth)},
        BitLength: 8,
    })

    if err != nil {
        Error.Fatalf("Can't serialize Extended Key Usage %s", err)
    }

Then I create a template and successful at generating and saving a CSR, well almost:


template := x509.CertificateRequest{
        RawSubject:         asn1Subj,
        EmailAddresses:     []string{emailAddress},
        SignatureAlgorithm: _sigAlg,
        ExtraExtensions: []pkix.Extension{
            {
                Id: OidExtensionExtendedKeyUsage,
                Value: asn1KeyUsageCAuth,
            },
            {
                Id:       OidExtensionKeyUsage,
                Critical: true,
                Value:    asn1KeyUsageDatEnc,
            },
            {
                Id:       OidExtensionKeyUsage,
                Critical: true,
                Value:    asn1KeyUsageDigSig,
            },
        },
    }

    csrBytes, _ := x509.CreateCertificateRequest(rand.Reader, &template, privateKey)

And here is an openssl req -in MY_OUTPUT.csr -text -noout

******
         ASN1 OID: prime256v1
                NIST CURVE: P-256
        Attributes:
        Requested Extensions:
            X509v3 Subject Alternative Name: 
                email:someone@somewhere.com
            X509v3 Extended Key Usage: 
                ....
            X509v3 Key Usage: critical
                Key Agreement
            X509v3 Key Usage: critical
                Encipher Only
    Signature Algorithm: ecdsa-with-SHA256

******

My ExtendedKeyUsage is empty, while it should be ClientAuthentication. What am I doing wrong?

I'm expecting to see:

X509v3 Extended Key Usage: ClientAuthentication

I'm seeing empty field. I tried using different set of bytes from another oid, but still nothing. It is as if ExtendedKeyUsage field doesn't allow anything to be written (while it should)

If it is imported:
go ver: go1.19.3 darwin/amd64

Solution

I think the problem is when you print the data. The key/value is actually present in the data.

From the code:

var OidExtensionExtendedKeyUsage = asn1.ObjectIdentifier{2, 5, 29, 37}

asn1KeyUsageCAuth, err := asn1.Marshal(asn1.BitString{
    Bytes:     []byte{byte(x509.ExtKeyUsageClientAuth)},
    BitLength: 8,
})

ExtraExtensions: []pkix.Extension{
    {
        Id: OidExtensionExtendedKeyUsage,
        //Critical: true,
        Value: asn1KeyUsageCAuth,
        //Value: {2, 5, 29, 15},
    },

OidExtensionExtendedKeyUsage is ASN.1 OID 2.5.29.37 and will be '55 1D 25' when encoded with a DER encoder

You can encode it online to see what binary it will produce (e.g. https://misc.daniel-marschall.de/asn.1/oid-converter/online.php)

asn1KeyUsageCAuth value is 2 (constant defined in x509.go) and will be '00 02' when encoded as ASN.1 BIT STRING with a DER encoder (first 00 is the number of padding bits (none), 02 is the value 2)

Now take the Base64 value of your Certificate Request and decode it with an ASN.1 DER Decoder (for example: https://asn1.io/asn1playground)

MIIBtzCCAV0CAQAwgZwxCzAJBgNVBAYTAkFVMQ8wDQYDVQQIEwZTeWRuZXkxDzAN BgNVBAcTBlN5ZG5leTETMBEGA1UEChMKc210aENsaWVudDELMAkGA1UECxMCSVQx JTAjBgNVBAMTHHNtdGgtQ2xpZW50LVk4cDg1bk1pSVNzMGlIZ0ExIjAgBgkqhkiG 9w0BCQEME3NtdGhjbGllbnRAc210aC5jb20wWTATBgcqhkjOPQIBBggqhkjOPQMB BwNCAAR4rIgUOxSyXdaML9F9e2gRjuMUK8Q0jiloTb2kAdmbz1RoCEdsZUUxKQcr 0Vud2aW3VIDph1ar4hkqWkM43hxqoF4wXAYJKoZIhvcNAQkOMU8wTTAeBgNVHREE FzAVgRNzbXRoY2xpZW50QHNtdGguY29tMAsGA1UdJQQEAwIAAjAOBgNVHQ8BAf8E BAMCAAgwDgYDVR0PAQH/BAQDAgABMAoGCCqGSM49BAMCA0gAMEUCIQDTBj+0Atjy F1GY8AM2Mv7/X3tsEbmMVdszKw8l6rVsEQIgMiH8CO9nkP0aXdMgp9x4kVjJZK9X rW3ROydT89D73OA=

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30 8201B7(439)
  30 82015D(349)
    02 01 00
    30 819C(156)
      31 0B
        30 09
          06 03 550406
          13 02 4155
      31 0F
        30 0D
          06 03 550408
          13 06 5379646E6579
      31 0F
        30 0D
          06 03 550407
          13 06 5379646E6579
      31 13
        30 11
          06 03 55040A
          13 0A 736D7468436C69656E74
      31 0B
        30 09
          06 03 55040B
          13 02 4954
      31 25
        30 23
          06 03 550403
          13 1C 736D74682D436C69656E742D59387038356E4D694953733069486741
      31 22
        30 20
          06 09 2A864886F70D010901
          0C 13 736D7468636C69656E7440736D74682E636F6D
    30 59
      30 13
        06 07 2A8648CE3D0201
        06 08 2A8648CE3D030107
      03 42 000478AC88143B14B25DD68C2FD17D7B68118EE3142BC4348E29684DBDA401D9...
    A0 5E
      30 5C
        06 09 2A864886F70D01090E
        31 4F
          30 4D
            30 1E
              06 03 551D11
              04 17 30158113736D7468636C69656E7440736D74682E636F6D
            30 0B -- HERE IT IS!
              06 03 551D25
              04 04 03020002
            30 0E
              06 03 551D0F
              01 01 FF
              04 04 03020008
            30 0E
              06 03 551D0F
              01 01 FF
              04 04 03020001
  30 0A
    06 08 2A8648CE3D040302
  03 48 003045022100D3063FB402D8F2175198F0033632FEFF5F7B6C11B98C55DB332B0F25...


Results
To get more details, please provide/compile a schema for your data.

Scroll down the output above until you find HERE IT IS!

Your key/value is:

30 0B    -- a SEQUENCE of 11 bytes
    06 03 551D25   -- an item of 3 bytes (551D25 ... OidExtensionExtendedKeyUsage)
    04 04 03020002 --  an item of 4 bytes (03 02 0002 ... an item of 2 bytes 0002 ... asn1KeyUsageCAuth)

I would have loved to decode the CSR againt the ASN.1 specification ... but I could not find it :(