Golang生成CSR时ExtendedKeyUsage无法正常写入求助

我在编写一个基于用户输入生成CSR的小型Golang工具时遇到异常：ExtendedKeyUsage字段无法正常生效。使用Go版本为go1.19.3 darwin/amd64。

相关ASN.1编码及CSR模板代码如下：

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)
}

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)

通过openssl req -in MY_OUTPUT.csr -text -noout查看生成的CSR时，ExtendedKeyUsage字段为空，显示如下：

******
         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

******

预期该字段应显示为X509v3 Extended Key Usage: ClientAuthentication，但实际为空，尝试更换OID对应的字节也无效。

问题出在ExtendedKeyUsage的ASN.1编码格式错误，同时KeyUsage的使用方式也存在问题：

核心错误点： #

1. ExtendedKeyUsage编码错误：你误用asn1.BitString序列化x509.ExtKeyUsageClientAuth，但ExtendedKeyUsage的ASN.1定义是KeyPurposeId的序列（SEQUENCE），即多个OID组成的列表，而非单个OID转成的比特串。
2. KeyUsage重复添加：KeyUsage是一个包含多个权限的单比特串，不应拆分多次添加同名扩展。

修正后的代码： #

1. 正确序列化ExtendedKeyUsage：

// ExtendedKeyUsage需要序列化OID的序列
extendedKeyUsages := []asn1.ObjectIdentifier{x509.ExtKeyUsageClientAuth}
asn1ExtendedKeyUsage, err := asn1.Marshal(extendedKeyUsages)
if err != nil {
    log.Fatalf("Can't serialize Extended Key Usage %s", err)
}

2. 合并KeyUsage权限并序列化：

// 合并多个KeyUsage到单个比特串
keyUsage := x509.KeyUsageDigitalSignature | x509.KeyUsageDataEncipherment
asn1KeyUsage, err := asn1.Marshal(asn1.BitString{
    Bytes:     []byte{byte(keyUsage)},
    BitLength: 8,
})
if err != nil {
    log.Fatalf("Can't serialize Key Usage %s", err)
}

3. 更新CSR模板：

template := x509.CertificateRequest{
    RawSubject:         asn1Subj,
    EmailAddresses:     []string{emailAddress},
    SignatureAlgorithm: _sigAlg,
    ExtraExtensions: []pkix.Extension{
        {
            Id: OidExtensionExtendedKeyUsage,
            Value: asn1ExtendedKeyUsage,
        },
        {
            Id:       OidExtensionKeyUsage,
            Critical: true,
            Value:    asn1KeyUsage,
        },
    },
}

csrBytes, err := x509.CreateCertificateRequest(rand.Reader, &template, privateKey)
if err != nil {
    log.Fatalf("Failed to create CSR: %s", err)
}

验证结果： #

重新生成CSR后，执行openssl req -in MY_OUTPUT.csr -text -noout，ExtendedKeyUsage字段将正常显示：

X509v3 Extended Key Usage: 
    Client Authentication

内容的提问来源于stack exchange，提问作者suxarik