1# Certificate Extension Development
2
3
4This topic walks you through on how to create a certificate extension (**CertExtension**) object, obtain the certificate extension information based on an object identifier (OID), and check whether the certificate is a CA certificate.
5
6
7## How to Develop
8
91. Import the [cert](../../reference/apis-device-certificate-kit/js-apis-cert.md) module.
10   ```ts
11   import { cert } from '@kit.DeviceCertificateKit';
12   ```
13
142. Use [cert.createCertExtension](../../reference/apis-device-certificate-kit/js-apis-cert.md#certcreatecertextension10) to create a **CertExtension** object.
15
163. Use [CertExtension.getEntry](../../reference/apis-device-certificate-kit/js-apis-cert.md#getentry10) to obtain the certificate extension of the specified OID.
17
184. Use [CertExtension.checkCA](../../reference/apis-device-certificate-kit/js-apis-cert.md#checkca10) to check whether the certificate is a CA certificate.
19
20```ts
21import { cert } from '@kit.DeviceCertificateKit';
22import { BusinessError } from '@kit.BasicServicesKit';
23import { util } from '@kit.ArkTS';
24
25// Certificate extension data. The following is only an example.
26let extData = new Uint8Array([
27  0x30, 0x40, 0x30, 0x0F, 0x06, 0x03, 0x55, 0x1D,
28  0x13, 0x01, 0x01, 0xFF, 0x04, 0x05, 0x30, 0x03,
29  0x01, 0x01, 0xFF, 0x30, 0x0E, 0x06, 0x03, 0x55,
30  0x1D, 0x0F, 0x01, 0x01, 0xFF, 0x04, 0x04, 0x03,
31  0x02, 0x01, 0xC6, 0x30, 0x1D, 0x06, 0x03, 0x55,
32  0x1D, 0x0E, 0x04, 0x16, 0x04, 0x14, 0xE0, 0x8C,
33  0x9B, 0xDB, 0x25, 0x49, 0xB3, 0xF1, 0x7C, 0x86,
34  0xD6, 0xB2, 0x42, 0x87, 0x0B, 0xD0, 0x6B, 0xA0,
35  0xD9, 0xE4
36]);
37
38// Certificate extension example.
39function certExtensionSample(): void {
40  let textEncoder = new util.TextEncoder();
41  let encodingBlob: cert.EncodingBlob = {
42    data: extData,
43    // Certificate extension format. Currently, only the DER format is supported.
44    encodingFormat: cert.EncodingFormat.FORMAT_DER
45  };
46
47  // Create a CertExtension object.
48  cert.createCertExtension(encodingBlob, (err, certExtension) => {
49    if (err != null) {
50      // The CertExtension object fails to be created.
51      console.error(`createCertExtension failed, errCode:${err.code}, errMsg:${err.message} `);
52      return;
53    }
54    // The CertExtension object is created.
55    console.log('createCertExtension success');
56
57    try {
58      // Obtain the certificate extension information based on an OID.
59      let oidData = '2.5.29.14';
60      let oid: cert.DataBlob = {
61        data: textEncoder.encodeInto(oidData),
62      }
63      let entry = certExtension.getEntry(cert.ExtensionEntryType.EXTENSION_ENTRY_TYPE_ENTRY, oid);
64
65      // Check whether the certificate is a CA certificate.
66      let pathLen = certExtension.checkCA();
67      console.log('test cert extension success');
68    } catch (err) {
69      let e: BusinessError = err as BusinessError;
70      console.error(`operation failed, message:${e.message} ,code:${e.code} `);
71    }
72  });
73}
74```
75