# Certificate Framework Overview


The certificate framework shields the differences between third-party certificate algorithm libraries. It provides the capabilities of parsing and validating a certificate, certificate extensions, and a certificate revocation list (CRL), and validating a certificate chain.


You can use the APIs provided by the certificate framework to easily complete your development.


> **NOTE**
>
> The certificate framework provides the capabilities of processing certificates and CRL data, but does not provide the capabilities of generating or issuing certificates or CRLs. Certificates and CRLs are issued by a certificate authority (CA) rather than a single application.


## Basic Concepts

The certificate framework provides APIs for parsing, serializing, signing an X.509 certificate, verifying the signature of an X.509 certificate, and operating X.509 CRLs and the certificate chain validator.

Before you get started, familiarity with the basic certificate concepts is helpful, which include but are not limited to the following:

- Digital certificate
- X.509 (also referred to as "X509" in this document)
- Certificate chain
- To Be Signed (TBS)
- CRL


## Certificate Specifications

The following describes certificate specifications.


### Certificate Chain Validation

The certificate chain validator does not verify the certificate validity period because the device system time is always untrusted. To check the validity of a certificate, use [checkValidityWithDate()](../../reference/apis-device-certificate-kit/js-apis-cert.md#checkvaliditywithdate) of **X509Cert**.


### Certificate Format

Currently, only the certificates in DER and PEM formats are supported.


### X.509 Certificate Structure 

![](figures/X509_certificate_structure.png)

Certificate example:

![](figures/certificate_example.png)


### X.509 CRL Structure

![](figures/CRL_structure.png)

CRL example:

![](figures/CRL_example.png)


## Constraints

The certificate framework depends on the basic algorithm capabilities of the Crypto Architecture Kit, which does not support multi-thread concurrent operations. For details, see [Constraints](../CryptoArchitectureKit/crypto-architecture-kit-intro.md#constraints).


## Development

You can use the certificate framework to implement the following functionalities. Before you start, be sure to familiarize yourself with [Certificate Specifications](#certificate-specifications).

- [Certificate Development](create-parse-verify-cert-object.md)
- [Certificate Extension Development](create-parse-verify-certextension-object.md)
- [CRL Development](create-parse-verify-crl-object.md)
- [Certificate Chain Validator Development](create-verify-cerchainvalidator-object.md)
- [Certificate and CRL Collection Development](create-get-cert-crl-object.md)
- [Certificate Chain Development](create-verify-certchain-object.md)
- [Creating a TrustAnchor Object Array from a p12 File](create-trustanchor-from-p12.md)

The following table lists the classes provided by the certificate framework. Based on the classes, you can learn the related APIs.

| Name| Class| Description|
| -------- | -------- | -------- |
| X.509 certificate| [X509Cert](../../reference/apis-device-certificate-kit/js-apis-cert.md#x509cert) | Provides APIs for parsing, serializing, and signing an X.509 certificate, verifying an X.509 certificate signature, and obtaining certificate information.|
| Certificate extension| [CertExtension](../../reference/apis-device-certificate-kit/js-apis-cert.md#certextension10) | Provides APIs for obtaining X.509 certificate extensions, such as CA and CRL distribution points.|
| X.509 CRL| [X509CRL](../../reference/apis-device-certificate-kit/js-apis-cert.md#x509crl11) | Provides APIs for parsing and serializing an X.509 CRL, and obtaining X.509 CRL information.|
| Certificate chain validator| [CertChainValidator](../../reference/apis-device-certificate-kit/js-apis-cert.md#certchainvalidator) | Provides APIs for validating a certificate chain (excluding the certificate validity period) and obtaining the certificate chain algorithm.|
| Certificate and CRL collection| [CertCRLCollection](../../reference/apis-device-certificate-kit/js-apis-cert.md#certcrlcollection11) | Provides APIs for obtaining information about the certificate and CRL collection.|
| X.509 certificate chain| [X509CertChain](../../reference/apis-device-certificate-kit/js-apis-cert.md#x509certchain11) | Provides APIs for obtaining the certificate list and validating a certificate chain.|