subtle.exportKey(format, key)

• format: <string> 必须是 'raw'、'pkcs8'、'spki' 或 'jwk' 之一。
• key: <CryptoKey>
• 返回：<Promise> 使用 <ArrayBuffer> | <Object> 完成。

如果支持，将给定的密钥导出为指定的格式。

【Exports the given key into the specified format, if supported.】

如果 <CryptoKey> 无法提取，返回的 Promise 将会被拒绝。

【If the <CryptoKey> is not extractable, the returned promise will reject.】

当 format 为 'pkcs8' 或 'spki' 且导出成功时，返回的 promise 将被解析为一个包含导出密钥数据的 <ArrayBuffer>。

【When format is either 'pkcs8' or 'spki' and the export is successful, the returned promise will be resolved with an <ArrayBuffer> containing the exported key data.】

当 format 为 'jwk' 且导出成功时，返回的 promise 将会以一个符合 JSON 网钥 规范的 JavaScript 对象来解析。

【When format is 'jwk' and the export is successful, the returned promise will be resolved with a JavaScript object conforming to the JSON Web Key specification.】

Key Type	'spki'	'pkcs8'	'jwk'	'raw'
'AES-CBC'			✔	✔
'AES-CTR'			✔	✔
'AES-GCM'			✔	✔
'AES-KW'			✔	✔
'ECDH'	✔	✔	✔	✔
'ECDSA'	✔	✔	✔	✔
'Ed25519'	✔	✔	✔	✔
'Ed448' 1	✔	✔	✔	✔
'HDKF'
'HMAC'			✔	✔
'PBKDF2'
'RSA-OAEP'	✔	✔	✔
'RSA-PSS'	✔	✔	✔
'RSASSA-PKCS1-v1_5'	✔	✔	✔