subtle.exportKey(format, key)
format：<string> 必须是'raw'、'pkcs8'、'spki'或'jwk'中的一个。key：<CryptoKey>- 返回：<Promise> 包含 <ArrayBuffer>。
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.
| 密钥类型 | 'spki' | 'pkcs8' | 'jwk' | 'raw' |
|---|---|---|---|---|
'AES-CBC' | ✔ | ✔ | ||
'AES-CTR' | ✔ | ✔ | ||
'AES-GCM' | ✔ | ✔ | ||
'AES-KW' | ✔ | ✔ | ||
'ECDH' | ✔ | ✔ | ✔ | ✔ |
'ECDSA' | ✔ | ✔ | ✔ | ✔ |
'Ed25519'1 | ✔ | ✔ | ✔ | ✔ |
'Ed448'1 | ✔ | ✔ | ✔ | ✔ |
'HDKF' | ||||
'HMAC' | ✔ | ✔ | ||
'PBKDF2' | ||||
'RSA-OAEP' | ✔ | ✔ | ✔ | |
'RSA-PSS' | ✔ | ✔ | ✔ | |
'RSASSA-PKCS1-v1_5' | ✔ | ✔ | ✔ |