Java解密电子邮件附件（.p7m文件）

我有一个.p7m格式的电子邮件附件和一个包含私钥和证书的.pem文件。 使用OpenSSL，我可以使用以下命令解密文件：

openssl smime -decrypt -inform DER -in fileToDecrypt.p7m -inkey privateKey.pem -out destinationFile

但是使用Java中的bouncycastle，我无法解密它。 我使用以下代码读取私钥：

    PEMReader pemReader = new PEMReader(new InputStreamReader(new FileInputStream(privateKeyName)));
    Object obj;
    PrivateKey key = null;
    X509Certificate cert1 = null;
    X509Certificate cert2 = null;

    obj = pemReader.readObject();
    if (obj instanceof PrivateKey) {
        key = (PrivateKey) obj;
        System.out.println("Private Key found");
    }
    obj = pemReader.readObject();
    if(obj instanceof X509Certificate){
        cert1 = (X509Certificate) obj;
        System.out.println("cert found");
    }
    obj = pemReader.readObject();
    if(obj instanceof X509Certificate){
        cert2 = (X509Certificate) obj;
        System.out.println("cert found");
    }

这将打印出：

Private Key Found
cert found
cert found

钥匙的类型为：

 System.out.println(key.getAlgorithm());
 System.out.println(cert1.getSigAlgName());
 System.out.println(cert2.getSigAlgName());

RSA
SHA256WithRSAEncryption
SHA256WithRSAEncryption

如果我尝试这样解密：

 Cipher cipher = Cipher.getInstance("RSA");
 cipher.init(Cipher.DECRYPT_MODE, key);
 Path path = Paths.get("fileToDecrypt.p7m");
 byte[] data = Files.readAllBytes(path);
 byte[] decryptedData = cipher.doFinal(data);

我得到：

javax.crypto.IllegalBlockSizeException:数据长度不得超过256字节

我有两个文件：

fileToDecrypt.p7m

privateKey.pem：包含RSA私钥和两个X508证书

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我不知道从哪里开始，用什么解密，如何解密？

问题的解决方案：

private static byte[] cmsDecrypt(byte[] message, PrivateKey key) throws
        Exception {
    CMSEnvelopedDataParser ep = new CMSEnvelopedDataParser(message);
    RecipientInformationStore recipients = ep.getRecipientInfos();
    Collection c = recipients.getRecipients();
    Iterator iter = c.iterator();
    RecipientInformation recipient = (RecipientInformation) iter.next();
    return recipient.getContent(key, new BouncyCastleProvider());
}

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问题的解决办法：

private static byte[] cmsDecrypt(byte[] message, PrivateKey key) throws
        Exception {
    CMSEnvelopedDataParser ep = new CMSEnvelopedDataParser(message);
    RecipientInformationStore recipients = ep.getRecipientInfos();
    Collection c = recipients.getRecipients();
    Iterator iter = c.iterator();
    RecipientInformation recipient = (RecipientInformation) iter.next();
    return recipient.getContent(key, new BouncyCastleProvider());
}

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