我正在使用DKIM for JavaMail与DKIM签署外发邮件.
我的私有DKIM密钥是使用opendkim-genkey -s default -d example.com生成的,如下所示：

-----BEGIN RSA PRIVATE KEY-----
ABCCXQ...[long string]...SdQaZw9
-----END RSA PRIVATE KEY-----

DKIM for JavaMail库需要DER格式的私有DKIM密钥,如其自述文件中所述：

DKIM for JavaMail needs the private key in DER format, you can
transform a PEM key
with openssl:

openssl pkcs8 -topk8 -nocrypt -in private.key.pem -out private.key.der -outform der

我正在寻找一种方法来避免使用openssl将我的密钥转换为DER格式.相反,我想直接在Java中进行转换.
我尝试了不同的建议(1,2,3)但到目前为止还没有任何工作.
DKIM for Java处理DER文件,如下所示：

    File privKeyFile = new File(privkeyFilename);

    // read private key DER file
    DataInputStream dis = new DataInputStream(new FileInputStream(privKeyFile));
    byte[] privKeyBytes = new byte[(int) privKeyFile.length()];
    dis.read(privKeyBytes);
    dis.close();

    KeyFactory keyFactory = KeyFactory.getInstance("RSA");

    // decode private key
    PKCS8EncodedKeySpec privSpec = new PKCS8EncodedKeySpec(privKeyBytes);
    RSAPrivateKey privKey = (RSAPrivateKey) keyFactory.generatePrivate(privSpec);

所以我最终需要的是RSAPrivateKey.

如何从我的RSA私钥轻松生成DKIM for JavaMail所需的RSAPrivateKey？

解决方法:

您的参考3(仅)是正确的;因为它说你的问题不只是转换
PEM到DER(正如@Jim所说的基本上只是base64到二进制)但转换
PEM包含openssl“传统”或“遗留”或“PKCS#1”格式的密钥数据
DER包含PKCS#8(特别是PKCS#8清除/未加密)格式的密钥数据.

阿利斯泰尔的回答指出了http://juliusdavies.ca/commons-ssl/pkcs8.html
看起来可能有可能,但我没有详细检查.自PKCS#8起
RSA的clear(PrivateKeyInfo)只是围绕PKCS#1的简单ASN.1包装器,
以下(有点)快速和(非常)脏代码提供了最小的解决方案.
改变输入读取逻辑(和错误处理)以尝试并替换可用的base64解码器.

    BufferedReader br = new BufferedReader (new FileReader (oldpem_file));
    StringBuilder b64 = null;
    String line;
    while( (line = br.readLine()) != null )
        if( line.equals("-----BEGIN RSA PRIVATE KEY-----") )
            b64 = new StringBuilder ();
        else if( line.equals("-----END RSA PRIVATE KEY-----" ) )
            break;
        else if( b64 != null ) b64.append(line);
    br.close();
    if( b64 == null || line == null ) 
        throw new Exception ("didn't find RSA PRIVATE KEY block in input");

    // b64 now contains the base64 "body" of the PEM-PKCS#1 file
    byte[] oldder = Base64.decode (b64.toString().toCharArray());

    // concatenate the mostly-fixed prefix plus the PKCS#1 data 
    final byte[] prefix = {0x30,(byte)0x82,0,0, 2,1,0, // SEQUENCE(lenTBD) and version INTEGER 
            0x30,0x0d, 6,9,0x2a,(byte)0x86,0x48,(byte)0x86,(byte)0xf7,0x0d,1,1,1, 5,0, // AlgID for rsaEncryption,NULL
            4,(byte)0x82,0,0 }; // OCTETSTRING(lenTBD) 
    byte[] newder = new byte [prefix.length + oldder.length];
    System.arraycopy (prefix,0, newder,0, prefix.length);
    System.arraycopy (oldder,0, newder,prefix.length, oldder.length);
    // and patch the (variable) lengths to be correct
    int len = oldder.length, loc = prefix.length-2; 
    newder[loc] = (byte)(len>>8); newder[loc+1] = (byte)len;
    len = newder.length-4; loc = 2;
    newder[loc] = (byte)(len>>8); newder[loc+1] = (byte)len;

    FileOutputStream fo = new FileOutputStream (newder_file);
    fo.write (newder); fo.close();
    System.out.println ("converted length " + newder.length);

除此之外：我假设您发布的数据中的ABCC已被编辑.任何有效和合理的
PKCS#1(清除)RSA密钥必须以字节0x30 0x82 x开头,其中x为2到9;
当转换为base64时,必须从MIIC到MIIJ开始.