java - 我们如何将字符串从 PEM 格式转换为 DER 格式

Question

以以下格式发送字符串:

-----BEGIN RSA PUBLIC KEY-----
MIGHAoGBANAahj75ZIz9nXqW2H83nGcUao4wNyYZ9Z1kiNTUYQl7ob/RBmDzs5rY
mUahXAg0qyS7+a55eU/csShf5ATGzAXv+DDPcz8HrSTcHMEFpuyYooX6PrIZ07Ma
XtsJ2J4mhlySI5uOZVRDoaFY53MPQx5gud2quDz759IN/0gnDEEVAgED
-----END RSA PUBLIC KEY-----

我如何从这个字符串构造一个 PublicKey 对象？尝试过以下去掉页眉和页脚并base64解码缓冲区

public static PublicKey getFromString(String keystr) throws Exception
  {
  //String S1= asciiToHex(keystr);
   byte[] keyBytes = new sun.misc.BASE64Decoder().decodeBuffer(keystr);
   X509EncodedKeySpec spec =
       new X509EncodedKeySpec(keyBytes);
     KeyFactory kf = KeyFactory.getInstance("RSA");
     return kf.generatePublic(spec);

  }

这会因为无效的 key 格式而失败，或者会出现以下错误

java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: IOException: algid parse error, not a sequence
 at sun.security.rsa.RSAKeyFactory.engineGeneratePublic(RSAKeyFactory.java:188)
 at java.security.KeyFactory.generatePublic(KeyFactory.java:304)
 at PublicKeyReader.getFromString(PublicKeyReader.java:30)
 at Tst.main(Tst.java:36)

key 是通过 openSSL 的 API PEM_write_bio_RSAPublicKey(bio, rsa);

Answer 1

通过调用 PEM_write_bio_RSAPublicKey，只有 key 模数和公共(public)指数被编码到输出 PEM 数据中。然而 X509EncodedKeySpec预计此 ASN.1 key 格式:

 SubjectPublicKeyInfo ::= SEQUENCE {
   algorithm AlgorithmIdentifier,
   subjectPublicKey BIT STRING }

您应该使用 PEM_write_bio_PUBKEY 函数，该函数使用 X509EncodedKeySpec 预期的 SubjectPublicKeyInfo 结构对公钥进行编码

解码 key 的另一种可能解决方案。不幸的是，我认为仅使用标准 JDK API 是不可能的，但可以使用 Bouncycastle 来完成。图书馆

import org.bouncycastle.asn1.*;
import org.bouncycastle.asn1.x509.RSAPublicKeyStructure;

public static PublicKey getFromString(String keystr) throws Exception
{
  //String S1= asciiToHex(keystr);
   byte[] keyBytes = new sun.misc.BASE64Decoder().decodeBuffer(keystr);
   ASN1InputStream in = new ASN1InputStream(keyBytes);
   DERObject obj = in.readObject();
   RSAPublicKeyStructure pStruct = RSAPublicKeyStructure.getInstance(obj);
   RSAPublicKeySpec spec = new RSAPublicKeySpec(pStrcut.getModulus(), pStruct.getPublicExponent());
   KeyFactory kf = KeyFactory.getInstance("RSA");
   return kf.generatePublic(spec);
}