http://blog.csdn.net/zhouyuqwert/article/details/8627952

一、DER编码密钥对

先说下DER编码，是因为JCE本身是支持DER编码密钥对的解析的，可以参见PKCS8EncodedKeySpec和X509EncodedKeySpec.

DER编码是ASN.1编码规则中的一个子集，具体格式如何编排没有去了解，但最终呈现肯定的是一堆有规律的二进制组合而成。

PKCS#8定义了私钥信息语法和加密私钥语法，而X509定义证书规范，通常都会用DER和PEM进行编码存储，而在JAVA中则使用的

DER。

接下来看看如果通过DER编码的密钥对分别获取JAVA的公私钥对象。

1.下面一段是生成私钥对象的，传入参数是DER编码的私钥内容。

 @Override
public PrivateKey generatePrivateKey(byte[] key) throws NoSuchAlgorithmException, InvalidKeySpecException {
KeySpec keySpec = new PKCS8EncodedKeySpec(key);
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
return keyFactory.generatePrivate(keySpec);
}

2.下面是生成公钥对象的，传入参数是DER编码公钥内容，可以看到和生成私钥的部分非常相似。

    public PublicKey geneneratePublicKey(byte[] key) throws InvalidKeySpecException, NoSuchAlgorithmException{
KeySpec keySpec = new X509EncodedKeySpec(key);
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
return keyFactory.generatePublic(keySpec);
}

二、PEM编码

PEM编码也是密钥对较常用的编码方式，openssl则是以PEM编码为主，相对DER对人可读性更强，以BASE64编码呈现，外围包上类似-----BEGIN RSA PRIVATE KEY-----。

JCE没有对PEM直接支持的方式，但是可以通过第三方包例如bouncycastle解析，当然如果想要自己理解pem编码结构，也可以自己写代码解析。

这里介绍下如何使用bouncycastle进行解析。

      FileInputStream fis = new FileInputStream("id_rsa");
byte[] key = PrivateKeyUtils.readStreamToBytes(fis);
Security.addProvider(new BouncyCastleProvider());
ByteArrayInputStream bais = new ByteArrayInputStream(key);
PEMReader reader = new PEMReader(new InputStreamReader(bais), new PasswordFinder() {
@Override
public char[] getPassword() {
return "".toCharArray();
}
});
KeyPair keyPair = (KeyPair) reader.readObject();
reader.close();
PublicKey pubk = keyPair.getPublic();
System.out.println(pubk);
PrivateKey prik = keyPair.getPrivate();
System.out.println(prik);
KeySpec keySpec = new X509EncodedKeySpec(pubk.getEncoded());
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
System.out.println(keyFactory.generatePublic(keySpec));
KeySpec keySpec2 = new PKCS8EncodedKeySpec(prik.getEncoded());
System.out.println(keyFactory.generatePrivate(keySpec2));

看下这个输出结果

RSA Public Key
modulus: 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
public exponent: 10001
RSA Private CRT Key
modulus: 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
public exponent: 10001
xxx
Sun RSA public key, 2048 bits
modulus: 25367925677263221630752072905429434117596189021449325931333193529363239091429133073657699480437320802724298965580526553453499379398405915207286949216370963889754756788008021698178495726807109888833039800230667805051637457878962812581009778614579379073430749907762728841603230968432191178635884450213875555645164935313884823663096624318071901833679005494934145072511042211644746801428698070096755012497436134537077746175344235590315572214836519284172251946833712681076781034466422251569387242330311670205489884189790153154281087401570994337126054046621401176808489895271448688335849540690562754961439975230588159770903
public exponent: 65537
Sun RSA private CRT key, 2048 bits
modulus:          25367925677263221630752072905429434117596189021449325931333193529363239091429133073657699480437320802724298965580526553453499379398405915207286949216370963889754756788008021698178495726807109888833039800230667805051637457878962812581009778614579379073430749907762728841603230968432191178635884450213875555645164935313884823663096624318071901833679005494934145072511042211644746801428698070096755012497436134537077746175344235590315572214836519284172251946833712681076781034466422251569387242330311670205489884189790153154281087401570994337126054046621401176808489895271448688335849540690562754961439975230588159770903
public exponent:  65537
xxx

中间内容太多，略去了一部分，看下重点的public exponent部分，发现不同，但其实是一个是10进制输出，一个是16进制输出，所以在这里提个醒，这里生成过程没有错的。

三、openssh公钥

很多SSH公钥习惯使用openssh格式的，下面介绍下openssh格式的公钥如何解析，目前好像是没有官方库或者第三方库提供支持的。

openssh公钥呈现形式如

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQCW6qYq6m8gVOWLyTB1JGl1aLrJDOCIfErXWNUsNeUXg4UdAtSbkiA+Ta9Nx6oMR4w+OkPbxyivnzkZt1YpmDxrm1z99z81/VyVw+lue+3neRjTgfGMascG+46b7DpEKLXlfS2hwOA+4ooRIeR+LbQZVovy5SP6ZTngskiqcySYqQ== RSA-1024

以ssh-rsa打头，描述“RSA-1024”结尾的形式，中间是Base64编码。

这里过滤掉除了Base64外的其他部分，解码Base64得到公钥二进制内容。

这里二进制编码格式如下：

前11字节固定

0 0 0 7  's' 's' 'h' '-' ‘r' 's' 'a'

紧接着4个字节为一个int值，表示public exponent所占字节长度

可通过移位符及相加或者BigInteger方式实现转换。

  public static int decodeUInt32(byte[] key, int start_index){
byte[] test = Arrays.copyOfRange(key, start_index, start_index + 4);
return new BigInteger(test).intValue();
//      int int_24 = (key[start_index++] << 24) & 0xff;
//      int int_16 = (key[start_index++] << 16) & 0xff;
//      int int_8 = (key[start_index++] << 8) & 0xff;
//      int int_0 = key[start_index++] & 0xff;
//      return int_24 + int_16 + int_8 + int_0;
}

得到长度后，再从后一字节开始读取该长度字节作为public exponent的值，构造相应BigInteger。

再紧接着4个字节也是一个int值，表示modulus所占字节长度，同理转换得到长度。

再根据长度读取字节数组得到modulus值即可。

相应代码如下

   public static RSAPublicKey decodePublicKey(byte[] key) throws NoSuchAlgorithmException, InvalidKeySpecException{
byte[] sshrsa = new byte[] { 0, 0, 0, 7, 's', 's', 'h', '-', 'r', 's',
'a' };
int start_index = sshrsa.length;
/* Decode the public exponent */
int len = decodeUInt32(key, start_index);
start_index += 4;
byte[] pe_b = new byte[len];
for(int i= 0 ; i < len; i++){
pe_b[i] = key[start_index++];
}
BigInteger pe = new BigInteger(pe_b);
/* Decode the modulus */
len = decodeUInt32(key, start_index);
start_index += 4;
byte[] md_b = new byte[len];
for(int i = 0 ; i < len; i++){
md_b[i] = key[start_index++];
}
BigInteger md = new BigInteger(md_b);
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
KeySpec ks = new RSAPublicKeySpec(md, pe);
return (RSAPublicKey) keyFactory.generatePublic(ks);
}

四、其他编码