Question

java中如何得到公钥的key size

File file = new File("d:\\cert\\test.cer");
byte[] certBytes = new byte[(int) file.length()];
FileInputStream input = new FileInputStream(file);
while (input.read(certBytes) != -1) {
}
X509Certificate cert1=(X509Certificate)pfCoder.getCertificate(certBytes);
PublicKey publickey=cert1.getPublicKey();

/**
* 得到证书
*
* @param certBytes
* @return
* @throws Exception
*/
public Certificate getCertificate(byte[] certBytes) throws Exception {
InputStream in = new ByteArrayInputStream(certBytes);
CertificateFactory cf = CertificateFactory.getInstance(X509);
return cf.generateCertificate(in);
}
怎么判断PublicKey是1024bits 还是2048bits啊，求，。。。。。。。

Answer 1

接你代码后面

String algorithm = publickey.getAlgorithm(); // 获取算法
KeyFactory keyFact = KeyFactory.getInstance(algorithm);
BigInteger prime = null;
if ("RSA".equals(algorithm)) { // 如果是RSA加密
	RSAPublicKeySpec keySpec = (RSAPublicKeySpec)keyFact.getKeySpec(publickey, RSAPublicKeySpec.class);
	prime = keySpec.getModulus();
} else if ("DSA".equals(algorithm)) { // 如果是DSA加密
	DSAPublicKeySpec keySpec = (DSAPublicKeySpec)keyFact.getKeySpec(publickey, DSAPublicKeySpec.class);
	prime = keySpec.getP();
}
int len = prime.toString(2).length(); // 转换为二进制，获取公钥长度

Answer 2

sun.security.util.KeyUtil.getKeySize(Key key), 支持RSA、DSA等算法的PrivateKey和PublicKey

Answer 3

X509Certificate的 toString()是这样的：

[
Version: V3
Subject: CN=XXXXXX, EMAILADDRESS=yyyyyyy@XXXXX.com, O=ZZZZZZZZZZ
Signature Algorithm: MD5withRSA, OID = 1.2.840.113549.1.1.4

Key: Sun RSA public key, 1024 bits

所以，可以知道是1024 bits

Answer 4

PublicKey的toString()的开头是这样的
Sun RSA public key, 1024 bits
modulus: 990645334079840363710713617513

所以，也能知道是1024 bits