Hello,
I don't know if it is possible to do what I want, so asking will save me some time. Maybe I am not understanding this correctly.
I have the exponent and the modulus of a RSA public Key. I have the values in hexadecimal.
Is it possible to create a public key .pem file (in Base64) from this to values? I would like to use this public key with openssl, and it accepts public keys in this format.
Thank you.
Here is a complete example
import java.io.*;
import java.math.BigInteger;
import java.security.*;
import java.security.interfaces.RSAPublicKey;
import java.security.spec.*;
public class MakePEMFromRSAPubkey
{
final private BigInteger modulus;
final private BigInteger pubExponent;
final private KeyFactory kf;
final private static String PEM_HEADER = "--BEGIN PUBLIC KEY--";
final private static String PEM_FOOTER = "--END PUBLIC KEY--";
public MakePEMFromRSAPubkey(final BigInteger modulus,
final BigInteger pubExponent) throws NoSuchAlgorithmException
{
this.modulus = modulus;
this.pubExponent = pubExponent;
kf = KeyFactory.getInstance("RSA");
}
public static void main(String[] args) throws InvalidKeySpecException,
NoSuchAlgorithmException, FileNotFoundException
{
BigInteger modulus = new BigInteger(
"DEA1A28CCAF2A670BCCCB567CAD3B44CBC420858FE7CC3DA187AE6BA9BDF3BAD3C68D203AE219895FB1CC19383F0E2662E09CDA6486F140DB39C81AC538940AB",
16);
BigInteger pubExponent = new BigInteger("10001", 16); // 65537
final String pemFileName = "example.pem";
MakePEMFromRSAPubkey keyMaker = new MakePEMFromRSAPubkey(modulus,
pubExponent);
keyMaker.writeKey(pemFileName);
}
private void writeKey(String pemFileName) throws InvalidKeySpecException,
FileNotFoundException
{
RSAPublicKeySpec rsaKS = new RSAPublicKeySpec(modulus, pubExponent);
RSAPublicKey rsaPubKey = (RSAPublicKey) kf.generatePublic(rsaKS);
byte[] encoded = rsaPubKey.getEncoded();
sun.misc.BASE64Encoder b64 = new sun.misc.BASE64Encoder();
String base64 = b64.encode(encoded);
PrintStream ps = new PrintStream(new FileOutputStream(pemFileName));
ps.println(PEM_HEADER);
ps.println(base64);
ps.println(PEM_FOOTER);
ps.close();
}
}
And some commands (DOS/Windows) showing the result
> type example.pem
--BEGIN PUBLIC KEY--
MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBAN6hoozK8qZwvMy1Z8rTtEy8QghY/nzD2hh65rqb3zut
PGjSA64hmJX7HMGTg/DiZi4JzaZIbxQNs5yBrFOJQKsCAwEAAQ==
--END PUBLIC KEY--
> openssl rsa -in example.pem -pubin -text -noout
Modulus (512 bit):
00:de:a1:a2:8c:ca:f2:a6:70:bc:cc:b5:67:ca:d3:
b4:4c:bc:42:08:58:fe:7c:c3:da:18:7a:e6:ba:9b:
df:3b:ad:3c:68:d2:03:ae:21:98:95:fb:1c:c1:93:
83:f0:e2:66:2e:09:cd:a6:48:6f:14:0d:b3:9c:81:
ac:53:89:40:ab
Exponent: 65537 (0x10001)