Get public key from private in Java

问题

I remember do this long time ago with OpenSSL, but I want to know if it's possible and how, I've never used Cryptography on java.

回答1:

You cannot generate either key directly from the other. It is mathematically impossible. If you had a key blob that contained both the public and private keys, you could extract either one of them with relative ease.

EDIT, 2017: Many years and a much better understanding of crypto later, and it's now clear to me that this answer isn't really correct.

To quote Wikipedia:

The public key consists of the modulus n and the public (or encryption) exponent e. The private key consists of the modulus n and the private (or decryption) exponent d, which must be kept secret. p, q, and λ(n) must also be kept secret because they can be used to calculate d.

The public modulus n can be computed as p × q. The only thing missing from a raw private key is e, but this value is usually selected as 65537, and if not you can still compute e from d and λ(n).

However, many private key storage formats actually contain the public modulus n alongside the other components, so you can just do a direct extraction of the values.

EDIT, 2018: Still getting downvotes for this, and rightly so! I'm leaving this answer up so people can see why I was originally wrong, and to remind myself not to be wrong in future.

回答2:

The assumption is that we are talking about RSA private and Public keys. Then, if you are working from a PEM format file, then first you need to read the private key from the file into a PrivateKey object:

    public PrivateKey readPemRsaPrivateKey(String pemFilename) throws
            java.io.IOException,
            java.security.NoSuchAlgorithmException,
            java.security.spec.InvalidKeySpecException
    {
            String pemString = File2String(pemFilename);

            pemString = pemString.replace("-----BEGIN RSA PRIVATE KEY-----\n", "");
            pemString = pemString.replace("-----END RSA PRIVATE KEY-----", "");

            byte[] decoded = Base64.decodeBase64(pemString);

            PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(decoded);
            KeyFactory kf = KeyFactory.getInstance("RSA");

            return kf.generatePrivate(keySpec);
    }

where File2String is something like:

    private static String File2String(String fileName) throws
            java.io.FileNotFoundException, java.io.IOException
    {
            File file = new File(fileName);

            char[] buffer = null;

            BufferedReader bufferedReader = new BufferedReader(new FileReader(file));

            buffer = new char[(int)file.length()];

            int i = 0;
            int c = bufferedReader.read();

            while (c != -1) {
                    buffer[i++] = (char)c;
                    c = bufferedReader.read();
            }
            return new String(buffer);
    }

Now you can generate the corresponding PublicKey with code like this:

    import java.security.interfaces.RSAPrivateCrtKey;
    import java.security.spec.RSAPublicKeySpec;

...

    PrivateKey myPrivateKey = readPemRsaPrivateKey(myPrivateKeyPemFileName);
    RSAPrivateCrtKey privk = (RSAPrivateCrtKey)myPrivateKey;

    RSAPublicKeySpec publicKeySpec = new java.security.spec.RSAPublicKeySpec(privk.getModulus(), privk.getPublicExponent());

    KeyFactory keyFactory = KeyFactory.getInstance("RSA");
    PublicKey myPublicKey = keyFactory.generatePublic(publicKeySpec);

Credits: How to get a RSA PublicKey by giving a PrivateKey?

回答3:

Please make sure that Eli Rosencruft answer is basically correct, but the order of the modulus and the public exponent are incorrect! This is the correct statement:

RSAPublicKeySpec publicKeySpec = new java.security.spec.RSAPublicKeySpec(privk.getModulus(), privk.getPublicExponent());

来源：https://stackoverflow.com/questions/8434428/get-public-key-from-private-in-java

标签

java

public-key

private-key