How to load RSA public key from String for signature verification in Java?

Peter :

I have the following public key, that is stored in the DB (PostgresSQL) as text. It's a String, in java:

I don't know how this key has been generated, I'm sorry. I have been told to take this key and verify the signature of another string that I'll call "object". I have been told that the algorithm that I have to use to verify "object" is SHA256withRSA.

So, I have written the following java method to read the key

private PublicKey getPublicKey(String publicKey) throws NoSuchAlgorithmException, InvalidKeySpecException, UnsupportedEncodingException {
    publicKey = publicKey.replaceAll("\\n", "");
    publicKey = publicKey.replace("-----BEGIN RSA PUBLIC KEY-----", "");
    publicKey = publicKey.replace("-----END RSA PUBLIC KEY-----", "");
    publicKey = publicKey.trim();
    byte[] keyDecoded = Base64.getDecoder().decode(publicKey.getBytes());
    X509EncodedKeySpec publicSpec = new X509EncodedKeySpec(keyDecoded);
    KeyFactory kf = KeyFactory.getInstance("RSA");
    PublicKey pubKey = kf.generatePublic(publicSpec);
    return pubKey;
}

The point is that I get the following exception:

java.security.InvalidKeyException: IOException: algid parse error, not a sequence

I have read plenty of qustions as mine in stackoverflow. The code written by other users is pretty similar (sometimes identical) to mine. So I definitely don't get why it doesn't work for me. Other developers (workmates) are doing the same in php and it works great, so I would discard the hypothesis of wrong public key. Maybe didn't I understood the process clearly? Do you have any clue, please?

I have also tried to cope with the problem using BouncyCastle library, as suggested here, but I get the same exception. The following is the code I have written:

private static PublicKey getPublicKey(String publicKey)
        throws IOException, NoSuchAlgorithmException, InvalidKeySpecException {
    Security.addProvider(new BouncyCastleProvider());
    PemReader pp = new PemReader(new StringReader(publicKey));
    PemObject pem = pp.readPemObject();
    byte[] content = pem.getContent();
    pp.close();

    X509EncodedKeySpec spec = new X509EncodedKeySpec(content);
    KeyFactory kf = KeyFactory.getInstance("RSA");
    return kf.generatePublic(spec);
}

Robert :

You can't load that key using an X509EncodedKeySpec. According to it's JavaDoc documentation it expects the following format:

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

Instead your key looks different. I used the data from your post, converted it to hex data and posted it into the online ASN.1 decoder.

The output is this:

SEQUENCE (2 elem)
  INTEGER (4096 bit) 873481340827968071893572683200799871431146795599597693981565010037737…
  INTEGER 65537

As you may recognize your key does not contain an AlgorithmIdentifier therefore it can not be loaded using X509EncodedKeySpec.

My suggestion would be to use the BouncyCastle library and it's PEMParser class for loading this key:

File pemFile = new File("test.pem");
try (PEMParser pp = new PEMParser(new InputStreamReader(new FileInputStream(pemFile)))) {
    SubjectPublicKeyInfo subjPubKeyInfo = (SubjectPublicKeyInfo) pp.readObject();
    RSAKeyParameters rsa = (RSAKeyParameters) PublicKeyFactory.createKey(subjPubKeyInfo);

    RSAPublicKeySpec rsaSpec = new RSAPublicKeySpec(rsa.getModulus(), rsa.getExponent());
    KeyFactory kf = KeyFactory.getInstance("RSA");
    java.security.PublicKey publicKey = kf.generatePublic(rsaSpec);
    System.out.println(publicKey);
}

Or you manually convert the key to PKCS#8 format via openssl.

How to load RSA public key from String for signature verification in Java?

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