CODEWITHSUNDEEP
javacryptographyrsapublic-key
HuXu7
HuXu7

Reputation: 315

Public Key Unknown Encoding

I have this public key:

MIGJAoGBAKv4OKlpY2oq9QZPMzAjbQfiqDqTnisSvdLP+mTswZJdbtk1J+4+qAySJuZjSQljzcUu0ANg+QG0VsvoU72zu5pErZKWubfe9HB/tq69bhP60qgP6/W2VebWlqUNGtsMedxuVaFBL3SoqU7e5RELIsuArCJJIgz86BQDX0x63VpXAgMBAAE=

I am trying to use it to decode this:

Zm/qR/FrkzawabBZYk7WfQJNMVZoZrwWTvfQwIhPMzAuqEO+y+sb/x9+TZwTbqmu45/GV4yhKv0bbDL8F6rif7RJap7iQUFQBDEIAraY42IGZ8pB6A0Q0RSnJWW+tLTLJg5cTrgZQ8sLoO+U03T6DE1wy73FU5h6XhXxZERo0tQ=

In which I know the unencrypted value is this:

2ABB43E83F7EC33D0D33F64BA5782E42

I have been trying several different things including Bouncy Castle (Java implementation) but I am unable to get the public key to work, mostly ending in invalid encoding errors.

This is my current implementation:

byte[] keyBytes = Base64.decodeBase64(PUB_KEY);
try {
        AlgorithmIdentifier rsaIdent = new AlgorithmIdentifier(PKCSObjectIdentifiers.rsaEncryption);
        SubjectPublicKeyInfo kInfo = new SubjectPublicKeyInfo(rsaIdent, keyBytes);
        ASN1Primitive primKey = kInfo.parsePublicKey();
        byte[] encoded = primKey.getEncoded();
        byte[] sessionBytes = Base64.decodeBase64("Zm/qR/FrkzawabBZYk7WfQJNMVZoZrwWTvfQwIhPMzAuqEO+y+sb/x9+TZwTbqmu45/GV4yhKv0bbDL8F6rif7RJap7iQUFQBDEIAraY42IGZ8pB6A0Q0RSnJWW+tLTLJg5cTrgZQ8sLoO+U03T6DE1wy73FU5h6XhXxZERo0tQ=");
        Security.addProvider(new BouncyCastleProvider());
        X509EncodedKeySpec spec = new X509EncodedKeySpec(encoded);
        KeyFactory factory = KeyFactory.getInstance(spec.getFormat());
        Cipher cipher = Cipher.getInstance("RSA", "BC");
        cipher.init(Cipher.DECRYPT_MODE, factory.generatePublic(spec));
        // ----- THIS IS WHERE IT BREAKS -----
        byte[] decrypted = cipher.doFinal(sessionBytes);
        String tada = new String(decrypted, StandardCharsets.UTF_8);
} catch (Exception e) { ... }

When I get to generate the public key from the factory I get 

 java.lang.IllegalArgumentException: unknown object in getInstance: org.bouncycastle.asn1.ASN1Integer

I have tried several other things but all result in the same error above.

Is there something wrong with my public key? What is the correct way to do this?

Upvotes: 1

Views: 8287

Answers (1)

Maarten Bodewes
Maarten Bodewes

Reputation: 94058

First of all, your key is PKCS#1 encoded. It's not a SubjectPublicKeyInfo structure required by Java. You can see how to decode it here.

Second, you cannot decrypt with a public key, you need a private key for that.

Upvotes: 2

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