Convert signature into human readable (bouncycastle)

Question

I want to sign a file I read in with JFileChooser. But when I print it to the terminal I can only read questonmarks and other not human readable characters. My code to sign is:

public static void sign() throws Exception{
    byte[] file = fileChooser();

    store = KeyStore.getInstance(storeType);
    FileInputStream in = new FileInputStream(new File(storePath));
    store.load(in, storePassword);
    in.close();

    Key priv = store.getKey("test", storePassword);
    System.out.println(priv.toString() + "priv string");
    X509Certificate cert = (X509Certificate) store.getCertificate("Subject");
    ContentSigner signer = new JcaContentSignerBuilder("SHA512withRSA").build((RSAPrivateKey) priv);        

    //Sign Data
    Signature signature = Signature.getInstance("SHA512WithRSA");
    signature.initSign((RSAPrivateKey) priv);
    signature.update(file);

    //Build cms
    CMSTypedData data = new CMSProcessableByteArray(signature.sign()); 
    CMSSignedDataGenerator gen = new CMSSignedDataGenerator();

    gen.addSignerInfoGenerator(
            new JcaSignerInfoGeneratorBuilder(
                    new JcaDigestCalculatorProviderBuilder().build())
                    .build(signer, cert));

    //Get signed data
    CMSSignedData sigData = gen.generate(data, false);

    byte[] sig = (byte[]) sigData.getSignedContent().getContent();
    sig.toString();
    String content = new String(sig);
    System.out.println("Signed content: " + content + "\n");
}

How can I format the signature to human readable?

Answer 1

The digital signature is an array of bytes that aren't in a human readable format, so creating a String with it won't work as you expect.

If you want to have a "readable" format, you can encode it to Base64, using BouncyCastle's org.bouncycastle.util.encoders.Base64 class (I'm using BouncyCastle 1.56, but previous versions might have similar classes for Base64 conversion):

byte[] base64ByteArray = Base64.encode(sig); // sig is your byte array
String humanReadableString = new String(base64ByteArray); // human readable string

To get the original sig array back, you must decode the humanReadableString:

byte[] originalSigBytes = Base64.decode(humanReadableString.getBytes());
// originalSigBytes will be the same as sig

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Notes: if you face any problem related to encoding, you can force the encoding with the java.nio.charset.Charset class (in this example, I'm using UTF-8 but you can use whatever encoding your system uses):

// same thing as above, but using a specific encoding (UTF-8 in this case)
String humanReadableString = new String(base64ByteArray, Charset.forName("UTF-8"));
byte[] originalSigBytes = Base64.decode(humanReadableString.getBytes(Charset.forName("UTF-8")));

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Alternative

If you prefer, you can also convert to a Hex-encoded string (each byte becomes a string representation of its value in base 16), using BouncyCastle's org.bouncycastle.util.encoders.Hex class:

byte[] hexEncodedArray = Hex.encode(sig);
String humanReadableString = new String(hexEncodedArray, Charset.forName("UTF-8"));

And to get your original sig array back:

byte[] originalSigBytes = Hex.decode(humanReadableString.getBytes(Charset.forName("UTF-8")));
// originalSigBytes will be the same as sig

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One more thing: when dealing with cryptography stuff, I suggest you to always manipulate the byte array instead of converting them to String. But if converting to a String is mandatory, then use Base64 (or Hex-encoded string).