Verifying a signature with a public key

Question

I have an external service which call me back after some defined event, and sign his request with its private key.

I have stored the public key which look like :

-----BEGIN PUBLIC KEY-----
........................................
-----END PUBLIC KEY-----

So my work is to check if request's content has not been alterned by verifying signature.

Here is my algorithm :

// 1 - reading public key :
Scanner scanner = new Scanner( new File( keyPath ) );


//            encodedPublicKey.toString( );
StringBuilder sb = new StringBuilder( );
while ( scanner.hasNextLine( ) )
{
    sb.append( scanner.nextLine( ) );
    sb.append( '\n' );
}

byte[] encodedPublicKey = sb.toString( ).getBytes( "utf-8" );

// 2 - loading public key in a relevant object :
X509EncodedKeySpec publicKeySpec = new X509EncodedKeySpec( publicKeyBytes );

KeyFactory keyFactory = KeyFactory.getInstance( "DSA" );

PublicKey publicKey = keyFactory.generatePublic( publicKeySpec );

// 3 - verifying content with signature and content :
Signature sig = Signature.getInstance( "SHA1withDSA" );
sig.initVerify( publicKey );
sig.update( message.getBytes( ) );
ret = sig.verify( sign.getBytes( ) );

But for now my algorithm is stoped at "PublicKey publicKey = keyFactory.generatePublic( publicKeySpec )" step by this message :

java.security.spec.InvalidKeySpecException: Inappropriate key specification: invalid key format

So how can I load my key in a way that is accepted by java api ?

Answer 1

Actually I've found the solution.

The problem was to load in the public key file in the right way.

I'va added bouncycastle library to my dependencies :

<dependency>
  <groupId>org.bouncycastle</groupId>
  <artifactId>bcprov-jdk15on</artifactId>
  <version>1.47</version>
</dependency>

It provides PemReader which allows to read and load non certificated public keys.

Here is my utility class :

import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.net.URLDecoder;
import java.security.InvalidKeyException;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PublicKey;
import java.security.Signature;
import java.security.SignatureException;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.X509EncodedKeySpec;

import org.bouncycastle.util.io.pem.PemReader;
import org.castor.util.Base64Decoder;

import fr.paris.lutece.portal.service.util.AppLogService;


/**
 * Classe d'aide à l'interfacage avec le service paybox.
 *
 * Toutes les informations parameterables sont sous la forme paybox.*
 */
public final class PayboxUtil
{

    /** The Constant CHARSET. */
    private static final String CHARSET = "utf-8";

    /** The Constant ENCRYPTION_ALGORITHM. */
    private static final String ENCRYPTION_ALGORITHM = "RSA";

    /** The Constant HASH_ENCRIPTION_ALGORITHM. */
    private static final String HASH_ENCRYPTION_ALGORITHM = "SHA1withRSA";

    /**
     * constructeur privé pour classe statique.
     */
    private PayboxUtil(  )
    {
    }

    /**
     * Controle si une signature est bien celle du message à l'aide de la clé
     * publique de l'emmeteur?.
     *
     * @param message le message
     * @param sign la signature
     * @param keyPath le chemin vers la clé publique.
     * @return true si la signature est bien celle du message avec la clé privé
     *         attendue.
     */
    public static boolean checkSign( String message, String sign, String keyPath )
    {
        boolean ret = false;

        try
        {
            ret = PayboxUtil.verify( message, sign, PayboxUtil.getKey( keyPath ) );
        }
        catch ( final FileNotFoundException e )
        {
            AppLogService.error( e );
        }
        catch ( final IOException e )
        {
            AppLogService.error( e );
        }
        catch ( final NoSuchAlgorithmException e )
        {
            AppLogService.error( e );
        }
        catch ( final InvalidKeySpecException e )
        {
            AppLogService.error( e );
        }
        catch ( final InvalidKeyException e )
        {
            AppLogService.error( e );
        }
        catch ( final SignatureException e )
        {
            AppLogService.error( e );
        }

        return ret;
    }


    /**
     * Récupère la clé publique à partir du chemin passé en paramètre.
     *
     * @param keyPath le chemin vers la clé.
     * @return la clé publique
     * @throws NoSuchAlgorithmException the no such algorithm exception
     * @throws IOException Signals that an I/O exception has occurred.
     * @throws InvalidKeySpecException the invalid key spec exception
     */
    private static PublicKey getKey( String keyPath )
        throws NoSuchAlgorithmException, IOException, InvalidKeySpecException
    {
        final KeyFactory keyFactory = KeyFactory.getInstance( PayboxUtil.ENCRYPTION_ALGORITHM );
        final PemReader reader = new PemReader( new FileReader( keyPath ) );
        final byte[] pubKey = reader.readPemObject(  ).getContent(  );
        final X509EncodedKeySpec publicKeySpec = new X509EncodedKeySpec( pubKey );

        return keyFactory.generatePublic( publicKeySpec );
    }

    /**
     * effectue la vérification du message en fonction de la signature et de la
     * clé.
     *
     * @param message le message
     * @param sign la signature
     * @param publicKey la clé publique.
     * @return true, if successful
     * @throws NoSuchAlgorithmException the no such algorithm exception
     * @throws InvalidKeyException the invalid key exception
     * @throws SignatureException the signature exception
     * @throws UnsupportedEncodingException the unsupported encoding exception
     */
    private static boolean verify( String message, String sign, PublicKey publicKey )
        throws NoSuchAlgorithmException, InvalidKeyException, SignatureException, UnsupportedEncodingException
    {
        final Signature sig = Signature.getInstance( PayboxUtil.HASH_ENCRYPTION_ALGORITHM );
        sig.initVerify( publicKey );
        sig.update( message.getBytes( PayboxUtil.CHARSET ) );

        final byte[] bytes = Base64Decoder.decode( URLDecoder.decode( sign, PayboxUtil.CHARSET ) );

        return sig.verify( bytes );
    }
}

You just have to pass signed content, signature and key path to checkSign method and it does all the work.