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Weaknesses and Alternatives

The GenSig and VerSig programs in this lesson illustrate the use of the JDK Security API to generate a digital signature for data, and to verify that a signature is authentic.

However, the actual scenario depicted by those programs, in which

is not necessarily realistic, and has a potential major flaw.

In many cases the keys do not need to be generated; they already exist, either as encoded keys in files or as entries in a keystore.

The potential major flaw is that there is nothing that guarantees the authenticity of the public key the receiver receives, and the VerSig program only correctly verifies the authenticity of a signature if the public key it is supplied is itself authentic!

These are not issues in some cases, for example, when a single program is doing both signing and verification. For example, in the Signing and Verifying the BINGO Cards(in the Learning the Java Language trail) part of the Putting It All Together(in the Learning the Java Language trail) trail, the NotaryPublic class constructor creates a key pair. Subsequently, whenever a Player joins a game, the Game class generates the cards for the Player to play with. Before sending the cards back to the Player, the Game digitally signs the cards using the private key. Later, when a Player claims to have a winning card, the Game verifies the signature using the public key, to make sure the card was actually created by this Game for the current game.

Working with Encoded Key Bytes

There are times when encoded key bytes already exist in files for the key pair to be used for signing and verification.

If that's the case, the GenSig program can import the encoded private key bytes and convert them to a PrivateKey needed for signing, via the following (assuming the name of the file containing the private key bytes is in in the privkeyfile String and the bytes represent a DSA key that has been encoded using the PKCS #8 standard):

    FileInputStream keyfis = new FileInputStream(privkeyfile);
    byte[] encKey = new byte[keyfis.available()];;

    PKCS8EncodedKeySpec privKeySpec = new PKCS8EncodedKeySpec(encKey);

    KeyFactory keyFactory = KeyFactory.getInstance("DSA");
    PrivateKey privKey = keyFactory.generatePrivate(privKeySpec);
GenSig no longer needs to save the public key bytes in a file (they're already in one).

In this case, the sender sends the receiver

The VerSig program remains unchanged, as it already expects encoded public key bytes in a file.

But what about the potential problem of a malicious user intercepting the files and replacing them all in such a way that their switch cannot be detected?

In some cases, this is not an issue because people have already exchanged public keys face-to-face, or via a trusted third party who does the face-to-face exchange. After that, multiple subsequent file and signature exchanges may be done remotely (that is, between two people in different locations), and the public keys may be used to verify their authenticity. If a malicious user tries to change the data or signature, this is detected by VerSig.

If a face-to-face key exchange is not possible, you can try other methods of increasing the likelihood of proper receipt. For example, you could send your public key via the most secure method you can prior to subsequent exchanges of data and signature files (perhaps using less secure mediums).

In general, if the data and signature are sent separately from your public key, the likelihood of an attack will be greatly reduced. Unless all three files are changed, and in a certain manner discussed in the next paragraph, VerSig will detect any tampering.

If all three files (data document, public key, and signature) were intercepted by a malicious user, that person could replace the document with whatever they want, sign it with a private key, and forward on to you the replaced document, the new signature, and the public key corresponding to the private key used to generate the new signature. Then VerSig would report a successful verification, and you'd think the document came from the original sender. Thus, you should take steps to ensure at least the public key is received intact (VerSig detects any tampering of the other files) or you can use certificates to facilitate authentication of the public key, as described in the next section.

Working with Certificates

It is more common in cryptography to exchange certificates containing public keys rather than the keys themselves.

One benefit is that a certificate is signed by one entity (the issuer) to verify that the enclosed public key is the actual public key of another entity (the subject or owner). Typically, a trusted third-party Certification Authority (CA) verifies the identity of the subject, and then vouches for it being the owner of the public key by signing the certificate.

Another benefit of using certificates is that you can check to ensure whether a certificate you received is valid by verifying its digital signature using its issuer's (signer's) public key, which itself may be stored in a certificate whose signature can be verified using that certificate's issuer's public key, which itself may be stored in a certificate, and so on, until you reach a public key that you already trust.

If you cannot establish a trust chain (for example, because the required issuer certificates are not available to you), the certificate fingerprint(s) can be calculated. Each fingerprint is a relatively short number that uniquely and reliably identifies the certificate. (Technically, it's a hash value of the certificate information, using a message digest, also known as a one-way hash function.) You can call up the certificate owner and compare the fingerprints of the certificate you received with the ones sent. If they're the same, the certificates are the same.

It would be more secure for GenSig to create a certificate containing the public key, and for VerSig to then import the certificate and extract the public key. However, currently there are not any public certificate APIs in the JDK that would allow you to create a certificate from a public key, so the GenSig program cannot create a certificate from the public key it generated. (There are public APIs for extracting a public key from a certificate, though.)

If you want, you can use the various security tools, not APIs, to sign your important document(s) and work with certificates from a keystore, as was done in the Exchanging Files(in the Java Security 1.2 trail) lesson.

Alternatively, you can use the API to modify your programs to work with an already-existing private key and corresponding public key (in a certificate) from your keystore.

To start, modify the GenSig program to extract a private key from a keystore rather than generate new keys. First, let's assume

Then you can extract the private key from the keystore via the following:
    KeyStore ks = KeyStore.getInstance("JKS");
    FileInputStream ksfis = new FileInputStream(ksName); 
    BufferedInputStream ksbufin = new BufferedInputStream(ksfis);  
    ks.load(ksbufin, spass);
    PrivateKey priv = (PrivateKey) ks.getKey(alias, kpass);
You can extract the public key certificate from the keystore, and save its encoded bytes to a file named suecert, via the following: cert = ks.getCertificate(alias);
    byte[] encodedCert = cert.getEncoded();
    /* save the certificate in a file named "suecert" */
    FileOutputStream certfos = new FileOutputStream("suecert");
Then you send the data file, the signature, and the certificate to the receiver. The receiver verifes the authenticity of the certificate by first getting the certificate's fingerprints, via the keytool -printcert command, as in:
  keytool -printcert -file suecert
    Owner: CN=Susan Jones, OU=Purchasing, O=ABC, L=Cupertino, ST=CA, C=US
    Issuer: CN=Susan Jones, OU=Purchasing, O=ABC, L=Cupertino, ST=CA, C=US
    Serial number: 35aaed17
    Valid from: Mon Jul 13 22:31:03 PDT 1998 until: Sun Oct 11 22:31:03 PDT 1998
    Certificate fingerprints:
         MD5:  1E:B8:04:59:86:7A:78:6B:40:AC:64:89:2C:0F:DD:13
         SHA1: 1C:79:BD:26:A1:34:C0:0A:30:63:11:6A:F2:B9:67:DF:E5:8D:7B:5E
Then the receiver verifies the fingerprints, for example, by calling the sender up and comparing them with those of the sender's certificate or by looking them up in a public repository.

The receiver's verification program (a modified VerSig) can then import the certificate and extract the public key from it via the following, assuming the certificate file name (for example, suecert) is in the String certName:

    FileInputStream certfis = new FileInputStream(certName);
    CertificateFactory cf = CertificateFactory.getInstance("X.509"); cert = cf.generateCertificate(certfis);
    PublicKey pub = cert.getPublicKey();

What About Confidentiality of the Data?

Suppose you want to keep the contents of the data confidential so people accidentally or maliciously trying to view it in transit (or on your own machine or disk) cannot do so. To keep the data confidential, you should encrypt it, and store and send only the encryption result (referred to as ciphertext). The receiver can decrypt the ciphertext to obtain a copy of the original data.

APIs for data encryption and decryption, together with some default algorithm implementations, are released separately in a "Java Cryptography Extension" (JCE) as an add-on package to the JDK, in accordance with U.S. export control regulations.

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