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Sensitive data must be protected from eavesdropping. All data that crosses a trust boundary must be protected from malicious tampering. An obfuscated transfer object [Steel 2005] that is strongly encrypted can protect data. This approach is known as sealing the object. To guarantee object integrity, apply a digital signature to the sealed object.

Sealing and signing

It is imperative that sensitive data should be protected from eavesdropping and malicious tampering during transit. An Obfuscated Transfer Object \[[Steel 05|AA. Java References#Steel 05]\] can be used to encrypt data in exchanges that involve multiple business tiers or end user systems. Obfuscation can be achieved, largely, by encrypting the sensitive object (sealing). This design pattern can further be supplemented to provide signature capabilities for guaranteeing object integrity.   

Signing and sealing objects is the preferred mechanism to secure data when:

• The data is sensitive but its serialization or transportation is necessaryTransporting sensitive data or serializing any data.
• A secure communication channel such as Secure Sockets Layer (SSL) is absent or is a too costly alternative for limited transactions.
• Some sensitive Sensitive data needs to must persist over an extended period of time (for example, on an external a hard drive).Implementing

Avoid using home-brewed cryptographic algorithms

...

; such algorithms will almost certainly introduce unnecessary vulnerabilities. Applications that apply home-brewed "cryptography" in the readObject() and writeObject

...

() methods are prime examples of anti-patterns. However, using existing cryptography libraries inside readObject() and writeObject() is perfrectly warranted.

This rule applies to the intentional serialization of sensitive information. SER03-J. Do not serialize unencrypted sensitive data is meant to prevent the unintentional serialization of sensitive information.

Noncompliant Code Example

The code examples for this rule are all based on the following code example:

class SerializableMap<K,V>

Noncompliant Code Example

This noncompliant code example is capable of being serialized and transferred across different business tiers. Unfortunately, there are no safeguards against byte stream manipulation attacks while the binary data is in transit. Likewise, anyone can reverse engineer the stream data from its hexadecimal notation to unravel the HashMap containing sensitive social security numbers.

class SimpleObject implements Serializable {
  final static long serialVersionUID = -2648720192864531932L;
  private HashMapMap<K,V> ssnMapmap;
  private SealedObject sealedSsnMap;

  public SimpleObjectSerializableMap() {
    ssnMapmap = new HashMapHashMap<K,V>();
  }

  public Object getdatagetData(ObjectK key) throws Exception {
    return ssnMapmap.get(key);
  }

  public void setData(ObjectK key, ObjectV data) throws Exception {
    ssnMapmap.put(key, data);
  }
}

public class MapSerializer {
  public static SerializableMap<String, Integer> buildMap() {
    SerializableMap<String, Integer> map = 
        new SerializableMap<String, Integer>();
    map.setData("John Doe", new Integer(123456789));
    map.setData("Richard Roe", new Integer(246813579));
    return map;
  }

  public static void InspectMap(SerializableMap<String, Integer> map) {
    System.out.println("John Doe's number is " + map.getData("John Doe"));
    System.out.println("Richard Roe's number is " +
                       map.getData("Richard Roe"));
  }

  public static void main(String[] args) {

Compliant Solution

    // ...
  }
}

This code sample defines a serializable map, a method to populate the map with values, and a method to check the map for those values.

This noncompliant code example simply serializes then deserializes the map. Consequently, the map can be serialized and transferred across different business tiers. Unfortunately, the example lacks any safeguards against byte stream manipulation attacks while the binary data is in transit. Likewise, anyone can reverse-engineer the serialized stream data to recover the data in the HashMap. Anyone would also be able to tamper with the map and produce an object that made the deserializer crash or hang.

public static void main(String[] args)
                        throws IOException, ClassNotFoundException {
  // Build map
  SerializableMap<String, Integer> map = buildMap();

  // Serialize map
  ObjectOutputStream out =
      new ObjectOutputStream(new FileOutputStream("data"));
  out.writeObject(map);
  out.close();

  // Deserialize map
  ObjectInputStream in =
      new ObjectInputStream(new FileInputStream("data"));
  map = (SerializableMap<String, Integer>) in.readObject();
  in.close();

  // Inspect map
  InspectMap(map);
}

If the data in the map were sensitive, this example would also violate SER03-J. Do not serialize unencrypted sensitive data.

Noncompliant Code Example (Seal)

This noncompliant code example uses To provide message confidentiality, use the javax.crypto.SealedObject class to provide message confidentiality. This class encapsulates a serialized object and encrypts (or seals) it. A strong cryptographic algorithm that uses a secure cryptographic key and padding scheme must be employed to initialize the Cipher object parameter. The seal() and unseal() utility methods provide the encryption and decryption facilities respectively.

This noncompliant code example encrypts the map into a SealedObject, rendering the data inaccessible to prying eyes. However, the program fails to sign the data, rendering it impossible to authenticate.

public static void main(String[] args)
                        throws IOException, GeneralSecurityException, 
                               ClassNotFoundException {
  // Build map
  SerializableMap<String, Integer> map = buildMap();

  // Generate sealing key & seal map
  KeyGenerator generator;
  generator = KeyGenerator.getInstance("AES");
  generator.init(new SecureRandom());
  Key key = generator.generateKey();
  Cipher cipher = Cipher.getInstance("AES");
  cipher.init(Cipher.ENCRYPT_MODE, key);
  SealedObject sealedMap = new SealedObject(map, cipher);

  // Serialize map
  ObjectOutputStream out =
      new ObjectOutputStream(new FileOutputStream("data"));
  out.writeObject(sealedMap);
  out.close();

  // Deserialize map
  ObjectInputStream in =
      new ObjectInputStream(new FileInputStream("data"));
  sealedMap = (SealedObject) in.readObject();
  in.close();

  // Unseal map
  cipher = Cipher.getInstance("AES");
  cipher.init(Cipher.DECRYPT_MODE, key);
  map = (SerializableMap<String, Integer>) sealedMap.getObject(cipher);

  // Inspect map
  InspectMap(map);
}

Noncompliant Code Example (Seal Then Sign)

This noncompliant code example uses the java.security.SignedObject class to sign an object In addition, use the sign and unsign utility methods when the integrity of the object is to must be ensured. The two new arguments passed in to the SignedObject() method to sign the object are Signature and a private key derived from a KeyPair object. To verify the signature, a PublicKey as well as a Signature argument is passed to the SignedObject.verify() method.

This noncompliant code example signs the object as well as seals it. According to Abadi and Needham [Abadi 1996],

When a principal signs material that has already been encrypted, it should not be inferred that the principal knows the content of the message. On the other hand, it is proper to infer that the principal that signs a message and then encrypts it for privacy knows the content of the message.

Any malicious party can intercept the originally signed encrypted message from the originator, strip the signature, and add its own signature to the encrypted message. Both the malicious party and the receiver have no information about the contents of the original message because it is encrypted and then signed (it can be decrypted only after verifying the signature). The receiver has no way of confirming the sender's identity unless the legitimate sender's public key is obtained over a secure channel. One of the three Internal Telegraph and Telephone Consultative Committee (CCITT) X.509 standard protocols was susceptible to such an attack [CCITT 1988].

Because the signing occurs after the sealing, it cannot be assumed that the signer is the true originator of the object.

class SignSealUtility implements Serializable {
  final long serialVersionUID = 2648720192864531932L;
  private HashMap ssnMap;
  private SealedObject sealedSsnMap;
  private SignedObject signedSsnMap;
  
  public SignSealUtility() {
    ssnMap = new HashMap();
  }

  public void seal(Cipher cipher) throws Exception {
    sealedSsnMap = new SealedObject(ssnMap, cipher);
    // Now set the Map to null so that original data does not remain in cleartext
    ssnMap = null; 
  }

  public void unseal(Cipher cipher) throws Exception {
    ssnMap = (HashMap)sealedSsnMap.getObject(cipher);
  }
  
  public void sign(Signature sig, PrivateKey key) throws Exception {
    signedSsnMap = new SignedObject(ssnMap, key, sig);
    ssnMap = null;
  }

  public void unsign(Signature sig, PublicKey key) throws Exception {
    if(signedSsnMap.verify(key, sig)) {
      ssnMap = (HashMap)signedSsnMap.getObject();
    }
  }

  public Object getdata(Object key) throws Exception {
    return ssnMap.get(key);
  }
 
  public void setData(Object key, Object data) throws Exception {
    ssnMap.put(key, data);
  }
}

Finally, refrain from signing encrypted (sealed) data. (See SEC32-J. Create and sign a SignedObject before creating a SealedObject)

Risk Assessment

public static void main(String[] args)
                        throws IOException, GeneralSecurityException, 
                               ClassNotFoundException {
  // Build map
  SerializableMap<String, Integer> map = buildMap();

  // Generate sealing key & seal map
  KeyGenerator generator;
  generator = KeyGenerator.getInstance("AES");
  generator.init(new SecureRandom());
  Key key = generator.generateKey();
  Cipher cipher = Cipher.getInstance("AES");
  cipher.init(Cipher.ENCRYPT_MODE, key);
  SealedObject sealedMap = new SealedObject(map, cipher);

  // Generate signing public/private key pair & sign map
  KeyPairGenerator kpg = KeyPairGenerator.getInstance("DSA");
  KeyPair kp = kpg.generateKeyPair();
  Signature sig = Signature.getInstance("SHA1withDSA");
  SignedObject signedMap =
      new SignedObject(sealedMap, kp.getPrivate(), sig);

  // Serialize map
  ObjectOutputStream out = 
      new ObjectOutputStream(new FileOutputStream("data"));
  out.writeObject(signedMap);
  out.close();

  // Deserialize map
  ObjectInputStream in =
      new ObjectInputStream(new FileInputStream("data"));
  signedMap = (SignedObject) in.readObject();
  in.close();

  // Verify signature and retrieve map
  if (!signedMap.verify(kp.getPublic(), sig)) {
    throw new GeneralSecurityException("Map failed verification");
  }
  sealedMap = (SealedObject) signedMap.getObject();

  // Unseal map
  cipher = Cipher.getInstance("AES");
  cipher.init(Cipher.DECRYPT_MODE, key);
  map = (SerializableMap<String, Integer>) sealedMap.getObject(cipher);

  // Inspect map
  InspectMap(map);
}

Compliant Solution (Sign Then Seal)

This compliant solution correctly signs the object before sealing it. This approach provides a guarantee of authenticity to the object in addition to protection from man-in-the-middle attacks.

public static void main(String[] args)
                        throws
  IOException, GeneralSecurityException, 
                               ClassNotFoundException {
  // Build map
  SerializableMap<String, Integer> map = buildMap();

  // Generate signing public/private key pair & sign map
  KeyPairGenerator kpg = KeyPairGenerator.getInstance("DSA");
  KeyPair kp = kpg.generateKeyPair();
  Signature sig = Signature.getInstance("SHA1withDSA");
  SignedObject signedMap = new SignedObject(map, kp.getPrivate(), sig);

  // Generate sealing key & seal map
  KeyGenerator generator;
  generator = KeyGenerator.getInstance("AES");
  generator.init(new SecureRandom());
  Key key = generator.generateKey();
  Cipher cipher = Cipher.getInstance("AES");
  cipher.init(Cipher.ENCRYPT_MODE, key);
  SealedObject sealedMap = new SealedObject(signedMap, cipher);

  // Serialize map
  ObjectOutputStream out = 
      new ObjectOutputStream(new FileOutputStream("data"));
  out.writeObject(sealedMap);
  out.close();

  // Deserialize map
  ObjectInputStream in =
      new ObjectInputStream(new FileInputStream("data"));
  sealedMap = (SealedObject) in.readObject();
  in.close();

  // Unseal map
  cipher = Cipher.getInstance("AES");
  cipher.init(Cipher.DECRYPT_MODE, key);
  signedMap = (SignedObject) sealedMap.getObject(cipher);

  // Verify signature and retrieve map
  if (!signedMap.verify(kp.getPublic(), sig)) {
    throw new GeneralSecurityException("Map failed verification");
  }
  map = (SerializableMap<String, Integer>) signedMap.getObject();

  // Inspect map
  InspectMap(map);
}

Exceptions

SER02-J-EX0: A reasonable use for signing a sealed object is to certify the authenticity of a sealed object passed from elsewhere. This use represents a commitment about the sealed object itself rather than about its content [Abadi 1996].

SER02-J-EX1: Signing and sealing is required only for objects that must cross a trust boundary. Objects that never leave the trust boundary need not be signed or sealed. For example, when an entire network is contained within a trust boundary, objects that never leave that network need not be signed or sealed. Another example is objects that are only sent down a signed binary stream.

Risk Assessment

Failure to sign and then Failure to sign and/or seal objects during transit can lead to loss of object integrity or confidentiality.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[API 06|AA. Java References#API 06]\] 
\[[Steel 05|AA. Java References#Steel 05]\] Chapter 10: Securing the Business Tier, Obfuscated Transfer Object
\[[Gong 03|AA. Java References#Gong 03]\] 9.10 Sealing Objects
\[[Harold 99|AA. Java References#Harold 99]\] Chapter 11: Object Serialization, Sealed Objects 
\[[Neward 04|AA. Java References#Neward 04]\] Item 64: Use SignedObject to provide integrity of Serialized objects and Item 65: Use SealedObject to provide confidentiality of Serializable objects
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 319|http://cwe.mitre.org/data/definitions/319.html] "Cleartext Transmission of Sensitive Information"

This rule is not amenable to static analysis in the general case.

Related Guidelines

Bibliography

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Image Added Image Added Image AddedSEC05-J. Minimize accessibility of classes and their members      01. Platform Security (SEC)      SEC32-J. Create and sign a SignedObject before creating a SealedObject