MSC18-J Store passwords using a hash function

Any time an application stores a password as cleartext (unencrypted text data), its value is potentially exposed in a variety of ways. To prevent this information from being inadvertently leaked, this exposure must be limited. While a program will receive the password from the user as cleartext, this should be the last time it is in this form.

Hash functions allow programs to indirectly compare an input password to the original, without storing a cleartext or decryptable version of the password. This approach minimizes the exposure of the password without presenting any practical disadvantages.

Cryptographic Hash Functions

The value that a hash function outputs is called the hash value. Another term for hash value is message digest. Hash functions are computationally feasible functions whose inverses are computationally infeasible. This means that in practice, one can encode a password to a hash value quickly, while they are also unable to decode it. The equality of the passwords can be tested through the equality of their hash values.

Java's MessageDigest class provides the functionality of various cryptographic hash functions. Be careful not to use any defective hash functions, such as MD5. How do I go about learning which hash functions are safe, and which are defective?

It is also important that you append a salt to the password you are hashing. A salt is a randomly generated piece of data that is stored along with the hash value. The use of a salt helps prevents dictionary attacks against the hash value, provided the salt is long enough what is long enough . Each password should have its own salt associated with it. If a single salt were used for more than one passwords, two users would be able to see if their passwords are the same.

Noncompliant Code Example

This noncompliant code example encrypts and decrypts the password stored in credentials.pw.

public final class Password {
  private void setPassword(byte[] pass) throws Exception {
    bytes[] encrypted = encrypt(pass); //arbitrary encryption scheme
    clearArray(pass);      
    saveBytes(encrypted,"credentials.pw"); //encrypted password to credentials.pw
  }

  private boolean checkPassword(byte[] pass) throws Exception {
    boolean arrays_equal;
    byte[] encrypted = loadBytes("credentials.pw"); //load the encrypted password
    byte[] decrypted = decrypt(encrypted);
    arrays_equal = Arrays.equal(decrypted, pass);
    clearArray(decrypted);
    clearArray(pass);
    return arrays_equal;
  }

  private clearArray(byte[] a) {
    //set all of the elements in a to zero
  }
}

An attacker could potentially decrypt this file to discover the password. This attacker could be someone knows or has figured out the encryption scheme being used by the program.

Noncompliant Code Example

This noncompliant code examples implements the SHA-1 hash function through the MessageDigest class to compare hash values instead of cleartext strings.

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public final class Password {
  private String salt = "ia0942980234241sadfaewvo32"; //Randomly generated

  private void setPassword(String pass) throws Exception {
    MessageDigest sha_1 = MessageDigest.getInstance("SHA-1");
    byte[] hashVal = sha_1.digest((pass+salt).getBytes()); //encode the string and salt
    saveBytes(hashVal,"credentials.pw"); //save the hash value to credentials.pw
  }

  private boolean checkPassword(String pass) throws Exception {
    MessageDigest sha_1 = MessageDigest.getInstance("SHA-1");
    byte[] hashVal1 = sha_1.digest((pass+salt).getBytes()); //encode the string and salt
    byte[] hashVal2 = loadBytes("credentials.pw"); //load the hash value stored in credentials.pw
    return Arrays.equals(hashVal1, hashVal2);
  }
}

While this fixes the decryption problem from the previous noncompliant code example, at runtime this code may inadvertently store the passwords as cleartext. This is because the pass arguments may not be cleared from memory by the Java garbage collector. See MSC10-J. Limit the lifetime of sensitive data for more information.

Compliant Solution

This compliant solution addresses the problems from the previous noncompliant examples.

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public final class Password {
  private byte[] salt = "ia0942980234241sadfaewvo32".getBytes(); //Randomly generated

  private void setPassword(byte[] pass) throws Exception {
    byte[] input = appendArrays(pass, salt);
    MessageDigest sha_1 = MessageDigest.getInstance("SHA-1");
    byte[] hashVal = sha_1.digest(input); //encode the string and salt    
    clearArray(pass);    
    clearArray(input);    
    saveBytes(hashVal,"credentials.pw"); //save the hash value to credentials.pw
  }

  private boolean checkPassword(byte[] pass) throws Exception {
    byte[] input = appendArrays(pass, salt);
    MessageDigest sha_1 = MessageDigest.getInstance("SHA-1");
    byte[] hashVal1 = sha_1.digest(input); //encode the string and salt
    clearArray(pass);
    clearArray(input);
    byte[] hashVal2 = loadBytes("credentials.pw"); //load the hash value stored in credentials.pw
    return Arrays.equals(hashVal1, hashVal2);
  }

  private appendArrays(byte[] a, byte[] b) {
    //Return a new array of a appended to b
  }

  private clearArray(byte[] a) {
    //set all of the elements in a to zero
  }
}

In both the setPassword() and checkPassword() methods, the cleartext representation of the password is erased as soon as it is converted into a hash value. After this happens, there is no way for an attacker to get the password as cleartext. 

It is important to note that only one password is being stored, which means it is valid to use a salt in the above manner. If this program were modified to handle more than one password, then it would have to be modified to randomly generate a salt for each password. 

It is also important that the salt is randomly generated for each implementation of the program. If this version is widely used, then a dictionary-attack with this particular salt might be feasible.

Exceptions

MSC18-EX0 You may be forced to encrypt passwords or store them as cleartext when you are extending code or an application that you cannot change. For example, a password manager may need to input passwords into other programs as cleartext.

MSC18-EX1 You may be forced to encrypt password when using a library that returns the password as a Java string object, resulting in the vulnerability described in the second noncompliant example. In these cases your best strategy may be to use somewhat vulnerable methods such as encryption, unless you can change the other code.

Risk Assessment

Violations of this rule have to be manually detected because it is a consequence of the overall design of the password storing mechanism. It is pretty unlikely, since it will occur around once or twice in a program that uses passwords. As demonstrated above, almost all violations of this rule have a clear exploit associated with them.

Bibliography

[[API 2006]] Class java.security.MessageDigest