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Cleartext refers to text data that is not encrypted.

Any time an application stores a password as cleartext, its value is potentially exposed Programs that store passwords as cleartext (unencrypted text data) risk exposure of those passwords in a variety of ways. Obviously this exposure must be limited. While a program will receive the password from the user Although programs generally receive passwords from users as cleartext, they should ensure that the passwords are not stored as cleartext.

An acceptable technique for limiting the exposure of passwords is the use of hash functions, which this should be the last time it is in this form. Hash functions allow programs to indirectly compare an input password to the original , password string without storing a cleartext or decryptable version of the password. This approach will therefore minimize minimizes the exposure of the password without presenting any practical disadvantages.

Cryptographic Hash Functions

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

A good practice is to always use a salt in addition to the password being hashed. A salt is a unique randomly generated piece of data that is stored and used to generate the hash value along with the password. Each password should have its own salt associated with it. If a single salt were used for more than one password an attacker could determine when a user has a commonly used password. Password specific salts are usually stored along with their corresponding hash values. In addition to password-unique salts, system-unique salts that are stored separately from the hash values may also be used to increase the difficulty of deriving passwords if a malicious actor obtains a copy of the hash values and salts.

The choice of hash function and salt length presents a trade-off between security and performance. Increasing the effort required for effective brute-force attacks by choosing a stronger hash function can also increase the time required to validate a password.  As time passes best practices around password management evolve to keep password derivation computationally infeasible.  The documents NIST 800-63 and OWASP ASVS are good places to consult for the current best practices around cryptographic hashing.

Java's javax.crypto package provides implementations Java's MessageDigest class provides the functionality of various cryptographic hash functions. Be careful not to use any defective hash functions, such as MD5.

It is also important that you append a salt to the password you are hashing. A salt is a piece of data that is randomly generated during the creation of the program (and consistent throughout the rest of its implementation). The use of a salt helps prevents dictionary attacks against the hash value, provided the salt is long enough.

Noncompliant Code Example

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Avoid functions that have known weaknesses, such as the Message-Digest Algorithm (MD5). 

Noncompliant Code Example

This noncompliant code example encrypts and decrypts the password stored in password.bin using a symmetric key algorithm:

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

  boolean checkPassword(byte[] pass) throws Exception {
    // Load the encrypted password
    byte[] encrypted = loadBytes("password.bin"); 
    byte[] decrypted = decrypt(encrypted);
    clearArray(encrypted);
    boolean arraysEqual = Arrays.equal(decrypted, pass);
    clearArray(decrypted);
    clearArray(pass);
    return arraysEqual;
  }

  private void clearArray(byte[] a) {
    for (int i = 0; i < a.length; i++) {
      a[i] = 0;
    }
  }

  private byte[] encrypt(byte[] clearValue) {
    // ... symmetric encryption of clearValue bytes, returning the encrypted value
  }

  private byte[] decrypt(byte[] encryptedValue) {
    // ... symmetric decryption of  encryptedValue bytes, returning clear value
  }

  private void saveBytes(byte[] bytes, String filename) throws IOException {
    // ... write bytes to the file
  }
  
  private byte[] loadBytes(String filename) throws IOException { 
    // ... read bytes to the file 
  }
}

This is a very simple password mechanism that only stores one password for the system.  The flaw in this approach is that the stored password is encrypted in a way that it can be decrypted to compare it to the user's password input. An attacker could potentially decrypt the password file to discover the password, particularly when the attacker has knowledge of the key and encryption scheme used by the program. Passwords should be protected even from system administrators and privileged users. Consequently, using encryption is only partly effective in mitigating password disclosure threats.

Noncompliant Code Example

This noncompliant code example uses the SHA-256 hash function through the MessageDigest class to compare hash values instead of cleartext strings. It uses SecureRandom to generate a strong salt, as recommended by MSC02-J. Generate strong random numbers.  However, it uses a String to store the password:

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

public final class Password {
  private SecureRandom random = new SecureRandom();

  private void setPassword(String pass) throws Exception {
    byte[] salt = new byte[12];
    random.nextBytes(salt);
    MessageDigest msgDigest = MessageDigest.getInstance("SHA-256");
    // Encode the string and salt
    byte[] hashVal = msgDigest.digest((pass+salt).getBytes());
    saveBytes(salt, "salt.bin");
    // Save the hash value to password.bin
    saveBytes(hashVal,"password.bin");
  }

  boolean checkPassword(String pass) throws Exception {
    byte[] salt = loadBytes("salt.bin");
    MessageDigest msgDigest = MessageDigest.getInstance("SHA-256");
    // Encode the string and salt
    byte[] hashVal1 = msgDigest.digest((pass+salt).getBytes());
    // Load the hash value stored in password.bin
    byte[] hashVal2 = loadBytes("password.bin");
    return Arrays.equals(hashVal1, hashVal2);
  }

  private void saveBytes(byte[] bytes, String filename) throws IOException {
    // ... write bytes to the file
  }   

  private byte[] loadBytes(String filename) throws IOException { 
    // ... read bytes to the file 
  }
}

This is a very simple password mechanism that only stores one password and one salt for the system. Even when an attacker knows that the program stores passwords using SHA-256 and a 12-byte salt, he or she will be unable to retrieve the actual password from password.bin and salt.bin.

Although this approach solves the decryption problem from the previous noncompliant code example, this program may inadvertently store the passwords as cleartext in memory. Java String objects are immutable and can be copied and internally stored by the Java Virtual Machine. Consequently, Java lacks a mechanism to securely erase a password once it has been stored in a String. See MSC59-J. Limit the lifetime of sensitive data for more information.

Compliant Solution

This compliant solution addresses the problems from the previous noncompliant code examples:

import java.security.GeneralSecurityException;
import java.security.SecureRandom;
import java.security.spec.KeySpec;
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.PBEKeySpec;
  
final class Password {
  private SecureRandom random = new SecureRandom();
  private final int SALT_BYTE_LENGTH = 12;
  private final int ITERATIONS = 100000;
  private final String ALGORITHM = "PBKDF2WithHmacSHA256";
    
  /* Set password to new value, zeroing out password */
  void setPassword(char[] pass)
      throws IOException, GeneralSecurityException  {
    byte[] salt = new byte[SALT_BYTE_LENGTH];
    random.nextBytes(salt);
    saveBytes(salt, "salt.bin");    
    byte[] hashVal = hashPassword(pass, salt); 
    saveBytes(hashVal,"password.bin");
    Arrays.fill(hashVal, (byte) 0);
  }

  /* Indicates if given password is correct */
  boolean checkPassword(char[] pass)
      throws IOException, GeneralSecurityException  {
    byte[] salt = loadBytes("salt.bin");
    byte[] hashVal1 = hashPassword(pass, salt);
    // Load the hash value stored in password.bin
    byte[] hashVal2 = loadBytes("password.bin");
    boolean arraysEqual = timingEquals(hashVal1, hashVal2);
    Arrays.fill(hashVal1, (byte) 0);
    Arrays.fill(hashVal2, (byte) 0);
    return arraysEqual;
  }
  
  /* Encrypts password & salt and zeroes both */
  private byte[] hashPassword(char[] pass, byte[] salt)
      throws GeneralSecurityException {
    KeySpec spec = new PBEKeySpec(pass, salt, ITERATIONS);
    Arrays.fill(pass, (char) 0);
    Arrays.fill(salt, (byte) 0);
    SecretKeyFactory f = SecretKeyFactory.getInstance(ALGORITHM);
    return f.generateSecret(spec).getEncoded();
  }

  /**
   * Indicates if both byte arrays are equal
   * but uses same amount of time if they are the same or different
   * to prevent timing attacks
   */
  public static boolean timingEquals(byte b1[], byte b2[]) {
    boolean result = true;
    int len = b1.length;
    if (len != b2.length) {
      result = false;
    }
    if (len > b2.length) {
      len = b2.length;
    }
    for (int i = 0; i < len; i++) {
      result &= (b1[i] == b2[i]);
    }
    return result;
  }

  private void saveBytes(byte[] bytes, String filename) throws IOException {
    // ... write bytes to the file
  }

  private byte[] loadBytes(String filename) throws IOException {
    // ... read bytes to the file
  }
}

This is a very simple password mechanism that only stores one password and one salt for the system.

First, this compliant solution uses byte array to store the password.

In both the setPassword() and checkPassword() methods, the cleartext representation of the password is erased immediately after it is converted into a hash value. Consequently, attackers must work harder to retrieve the cleartext password after the erasure. Providing guaranteed erasure is extremely challenging, is likely to be platform specific, and may even be impossible because of copying garbage collectors, dynamic paging, and other platform features that operate below the level of the Java language.

Furthermore, we use a timingEquals() method to validate the password. While doing a simple byte comparison, it takes the same time for both successful matches and unsuccessful ones; consequently thwarting timing attacks.

Finally, it uses PBKDF2 which, unlike MessageDigest, is specifically designed for hashing passwords.

The parametric values (SALT_BYTE_LENGTH, ITERATIONS, ALGORITHM) should be set to values that reflect current best practices.  It should also be noted that once these parametric values are set they can not be changed without having to re-hash all passwords with the new parametric values.  

Applicability

Passwords stored without a secure hash are exposed to malicious users. Violations of this guideline generally have a clear exploit associated with them.

Applications such as password managers may need to retrieve the original password to enter it into a third-party application. This is permitted even though it violates this guideline. The password manager is accessed by a single user and always has the user's permission to store his or her passwords and to display those passwords on command. Consequently, the limiting factor to safety and security is the user's competence rather than the program's operation.

Automated Detection

Bibliography

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