MSC66-JG. Store passwords using a hash function

Programs that store a password as cleartext (unencrypted text data) risk exposure of the password in a variety of ways. Programs must prevent this information from being leaked. Although a program generally receives the password from the user as cleartext, this should be the last time the password is in this form.

An acceptable technique to limit the exposure of passwords is the use of hash functions, which 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. In practice, a password can be encoded to a hash value, but decoding remains infeasible. The equality of the passwords can be tested through the equality of their hash values.

It is 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 prevent brute-force attacks against the hash value, provided the salt is long enough. Each password should have its own salt associated with it. If a single salt were used for more than one password, two users would be able to see if their passwords are the same.

The choice of hash function and salt length presents a trade-off between security and performance. If it takes longer to compute a hash value, then the performance of a brute-force attack is lowered. This come at the cost of slowing down the program when it validates passwords. If a longer salt length is used, then the performance of a brute-force attack is lowered at the cost of the extra storage space required.

Java's MessageDigest class provides the functionality of various cryptographic hash functions. Be careful not to pick a defective function such as MD5. Hash functions such as SHA-1 and SHA-2 are maintained by the NSA and are currently considered safe.

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,"password.bin"); //encrypted password to password.bin
  }

  private boolean checkPassword(byte[] pass) throws Exception {
    boolean arrays_equal;
    byte[] encrypted = loadBytes("password.bin"); //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. The attacker could be someone who knows or has figured out the encryption scheme being used by the program.

Noncompliant Code Example

This noncompliant code example 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 void setPassword(String pass) throws Exception {
    byte[] salt = generateSalt(12);
    MessageDigest sha_1 = MessageDigest.getInstance("SHA-1");
    byte[] hashVal = sha_1.digest((pass+salt).getBytes()); //encode the string and salt
    saveBytes(salt, "salt.bin");
    saveBytes(hashVal,"password.bin"); //save the hash value to credentials.bin
  }

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

  private byte[] generateSalt(int n) {
    //Generate a random byte array of length n
  }
}

Even when an attacker knows that the program stores passwords using SHA-1 and a 12-byte salt, she will be unable to retrieve the unencrypted password from password.bin and salt.bin.

Although this approach fixes the decryption problem from the previous noncompliant code example, at runtime this code may inadvertently store the passwords as cleartext. Java string objects are immutable and can be copied and internally stored by the Java Virtual Machine (JVM). Consequently, Java lacks a mechanism to securely erase a password once it has been stored in a String. See MSC63-JG. Limit the lifetime of sensitive data for more information.

Compliant Solution

This compliant solution addresses the problems from the previous noncompliant code example by using a byte array to store the password.

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

public final class Password {

  private void setPassword(byte[] pass) throws Exception {
    byte[] salt = generateSalt(12);
    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(salt, "salt.bin");   
    saveBytes(hashVal,"password.bin"); //save the hash value to credentials.pw
  }

  private boolean checkPassword(byte[] pass) throws Exception {
    byte[] salt = loadBytes("salt.bin");
    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 byte[] generateSalt(int n) {
    //Generate a random byte array of length n
  }

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

  private void 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 immediately after is converted into a hash value. Consequently, an attacker cannot get the password as cleartext after the erasure.

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 in order to enter it into a third-party application. This is permitted, even though it violates the guidline. The password manager is accessed by a single user and always has the user's permission to store his passwords and to display those passwords on command. As a result, provided the user is competent, the program's operation will be safe.

Related Guidelines

Bibliography

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