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An acceptable technique for limiting the exposure of passwords is the use of hash functions, which 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 minimizes the exposure of the password without presenting any practical disadvantages.

Cryptographic Hash Functions

The value produced by a hash function is the hash value or 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.

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Java's MessageDigest class provides implementations of various cryptographic hash functions. Avoid defective functions such as the Message-Digest Algorithm (MD5). Hash functions such as Secure Hash Algorithm (SHA)-1 and SHA-2 are maintained by the National Security Agency and are currently considered safe.

Noncompliant Code Example

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

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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, but it uses a String to store the password:

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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. Consequently, Java lacks a mechanism to securely erase a password once it has been stored in a String. See 1.02. 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:

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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 much harder to retrieve the cleartext password after the erasure. Providing truly guaranteed erasure is extremely challenging, is likely to be platform-specific, and may even be impossible because of the involvement of copying garbage collectors, dynamic paging, and other platform features that operate below the level of the Java language. Note, however, that most other languages share these complications (with the exception of garbage collection).

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 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 limit to safety and security is the user's competence rather than the program's operation.

Bibliography

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