Revision as of 01:42, 28 May 2013

Introduction

Cryptographic hashing functions are used to create digital signatures, message authentication codes (MACs) and other forms of authentication. They are also used to store user passwords in databases instead of storing the password in clear text and help prevent data leakage in session management for web applications. The actual algorithm used to create a cryptology function varies per implementation (MD5, SHA-512, etc.) but the main function is to take arbitrary block of data and return a fixed-size bit string.

The code reviewer needs to be aware of three main things when reviewing code that uses cryptographic hashing functions.

• Legality of the cryptographic hashing functions if the source code is being exported to another country.

• The life cycle of the cryptographic hashing function being used.

• Basic programming of cryptographic hashing functions.

Legal

In the United States in 2000, the department of Commerce Bureau of Export revised encryption export regulations. The results of the new export regulations it that the regulations have been greatly relaxed. However if the code is to be exported outside of the source country current export laws for the export and import counties should be reviewed for compliance.

Case in point is if the entire message is hashed instead of a digital signature of the of message the National Security Agency (NSA) considers this a quasi-encryption and State controls would apply.

It is always a valid choice to seek legal advice within the organization that the code review is being done to ensure legal compliance.

Lifecycle

With Security nothing is secure forever. This is especially true with cryptographic hashing functions. MD5 is now consider completely broken since 2004 and should not be used. The code reviewer needs to understand the implementation of the hashing function and it’s life cycle.

Programming/Vulnerabilities

The most common programmatic issue with hashing is not using a salt value or if using a salt the salt value is to short and or the same salt value is used in multiple hashes.

The salt value should be generated by using a cryptographically secure pseudo-random number generator.

• Java – java.security.SecureRandom

.* Net (C#,VB) - System.Security.Cryptography.RNGCryptoServiceProvider

• PHP - ???
• Ruby - ???
• Perl - ???
• C++ none managed code on CLR or none windows ????
• Javascript ?????

The salt value does not need to be secrete and can be stored along with the hash value.

Lastly, never accept in a code review an algorithm created by the programmer for hashing or copy a hashing function taken from the Internet. Always use cryptographic functions that are provided by the language framework the code is written in. These functions are well vetted and well tested by experience cryptographers.

References:

• http://valerieaurora.org/hash.html (Lifetimes of cryptographic hash functions)
• http://docs.oracle.com/javase/6/docs/api/java/security/SecureRandom.html
• http://msdn.microsoft.com/en-us/library/system.security.cryptography.rngcryptoserviceprovider.aspx