Question

I’ve read several stackoverflow posts about this topic, particularly this one:

Secure hash and salt for PHP passwords

but I still have a few questions, I need some clarification, please let me know if the following statements are true and explain your comments:

1. If someone has access to your database/data, then they would still have to figure out your hashing algorithm and your data would still be somewhat secure, depending on your algorithm? All they would have is the hash and the salt.

2. If someone has access to your database/data and your source code, then it seems like no matter what your do, your hashing algorithm can be reversed engineered, the only thing you would have on your side would be how complex and time consuming your algorithm is?

3. It seems like the weakest link is: how secure your own systems are and who has access to it?

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Lasse V. Karlsen … brings up a good point, if your data is compromised then game over … my follow up question is: what types of attacks are these hashes trying to protect against? I’ve read about rainbow table and dictionary attacks (brute force), but how are these attacks administered?

Answer 1

You question is about using passwords as an authentication mechanism and how to securely store these passwords in a database using a hash. As you probably already know the goal is to be able to verify passwords without storing these passwords i clear text in the database. In this context let me try to answer each of your questions:

If someone has access to your database/data, then they would still have to figure out your hashing algorithm and your data would still be somewhat secure, depending on your algorithm? All they would have is the hash and the salt.

The basic idea of hashing passwords is that the attacker has knowledge of the hashing algorithm and has access to both the hash and the salt. By selecting a cryptographic strong hash function and a suitable salt value that is different for each password the computational effort required to guess the password is so high that the cost exceeds the possible gain the attacker can get from guessing the password. So to answer your question, hiding the hash function does not improve the security.

If someone has access to your database/data and your source code, then it seems like no matter what your do, your hashing algorithm can be reversed engineered, the only thing you would have on your side would be how complex and time consuming your algorithm is?

You should always use a well-known (and suitably strong) hashing algorithm, and reverse engineering this algorithm is not meaningful as there is nothing hidden in your code. If you didn’t mean reverse engineer but actually reverse then, yes, the passwords are protected by the complexity of reversing the hash function (or guessing a password that matches a hash value). Good hash functions makes this very hard.

It seems like the weakest link is: how secure your own systems are and who has access to it?

In general this is true, but when it comes to securing passwords by storing them as hashes you should still assume that the attacker has full access to the hashes and design your system accordingly by choosing an appropriate hash function and using salts.

What types of attacks are these hashes trying to protect against? I’ve read about rainbow table and dictionary attacks (brute force), but how are these attacks administered?

The basic attack that password hashing protects against is when the attacker gets access to your database. The clear text password cannot be read from the database and the password is protected.

A more sophisticated attacker can generate a list of possible passwords and compute the hash using the same algorithm as you. He can then compare the computed hash to the stored hash and if he finds a match he has a valid password. This is a brute force attack and it is generally assumed that the attacker has “offline” access to your database. By requiring the users to use long and complex passwords the effort required to “brute force” a password is significantly increased.

When the attacker wants to attack not one password, but all the passwords in the database a large table of passwords and hash value pairs can be precomputed and further improved by using what is called hash chains. Rainbow tables is an application of this idea and can be used to brute force many passwords simultaneously without increasing the effort significantly. However, if a unique salt is used to compute the hash for each password a precomputed table becomes useless as it is different for each salt and cannot be reused.

To sum it up: Security by obscurity is not a good strategy for protecting sensitive information and modern cryptography allows you to secure information without having to resort to obscurity.