Hashing passwords safely and securely

This section explains the reasons behind using hashing functions to secure passwords, as well as how to do so effectively.

Why should passwords supplied by users be hashed?

Password hashing is one of the most basic security considerations that must be made when designing any application or service that accepts passwords from users. Without hashing, any passwords that are stored can be stolen if the data store is compromised, and then immediately used to compromise not only the application or service, but also the accounts of users on other services, if they do not use unique passwords.

By applying a hashing algorithm to the user's passwords before storing them, it becomes implausible for any attacker to determine the original password, while still being able to compare the resulting hash to the original password in the future.

It is important to note, however, that hashing passwords only protects them from being compromised in the data store, but does not necessarily protect them from being intercepted by malicious code injected into the application or service itself.

Why are common hashing functions such as md5() and sha1() unsuitable for passwords?

Hashing algorithms such as MD5, SHA1 and SHA256 are designed to be very fast and efficient. With modern techniques and computer equipment, it has become trivial to brute force the output of these algorithms, in order to determine the original input.

Because of how quickly a modern computer can reverse these hashing algorithms, many security professionals strongly suggest against their use for password hashing.

How should passwords be hashed, if the common hash functions are not suitable?

When hashing passwords, the two most important considerations are the computational expense, and the salt. The more computationally expensive the hashing algorithm, the longer it will take to brute force its output.

PHP provides a native password hashing API that safely handles both hashing and verifying passwords in a secure manner.

The suggested algorithm to use when hashing passwords is Blowfish, which is also the default used by the password hashing API, as it is significantly more computationally expensive than MD5 or SHA1, while still being scalable.

The crypt() function is also available for password hashing, but it is only recommended for interoperability with other systems. Instead, it is strongly encouraged to use the native password hashing API whenever possible.

What is a salt?

A cryptographic salt is data which is applied during the hashing process in order to eliminate the possibility of the output being looked up in a list of pre-calculated pairs of hashes and their input, known as a rainbow table.

In more simple terms, a salt is a bit of additional data which makes hashes significantly more difficult to crack. There are a number of services online which provide extensive lists of pre-computed hashes, as well as the original input for those hashes. The use of a salt makes it implausible or impossible to find the resulting hash in one of these lists.

password_hash() will create a random salt if one isn't provided, and this is generally the easiest and most secure approach.

How are salts stored?

When using password_hash() or crypt(), the return value includes the salt as part of the generated hash. This value should be stored verbatim in the database, as it includes information about the hash function that was used and can then be given directly to password_verify() when verifying passwords.

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password_verify() should always be used instead of re-hashing and comparing the result to a stored hash in order to avoid timing attacks.

The following diagram shows the format of a return value from crypt() or password_hash(). As can be seen, they are self-contained, with all the information on the algorithm and salt required for future password verification.