ISC Diary | Hashing Passwords（情報元のブックマーク数）
SANSの記事にも、ストレッチングとSALTの記事がのってるな。
After talking about SQL, this is the second part of the mini series to help you protect yourself from simple persistent attacks as we have seen them in the last couple months. A common MO employed in these attacks is to steal passwords from a database via sql injection. Later, the attacker will try to use these passwords to break into other sites for which users may choose the same password. Of course, part of the problem is password reuse. But for now, we will focus on the hashing of passwords to make it harder for an attacker to retrieve a users plain text password.
InfoSec Handlers Diary Blog - Hashing Passwords
First of all: What is hashing? According to NIST, "A hash algorithm (alternatively, hash "function") takes binary data, called the message, and produces a condensed representation, called the message digest. A cryptographic hash algorithm is a hash algorithm that is designed to achieve certain security properties." [1] A good cryptographic hash will make it hard to find two messages with the same hash, or find clear text for a specific hash value. Common hashing algorithms are MD-5 (old), or the "Secure Hashing Algorithms" (SHA) family of hashing algorithms (SHA-1, SHA-256...). Another common but less popular algorithm is RIPE-MD.
Probably the most important defense against rainbow tables is the idea of introducing a "salt". First of all a "salt" will ensure that two users who happen to use the same password, will end up with a different hash. A salt can also be used to increase the length of the plain text beyond the point where rainbow tables become practical.
InfoSec Handlers Diary Blog - Hashing Passwords
In order to use a "salt", the salt value and the users password are first concatenated, then the string is hashed.
Another trick to harden a hash is to just apply the same algorithm multiple times. For example, if we take the SHA-1 algorithm, and apply it 100 times, we will slow down a brute force attempt by a factor of 100. However, the delay in validating an individual password will be hard to notice.
When selecting an algorithm to hash passwords, it is important to select carefully as it is difficult to change the algorithm later. You will have to ask users to change their password if you do as you no longer know what password they picked.
