Hash

March 15, 2005
coldfusion

After a long break in my series of the little enhancements in ColdFusion MX 7 (CFMX 7 Little Things), I am back today with another article, this time with the Hash function.

In versions of ColdFusion prior to 7, the Hash function used the MD5 algorithm to generate hash values. In version 7 you can specify which algorithm to use, and the new choices are:

  • SHA - Generates a 28 character hash string using the Secure Hash Standard SHA-1 algorithm
  • SHA-256 - Generates a 44 character hash string using the SHA-256 algorithm
  • SHA-384 - Generates a 64 character hash string using the SHA-384 algorithm
  • SHA-512 - Generates a 88 character hash string using the SHA-512 algorithm

Here's a code example that generates an 88 character hash, which is pretty large:

<cfoutput>#Hash("myPassword", "SHA-512")#</cfoutput>

CF 7 Also adds an encoding argument, which according to the docs:

Must be a character encoding name recognized by the Java runtime. The default value is the value specified by the defaultCharset entry in the neo-runtime.xml file, which is normally UTF-8

The Hash function is most commonly used as a one way encryption for passwords. If you don't want to store a users password in your database in plain text, you can store the Hash of the password. Then when the user logs in instead of comparing the password with a value from your database, you compare a Hash of the input password, with the Hash of the users password in the database.

There is no known way to reverse a hash, so if your user forgets their password, you cannot email it to them, you have to come up with another way to authenticate the user (secret questions is one good way), in order to reset the password.



Related Entries

7 people found this page useful, what do you think?

Comments

Note that in addition to the new hashes listed, you can still specify MD5 (which is the default) to use the same algorithm as prior versions of CF. It's helpful to be able to specify the default, because it will allow you to store the hash algorithm in the database for a given password, and then migrate passwords to a different hash algorithm as they are updated, for example.
Pete, One other thing to have developers consider is using a salt (a random string stored in an additional db column, and prepended to the password before hashing) along with the Hash. Salting the password before hashing it makes it virtually impossible to launch a successful dictionary style attack against the hashed password values stored in the database because an attacker would have to try all of the possible salt values for each hash value in their dictionary. For example, if you use a 12-character string consisting of upper case letters from A to Z, there are 26^12 possible salt combinations for each password.
Rob that is a great idea, i've never heard of that. The attacker would not necessarily even know if the hash had been salted. Thanks for the tip.
Ryan, an attacker will know salt is being used, because they'll see the 'salt' column in the table. But that's fairly irrelevant, and may actually deter the attacker from continuing, because the chance of success is so greatly reduced.
You actually can reverse a hash...but it takes some time. See http://passcracking.com/. The site uses Rainbow tables (http://www.antsight.com/zsl/rainbowcrack/) as outlined by Philippe Oechslin's faster time-memory trade-off technique. Generating the tables takes forever (several days), but once you get them created, cracking a hash (of the MD5 variety) takes less than half-an-hour. I think hashing is a very good technique for making it more difficult for attackers to get data, but I've seen developers lean on this as a crutch thinking their passwords are safe. I'm not an alarmist, but hashing passwords is just one part of keeping your information safe. Monitoring your server, enforcing proper folder-level security, and a general understanding of how hackers perform their attacks is the best way to keep your data safe.
I've been using randomly generated salt values (like Rob described) since CF5. If anyone would like a copy of the code I'd be happy to share it.
Hey Jeff, I'd certainly be interested in knowing more! T
Wayne, it would be more accurate to say that you can get potential values for the hash rather than say that you can reverse it. It's a small, but important difference.
SHA-512 generates a 128 byte character sequence
People can use a site like http://www.hashreverse.com in order to do a lookup of the md5. When your password is something like billy123, it takes a fraction of a second to look up. As an admin, it would behoove you to do something like this: "md5(Password & Salt) & Salt)" so that it is much more unlikely that your users' passwords are stored precomputed in a database someplace.
I tried many different things, but sha1 results in 40 characters (I need the 28 you mention in your article). Any idea why this is and how to keep it to 28 characters?
Hi Any one have the idea how to encode any string into 88 char ex ->input aa output QMVVmHphUFduLwH0nFj751jAyrZdghNUxcg0PPKuziuTja7ZE9tf5YO488ciQJ2Wee9cXE86SidVNkz2WmjKnw== Is this base 64 encription or SHA1? please hlep me out

Post a Comment




  



Spell Checker by Foundeo

Recent Entries



foundeo


did you hack my cf?