I'm generating an encryption key to encrypt some sensitive data with the Rijndael (AES) encryption algoritm. I'm using a guid as key generator. Are these keys "strong" enough?
Note: it is only sensitive for 20 minutes.
18
votes
Reminds me of this: blogs.msdn.com/b/oldnewthing/archive/2004/02/11/71307.aspx. On a serious note, you may wish to generate keys using a tool that has a less predictable pattern and format.
– jamiecon
You might want to take a look at stackoverflow.com/questions/643445/…
– Zruty
The only thing in common between Raymond's blog post and this question is the fact that they both talk about GUIDs...
– Cody Gray
@Jamie: GUIDs are no longer based on MAC addresses, and haven't been for quite a long time. Conspiracy theorists thought that was a way for Big Brother to track them.
– Cody Gray
@Jamie: Haha, sorry about that! Wasn't my intention. Respect for reading Raymond's blog, though. I too take every opportunity to link to it. If more developers read it, we'd really be a lot better off. Some things (like that particular post) are of historical interest only. But other things have practical, real-world application and should be taken to heart by anyone who does Win32 programming.
– Cody Gray
4 Answers
20
votes
No. The GUID keys can be predicted, at least those generated by .NET / WinAPI. Also keep in mind that the GUID does not even have a true 128bit randomness, because the version number is fixed. This gives you a very weak key in the first place.
To make matters worse, several versions of the GUID algorithm suffer from predictability. The point is that GUIDs are not created at random, but they follow certain rules to make it practically impossible for GUIDs to collide.
As discussed in the comments, GUID V1 suffered from privacy issues (or, the other way around, weaker keys) because the MAC address was used to generate them. With GUID V4, there are still ways to predict the sequence according to the (russian) source below.
Fortunately, .NET has cryptographically strong random generators on board. The RNGCryptoServiceProvider is your friend:
RNGCryptoServiceProvider _cryptoProvider = new RNGCryptoServiceProvider();
int fileLength = 8 * 1024;
var randomBytes = new byte[fileLength];
_cryptoProvider.GetBytes(randomBytes);
You might want to refer to:
How can I generate a cryptographically secure pseudorandom number in C#? -- shows alternatives and in a comment, the link to Wikipedia is given:
http://en.wikipedia.org/wiki/Globally_Unique_Identifier
In there, it is claimed (according to wikipedia, the page is in Russian)that one can predict previous and future numbers generated:
17
votes
No, GUIDs are not cryptographically secure. They follow an extremely predictable and well-documented pattern, and they're fairly short as far as truly secure keys go. But more to the point, you're misusing GUIDs by doing this. This is not what they were designed for. They're globally unique identifiers. The only guarantee you get is that each of them is unique. A sophisticated hacker will make child's play of reverse engineering a GUID.
Use the functions provided by the System.Security.Cryptography namespace, instead. That's what they're designed for. Read up on cryptographically secure pseudo-random number generators.
1
votes
I would not use a GUID for the key to encrypt data. Look at some of the implementations of the UUID protocol: UUID they can be predicted as they're computed to be unique, not random. I'd look into the using System.Security.Cryptography namespace for objects like "TripleDESCryptoServiceProvider" for sensitive data personally.
0
votes
Consider using this, or an equivalent random string generator: http://msdn.microsoft.com/en-us/library/aa379942%28VS.85%29.aspx
