
Given a good enough programmer/cryptographer with a good PC you could decrypt just about any thing. The reason is that anyone that spends enough time to learn how to really encrypt something must have some sort of logical mind….so all you would need to do is to find a key…witch takes you back to the computer…just think what would have happened in WWII if they had the tech base that we have know and all we can do is get better from here.
sorry if this does not make sence as I have had toooooo much beer for my own good.

Enigma in WWII was the classic example of how important it was to know what hardware was used to do the encryption. In fact Poland provided an Enigma machine to the British at the start of the war, together with their cryptographic expertise, but no one at that time realised how important this was.
Later on, one or two enigma machines were captured, which meant that the team of mathematicians at Bletchley Park using primitive computers were able to continue to crack it.
In todays world, if you know that the encryption is being done on a normal PC, that should give you a head start, as the sort of questions you would ask is what OS is it using?
What are the weaknesses of this OS?
Is it a commercial algorithm or something else?
Etc.. etc ...

I'm not even into cryptography, but i'll throw in my take on this.
Give someone a string of random characters and they'll never break the encryption, or "crack it" so to speak. As far as I understand encryption, it relies on an algorithm, be it mathematical or otherwise, it's still an algorithm. The data is encrypted either using the algorithm alone, or more securely using a key/password that the algorithm incorporates. There are only a few ways to decrypt/crack the encrypted data.
1. Have a keyed algorithm and the proper key/password (decrypt)
2. Have an unkeyed algorithm and encrypted text (crack)
3. Have unkeyed encrypted text and the decrypted data (crack) You could determine the algorithm
4. Have the keyed algorithm (crack)
Beyond those 4 methods, your only way would be to brute force your way in. Even then, it'd be nearly impossible. Even with method 4 you would have to brute force your way threw every possible key, until the text made sense, depending on the length of the key this could take ages.
So when someone says here's text I encrypted, decrypt it for me. Tell them to F#$% Off. There are a lot more useful uses of your time. If I'm wrong on this, feel free to correct me, but this is how I see it.

In reply to HTRegz,
I don't really agree with you.
Let's take a simple example where you are running PGP on your PC.
Unless you are very careful, there will be some traces of the keys left on your PC.
So your encryption is worthless if either:
Your PC is infected with a trojan/virus  obviously shouldn't happen to people here :),
or you allow someone else physical access to your PC, in which case anything goes, or something you are doing leads the relevant authorities to make a raid, and seize your PC.
Brute force doesn't work, but how do you know what the mathematicians are getting up to?
ru absolutely sure prime numbers are randomly spread (which is what PGP relies on) ??
Just some thoughts, as this isn't as simple as you might think.
I agree with you it is normally a waste of time trying these challenges.
I think you will find that the likes of NSA have seen them all before ...

HTRegz
You have a point, if the string is truly random it will not be broken, not even by the intended recipient :) For this to be possible there has to be an algorithm/key.................that means it can be broken.
This follows on from my previous comment that if the string is nonsense, no one will find a message in it :)
In the old days of transmissions by radio, in blocks of numbers, some of these blocks would be dummies, to introduce an element of randomness. The recipient would know which ones to ignore depending on the time/date or whatever.
After a while, the codebreakers would probably identify these as well, but by that time the information in the message was past its merchandising/sell by date. This detection was based on hunch and experience, as opposed to brute force methods, which I would have thought would be vulnerable to this sort of subtrefuge?
Cheers

if you did notice in my example in the first post the clear text is also used as a key to the encryption and decryption (or better yet... the properties of the text). which means that if i change the clear text i'm also changing the key... therefore the method MUST be know or at least aproximated in order to have any valid starting point to start cracking. if you combine that w/ a techniqe that results in different (readable) plain text depending on the method you use .. that results for example, in deniability for certain cases where encryption key is used as proof of identity.... imagine an encrypted text that will give you a Abomb plans or Cat in a Hat depending on a method of encryption... but that as well as everything else is old news..

etruscan I absolutely agree with you. There HAS to be a starting point for any real hope of deciphering any kind of encryption.  Short of me giving an example of shifting the alphabet or any kind of child play attempt at hiding a message.
Popular belief that someone can take a set of data and decrypt it on a pc is science fiction or fantasy. You can site examples of previous successful hacks of the Enigma Device and DES, but none of those were decrypted using just an example of each devices data output. I am talking about random data only as intercepted by some means. Not physical access to the encryption/decryption engine.  Just plain old data only.
To say you could decrypt:
ewqpoicrqpweoirupq23804n v284ty 4v8qyq 89pewr p89q2b v3q
out of thin air is not possible. I like your bud can analogy. Sure you could look for key information like repeated patterns etc. Then start plugging away with mathematical solutions and complex arrangements of random data and try and interpret the result and in 1000 years come up with something, but what? You couldn’t have a gauge to verify anything. It could say “white bunny slippers” or “I want a peppermint” It would just be a guess. Without a stating point it's pointless.