What is the difference between a randomly generated number and secure randomly generated number?

Question

As the title says What is the difference between a randomly generated number and secure randomly generated number?

Answer 1

No computationally feasible algorithm should:

• recover the seed, or
• predict the "next bit"

for a secure random number generator.

Example: a linear feedback shift register produces lots of random numbers out there, but given enough output, the seed can be discovered and all subsequent numbers predicted.

Answer 2

With just a "random number" one usually means a pseudo random number. Because it's a pseudo random number it can be (easily) predicted by an attacker.

A secure random number is a random number from a truly random data source, ie. involving an entropy pool of some sorts.

Answer 3

It probably depends on the context, but when you are comparing them like this, I'd say "random number" is a pseduo random number and a "secure random number" is truly random. The former gives you a number based on a seed and an algorithm, the other on some inherintly random function.

Answer 4

It's like the difference between AES and ROT13.

To be less flippant, there is generally a tradeoff when generating random numbers between how hard it is and how predictable the next one in the sequence is once you've seen a few. A random number returned by your language's built-in rand() will usually be of the cheap, predictable variety.

Answer 5

Agree with Purfiedeas. There is also nice article about that, called Cheat Online Poker

Answer 6

A secure random number should not be predictable even given the list of previously generated random numbers. You'd typically use it for a key to an encryption routine, so you wouldn't want it guessable or predictable. Of course, guessable depends on the context, but you should assume the attacker knows all the things you know and might use to produce your random number.

There are various web sites that generate secure random numbers, one trusted one is hotbits. If you are only doing the random number generation as a one off activity, why not use a lottery draw result, since it's provably random. Of course, don't tell anyone which lottery and which draw, and put those numbers through a suitable mangle to get the range you want.

Answer 7

A random number would probably mean a pseudo random number returned by an algorithm using a 'seed'.

A secure random number would be a true random number returned from a device such as a caesium based random number generator (which uses the decay rate of the caesium to return numbers). This is naturally occurring and can't be predicted.

Answer 8

Note that numbers aren't random. "Randomness" simply isn't a property of any given number (not even when XKCD tells you otherwise).

Only the process of generating the numbers could possibly be rated on "randomness". Read What Colour are your bits for some discussion on this topic.