Frazer Jolly Goodfellow <(E-Mail Removed)> wrote in

news:Xns97C3A7C0B653frz@62.253.170.163:

> "nemo_outis" <(E-Mail Removed)> wrote in

> news:Xns97C2A6B65D746abcxyzcom@204.153.244.170:

>

>> Zak <(E-Mail Removed)> wrote in

>> news:Xns97C2C5EBF7A9764A18E@127.0.0.1:

>>

>>> Winzip offers 256 bit AES. So do other apps.

>>>

>>> If I use a password made up of ordinary characters (A-Z, a-z,

>>> 0-9) with no specials then how many characters do I need to use

>>> to make AES 256 uncrackable by a brute force attack?

>>>

>>> The info out there talks mainly of key length but I am not

>>> familiar with this field and I can sense they are not talking

>>> about the length of the password I am using.

>>>

>>> There is a little bit here but it seems out of date:

>>>

>>> <http://www.dekart.com/howto/howto_di.../howto_recover

>>> _lost_pa ssword/>

>>>

>>

>> In general you want to make the password/passphrase as strong as

>> the underlying algorithm (256 bits in this case).

>

> Please would you explain 'strong' in this context?
Strong for a password means resistant to being found. If a password is

truly random there is no more efficient way to find it than brute force

(i.e., exhaustive search). While one could be unbelievably lucky and get

it on the first guess, in general (i.e., the expectational value) one

would need 2^255 guesses. There is NO possibility of doing that with any

computer that now exists or that will exist for the foreseeable future.

To illustrate, Let's say, overly generously, that the fastest computer

today is capable of 1 petaflop (a quadrillion ops/second). Let's say it

could try one password guess per op. A trillion, trillion,trillion such

computers working for the 15 billion years the universs has been in

existence (since the big bang) would not have made a dent in the problem

(i.e., would only have looked at 1 one-billionth of 1 percent of the

possible passwords)! To me that seems strong enough!

>> With a

>> character set of 62 characters (a-z upper & lower case plus 0-9)

>> you want 62^n >= 2^256, where n (an integer) is the number of

>> random characters in the password.

>

> Why?

>> A little math results in n = 43.

>

> AIUI: given enough time a brute force attack will always succeed

> eventually. What time frame is your estimation method based upon?
No, brute force will NOT succeed! There isn't nearly enough time before

the heat death of the universe!

The fastest known computer would need a 100 billion, trillion, trillion,

trillion times the entire life of the universe!

> Other sources suggest very much lower numbers, including the OP

> quoted source. Another example is

> http://lastbit.com/rm_bruteforce.asp, which estimates that assuming

> a brute force trisl speed is 500,000 passwords per second, a random

> 9-character key of both lowercase and uppercase letters (i.e. 52

> possibilities) would on average take 178 years to crack. Why is

> there such a large discrepancy vs. your estimate?
The explanation in two words, m'boy: Logarithms and exponents.

It's time you refreshed your memory regarding them.

A 43-character password (drawn from 52 possible characters) is NOT 5

times as hard to guess as a 9-character one. No, it is approximately ten

billion, trillion, trillion, trillion, trillion times as hard!

Regards,