Uh, no.
If u r not a schoolboy u should use mathematical notation instead of vague words. Try again.
The
wikipedia page gives a little more detail about 3DES. Encrypting 3 times does not triple the strength of the cipher. However, it also does not weaken it.
double sha2 is weaker then sha2 in some aspects.
im not sure that anyone have ever proven that sha2 hits its whole 'probability' space(2^256), if doesn't do that it will be loosing entropy by repeated applications.
more data in(a big fat block of data), means more random out. less data in(a single 256-bit sha2 hash), means less random out.
So kokjo is pointing out that nobody has proven that SHA-256 has a completely uniform probability distribution.
That does not imply that a second iteration makes the combined hash weaker for the reason fpgaminer pointed out. The example he used was to assume double-SHA-256 has about the same cryptographic strength as MD5. I will make a weaker assumption: assume the second hash has reduced variability because of the limited input size.
Once the attacker determines the intermediate hash in 2
80 time, they have a problem: they must now break the remaining 'single' hash. I suppose I should prove that later rounds don't undo the work of earlier rounds: but frankly, I don't have the time right now.
