Seemingly Inefficient Hashing Question???

[Etlase2]

SHA-256's way of hashing arbitrarily-sized data allows for trivial "extension attacks". Given only the hash of "password; command", an attacker can trivially produce the hash of "password; command; attacker'sArbitraryData" (ie. he can append any data he wants without knowing what he's appending to).

Isn't this the likely reasoning behind using a hash of the hash?

[1715457476]

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[1715457476]

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[theymos]

Isn't this the likely reasoning behind using a hash of the hash?

I doubt it. Bitcoin doesn't hash anything secret.

[Hawkix]

....

Code:

Current Target: 00000000000009AE020000000000000000000000000000000000000000000000
Block Hash:     00000000000000001e8d6829a8a21adc5d38d0a473b144b6765798e61f98bd1d

This block hash is smaller than the target so it "solves" the block (if and only if all the inputs are still valid - which they aren't).  
Actually this block hash is really small.  It would be below the target even if difficulty was in the hundred million range.

Umm, that's quite interesting for me, how the hell you did get that hash?

[DeathAndTaxes]

....

Code:

Current Target: 00000000000009AE020000000000000000000000000000000000000000000000
Block Hash:     00000000000000001e8d6829a8a21adc5d38d0a473b144b6765798e61f98bd1d

This block hash is smaller than the target so it "solves" the block (if and only if all the inputs are still valid - which they aren't).  
Actually this block hash is really small.  It would be below the target even if difficulty was in the hundred million range.

Umm, that's quite interesting for me, how the hell you did get that hash?

Random luck.  Many block hashes are significantly smaller than their target.  The requirement is that they simply be equal to or smaller than the current target. 

Hash values are going to be randomly distributed between

0000000000000000000000000000000000000000000000000000000000000000

and

ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff

[etotheipi]

By the way, just for reference the block in the main chain with the lowest difficulty is:

Block with the lowest difficulty:
   Block Num:        125552
   Block Hash:       00000000000000001e8d6829a8a21adc5d38d0a473b144b6765798e61f98bd1d
   Equiv Difficulty: 35,987,768,035
   Equiv Diff bits:  67.07

That block would've been valid even if the network difficulty was 35,900,000,000  (yes, billion), even though the actual difficulty was 244,000.   

That may seem ludicrous, but it's actually on par with the approximately 2^68 hashes that the network has done up until now.  The last part of it (the 67.07) says that if you were to do 2^67.07 hashes, you'd have a 50% chance of finding a hash just as good...

[Sergio_Demian_Lerner]

There is a use for storing previous hashes and that use is attacking the hash function using the birthday paradox. http://en.wikipedia.org/wiki/Birthday_problem

Nevertheless, the attack is not feasible today for SHA256.