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Now i wonder, why it takes "so long" to compute a block then? My Computer is doing 120 Mhashes/s, which in turn means,
the 2^32-1 hashes resulting from the different nonces should be done in like 35 seconds? (2^32-1 / 120 * 1000 * 1000)
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Because the difficulty is high enough that most of the time you won't successfully find a block after running through all possible nonces. You then have to alter the header and try again. This can be done by modifying the timestamp, by changing the list of transactions, or by using the "input" portion of the coinbase transaction as another nonce. This is explained in the page you linked to where it says:
Whenever Nonce overflows (which it does frequently), the extraNonce portion of the generation transaction is incremented, which changes the Merkle root.
So, i guess, the header is updating other values (timestamp?) during the time its beeing processed?
After each cycle through all possible nonces, yes. Generally the "extraNonce" is used, but timestamp and/or other transactions are also valid possibilities.
Who (or "what") is delivering the updated header, when theres no central authority?
If you are solo mining, then you are delivering the updated header. You can either write your own program to do so, or you can do as many others do and accept the header that Bitcoin-Qt creates for you. In that case you are relying on the "rules" that the developers of Bitcoin-Qt have chosen for building a header.
If you are mining in a pool then you are voluntarily accepting the centralization of the pool operator. In that case you are relying on the pool operator to provide you with valid headers to hash.
Also: If THAT is the case, it seems like a computer only needs to be like 35 times faster than mine to be able to calculate a
block in One Second, and therefore ALWAYS finding the valid hash, for given timestamp and ALL nonces...
A given timestamp and all nonces will not typically result in a valid header, but if a computer was 35 times faster than yours then it would calculate 35 times more hashes than yours in any given timeframe making it 35 times more likely to solve the next block than you.
Can s.b. provide some additional details about the block chain, block hashing and the actuall hash operations that
need to be performed?
Sure. What do you want to know?
