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bitcoin's mining time is about 15 minutes while ethereum's mining time is 10 seconds. Is not it bad for ethereum's mining time to be too short? (from security aspect). Why is it so important for ethereum's proof of work to be ASIC resistant function?

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Is not it bad for ethereum's mining time to be too short? (from security aspect)

It could have been bad for security, yes. A shorter block time means potentially less hashing power has gone into solving the proof of work for a given block, meaning less hashing power would be required to reverse the block. (By creating an alternative block with the same number.)

However, Ethereum uses uncles (see: What is an uncle/ommer block?) to help increase security by incorporating the hashing power spent on mining orphaned blocks, which are themselves more likely due to the shorter block time. (Also have a look at Ethereum's GHOST selection rule: What is GHOST and what is its relationship to Frontier and Casper?)

(It's possible there are other reasons that security is affected. I can't immediately think what they are. Others may know more.)

Why is it so important for ethereum's proof of work to be ASIC resistant function?

This is a different question. Originally ASIC-resistance was intended to level the playing field, allowing anyone with a GPU to mine, rather than skewing the advantage towards people who could afford ASICs, which in turn would lead to mining centralisation.

Is this important? That depends who you ask...

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  • I see, and why are uncles help to the ethereum's security? (only by giving reward to miners so they will keep mining? )
    – adi
    Commented Apr 19, 2018 at 21:59
  • When a miner adds one or more uncles to a block, the block's difficulty value is increased accordingly. (The difficulty field in the block's header is proportional to the hashing power that was used to mine the block.) If an attacker wanted to mine another version of the same block (so he could perform a double spend attack), then he'd have to expend more hashing power than the original block. So if we increase the difficulty of the original block by incorporating uncles, we make it harder to perform a double spend attack.
    – Richard Horrocks
    Commented Apr 19, 2018 at 22:09
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Is not it bad for ethereum's mining time to be too short? (from security aspect)

In itself, yes you are right. But users can wait more blocks before considering a transaction confirmed. This will mitigate that problem. The conservative, secure option has been stated to be to wait for 6 confirmations on the Bitcoin blockchain and to wait for 12 confirmations on the Ethereum blockchain.

Some people also say 40 and some 20-25. Have a look at What number of confirmations is considered secure in Ethereum?.

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  • Can you elaborate please why waiting for 12 confirmations is safe?
    – adi
    Commented Apr 19, 2018 at 21:52
  • 12 blocks is just a convention. The probability that someone can reverse the work is an exponentially falling curve as a function of the number of confirmations, so you have to put the limit somewhere.
    – Thorkil Værge
    Commented Apr 20, 2018 at 8:50

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