Re: What if a large number of miners were suddenly forced to shut down?

95% of miners are suddenly offline.
Be it a new law or an EMP strike. What would happen to bitcoin? Huh

Confirmation of transactions would take longer untill we hit the next difficulty adjustment. After that difficulty would be decreased. I am not 100% certain if there is a maximum the difficulty can change per adjustment, but I suspect there is.

There is. The difficulty may not increase or decrease more than 4-fold. If the hashrate drop occurs when we're more than about 400 blocks from a difficulty change and the hashrate stays down then we'll endure a few months of 8-block days (1 block every 3 hours on average) before retargeting to grind through 2 months of 45-minute blocks.

In its 6-year life Bitcoin has only once experienced a full 4-fold difficulty change.

Are you sure about the difficulty not being able to decrease by more then 75% in one adjustment? I know that one of the early adjustments almost was 4x and satoshi mentioned that it was close to the limit, however I have never seen anything regarding there being any limits as to how much the difficulty can go down.

Nice to know someone's paying attention.

Well, certainly the intent is for a difficulty change to be limited between 25% and 400%. Here's the code in the reference client responsible for this. Technically, a difficulty rise might ever so slightly exceed 400% due to some rounding which is done to keep block-header size down, but this won't be more than by about 1 part in 10 million and will not allow a breach of the 75%-drop limit.

Bitcoin's difficulty increased precisely 4-fold on 16th July, 2010 with block #68544.

Block #68543 has a "bits" value of 0x1c05a3f4 (470131700 expressed in hexadecimal) where block #68544's bits value is 0x1c0168fd. The bits value is a 32-bit compact representation of the 256-bit target (the value below which a block's hash must be for it to pass the proof-of-work requirement). The corresponding target values are:

Code:

0x0.05a3f4 * (0x100 ^ 0x1c) = 0x0000000005a3f400000000000000000000000000000000000000000000000000 = 92413 * 2^202

and

Code:

0x0.0168fd * (0x100 ^ 0x1c) = 0x000000000168fd00000000000000000000000000000000000000000000000000 = 92413 * 2^200

The target dropped precisely 4-fold here. This corresponds to a 4-fold increase in difficulty from

Code:

45.3858223410126280934500557280902037592113663662038890632270351573...

Code:

181.5432893640505123738002229123608150368454654648155562529081406295...