公开课地址：https://www.coursera.org/learn/cryptocurrency/home/welcome
3.6 limitations & improvements
目前比特币硬编码的限制：
1- 10min/block
2- 1MB/block
3- 20,000 signature operations/block
4- 100M satoshis/block
5- 21M total bitcoins max
6- 50,25,12.5,... mining reward
4~6对经济平衡性影响巨大，无法改变。
throughputs limit of bitcoins:
- 1MB/block (10min)
250B/txn
- 7 txn/min :(
VISA: 2-10k txn/sec
Paypal: 50-100 txn/sec
cryptographic limit of btc:
- only 1 signature algorithm (ECDSA/P256)
- hard-coded hash functions
crypto primitives might be broken by 2040...
版本升级导致硬分叉。详细讲解硬分叉发生过程。
另一种解决方案：软分叉。new nodes enforced. old nodes will approve.
风险：old nodes might mine now-invalid blocks.
软分叉举例讲解：P2SH
其他可能通过软分叉实现的feature：
- 新的签名scheme
- extra per-block metadata
-- shove in the coinbase parameter
-- commit to UTXO tree in each block
必须通过硬分叉：
- new op codes
- changes to size limits
- changes to mining rate
- many small bug fixes
目前很不可能发生。
下节预告：人。人不是比特币节点：
- 人如何和比特币网络互动
- 人怎么用比特币交换现金
- 人怎么安全存储比特币
问题：哪个需要硬分叉：
A- Disabling the OP_SHA1 instruction
B- A requirement that each transaction have its outputs sorted by value in ascending (or non-decreasing) order
C- Increasing the maximum permitted size of blocks
D- Decreasing the maximum permitted size of blocks
E- Adding a new OP_SHA3 script instruction
正确答案是：只有C和E才需要硬分叉！
