Specification（python）

from ethereum import utils

def checksum_encode(addr): # Takes a 20-byte binary address as input
    o = ‘‘
    v = utils.big_endian_to_int(utils.sha3(addr.hex()))
    for i, c in enumerate(addr.hex()):
        if c in ‘0123456789‘://就是如果address在i位置上的值是数字的话，就不做任何改变
            o += c
        else: //但是如果是字符的话，就要另进行判断，(2**(255 - 4*i))这个的二进制的结果就是从后向前数的255 - 4*i个位置上的值为1，即是为了实现从前往后数为4*i位置的数字
            o += c.upper() if (v & (2**(255 - 4*i))) else c.lower()//255的原因是hash的v有32bytes,即32*8=256，从0开始                                                                   //所以意思是如果小写十六进制地址的散列v的第4*i位也是1，则以大写形式打印，否则以小写形式打印。
    return ‘0x‘+o

def test(addrstr):
    assert(addrstr == checksum_encode(bytes.fromhex(addrstr[2:])))

test(‘0x5aAeb6053F3E94C9b9A09f33669435E7Ef1BeAed‘)
test(‘0xfB6916095ca1df60bB79Ce92cE3Ea74c37c5d359‘)
test(‘0xdbF03B407c01E7cD3CBea99509d93f8DDDC8C6FB‘)
test(‘0xD1220A0cf47c7B9Be7A2E6BA89F429762e7b9aDb‘)

注意：v = utils.big_endian_to_int(utils.sha3(addr.hex()))

所谓的大端模式（Big-endian），是指数据的高字节，保存在内存的低地址中，而数据的低字节，保存在内存的高地址中，这样的存储模式有点儿类似于把数据当作字符串顺序处理：地址由小向大增加，而数据从高位往低位放；比如1234，如果是数字的话应该从低位开始读4321（即小端模式）；但是这里为大端模式，所以读法就是1234。这是为了顺序处理addr的hash值

In English, convert the address to hex, but if the ith digit is a letter (ie. it‘s one of abcdef) print it in uppercase if the 4*ith bit of the hash of the lowercase hexadecimal address is 1 otherwise print it in lowercase.

Rationale

Benefits:

• Backwards compatible with many hex parsers that accept mixed case, allowing it to be easily introduced over time
• Keeps the length at 40 characters。 address长度为40个字符
• On average there will be 15 check bits per address, and the net probability that a randomly generated address if mistyped will accidentally pass a check is 0.0247%. This is a ~50x improvement over ICAP, but not as good as a 4-byte check code.  上面bit的转换效率没有字节的转化效率快，所以下面实现的是半字节（16进制一字符4bits）形式的转换方法：

例子：

const createKeccakHash = require(‘keccak‘);

function toChecksumAddress (address) {
  address = address.toLowerCase().replace(‘0x‘, ‘‘);
  console.log(address);
  var hash = createKeccakHash(‘keccak256‘).update(address).digest(‘hex‘);//update就是输入要加密的值，digest就是将加密好的hash值以16进制的形式输出
  console.log(hash);//5cfac663f45837b409c4d3dc1cef5f4759734f4989dd53a31b1265734c0b28f4，64个
  var ret = ‘0x‘;

  for (var i = 0; i < address.length; i++) {//所以只用得到hash的前40个字符
    if (parseInt(hash[i], 16) >= 8) {//即将16进制的hash[i]转化成10进制的数值后与8进行比较，如果在i位置的hash的值大于或等于8，相应位置的address的值就换成大写，否则就还是小写
      ret += address[i].toUpperCase();//其实就是根据得到的hash值来相应将address转换成大小写皆有的形式
      console.log(‘if‘);
      console.log(ret);
    } else {
      ret += address[i];
      console.log(‘else‘);
      console.log(ret);
    }
  }

  return ret;
}

var addr = ‘0xfb6916095ca1df60bb79ce92ce3ea74c37c5d359‘;//去掉0x是40个
console.log(toChecksumAddress(addr));

这里的例子address是16进制的，当然，如果the hex address encoded as ASCII，那么就写成：

var hash = createKeccakHash(‘keccak256‘).update(Buffer.from(address.toLowerCase(), ‘ascii‘)).digest()

Adoption

Exchange support for mixed-case address checksums, as of 2017-05-27:

References

1. EIP 55 issue and discussion https://github.com/ethereum/eips/issues/55
2. Python implementation in ethereum-utils
3. Ethereumjs-util implementation https://github.com/ethereumjs/ethereumjs-util/blob/75f529458bc7dc84f85fd0446d0fac92d991c262/index.js#L452-L466

/**
 * Returns a checksummed address
 * @param {String} address
 * @return {String}
 */
exports.toChecksumAddress = function (address) {
  address = exports.stripHexPrefix(address).toLowerCase()
  var hash = exports.sha3(address).toString(‘hex‘)
  var ret = ‘0x‘

  for (var i = 0; i < address.length; i++) {
    if (parseInt(hash[i], 16) >= 8) {
      ret += address[i].toUpperCase()
    } else {
      ret += address[i]
    }
  }

  return ret
}

　　4.Swift implementation in EthereumKit

原文地址：https://www.cnblogs.com/wanghui-garcia/p/9709389.html