EIPs/EIPS/eip-55.md

```
EIP: 55
Title: Mixed-case checksum address encoding
Author: Vitalik Buterin
Type: Standard Track
Category: ERC
Status: Accepted
Created: 2016-01-14
```

# Specification

Code:

``` 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':
            o += c
        else:
            o += c.upper() if (v & (2**(255 - 4*i))) else c.lower()
    return '0x'+o

def test(addrstr):
    assert(addrstr == checksum_encode2(bytes.fromhex(addrstr[2:])))

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

```

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

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
- 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.

# References

1. EIP 55 issue and discussion https://github.com/ethereum/eips/issues/55
2. Python example by @Recmo https://github.com/ethereum/eips/issues/55#issuecomment-261521584