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+---
+snip: 55
+title: Mixed-case checksum address encoding
+author: Clément Walter <clement@kakarot.org>
+discussions-to: https://community.starknet.io/t/snip-55-mixed-case-checksum-address-encoding/114949
+type: Standards Track
+category: SRC
+status: Discussion
+created: 2024-10-24
+---
+
+## Abstract
+
+This SNIP proposes a checksum encoding for Starknet addresses.
+
+## Motivation
+
+Addresses are often mistyped, and this can lead to loss of funds or other issues. This SNIP aims to reduce the risk of mistyped addresses by encoding them in a way that makes it easier to detect typos. Furthermore, it enforces addresses to be represented as 32-bytes hex strings, which removes the risk of confusion between EVM and Starknet addresses.
+
+Benefits:
+
+- Enforce addresses to be represented as 32-bytes hex strings
+- Reduce the probability of mistyped addresses
+- Remove the risk to mix EVM and Starknet addresses
+
+See also the [ERC-55](https://github.com/ethereum/ERCs/blob/master/ERCS/erc-55.md) standard for reference.
+
+## Specification
+
+Code:
+
+```python
+from starkware.starknet.public.abi import starknet_keccak
+
+def checksum_encode(addr: int) -> str: # Takes a 32-byte binary address as input
+    addr_bytes = int.to_bytes(addr, 32, "big")
+    hex_addr = addr_bytes.hex()
+    checksummed_buffer = ""
+
+    # Treat the hex address as ascii/utf-8 for hashing
+    hashed_address = f"{starknet_keccak(addr_bytes):064x}"
+
+    # Iterate over each character in the hex address
+    for nibble_index, character in enumerate(hex_addr):
+
+        if character in "0123456789":
+            # We can't upper-case the decimal digits
+            checksummed_buffer += character
+        elif character in "abcdef":
+            # Check if the corresponding hex digit (nibble) in the hash is 8 or higher
+            hashed_address_nibble = int(hashed_address[nibble_index], 16)
+            if hashed_address_nibble > 7:
+                checksummed_buffer += character.upper()
+            else:
+                checksummed_buffer += character
+        else:
+            raise eth_utils.ValidationError(
+                f"Unrecognized hex character {character!r} at position {nibble_index}"
+            )
+
+    return "0x" + checksummed_buffer
+
+checksum_encode(0x49d36570d4e46f48e99674bd3fcc84644ddd6b96f7c741b1562b82f9e004dc7)
+# '0x049D36570D4e46f48e99674bd3fcc84644DdD6b96F7C741B1562B82f9e004dC7'
+```
+
+In English, convert the address to hex, but if the `i`th digit is a letter (i.e. it's one of `abcdef`) print it in uppercase if the `4*i`th bit of the keccak hash of the lowercase hexadecimal address is 1 otherwise print it in lowercase.
+
+## Backwards Compatibility
+
+This SNIP is fully backwards compatible: existing tools will accept both non-checksummed and checksummed addresses.
+
+## Security considerations
+
+No security considerations.
+
+## Copyright
+
+Copyright and related rights waived via [MIT](../LICENSE).