Create caip-draft-evm-xaddress.md

CAIPs/caip-draft-evm-xaddress.md

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+---
+# Every document starts with a front matter in YAML enclosed by triple dashes.
+# See https://jekyllrb.com/docs/front-matter/ to learn more about this concept.
+caip: CAIP-X
+title: Universal EVM-Cross-Chain Address
+author: Zainan Victor Zhou (@xinbenlv)
+discussions-to: TBD
+status: Draft
+type: Standard
+created: 2024-07-10
+updated: 2024-07-10
+---
+
+## Simple Summary
+A standard for an extended address format for EVM chain that includes chainId with consideration of future extensions, using 4bytes prior the 20bytes of normal address.
+
+
+## Abstract
+Currently EVM address are single chain, and is 20bytes. We envision native support to make contract call between EVM chains with different chainIds.
+This proposal presents one possible solutions: 
+1. limiting chainId number space hence the length of its possible value; 
+2. pack it into the 32bytes/u256 native word length for maximum compatibility; and
+3. reserve additional bytes for future extension
+
+## Motivation
+We envision one day EVM contract can make contract calls to other EVM chains (which have different chainIds).
+For a contract to make such call, they needs to be able to specify a contract when they are on another EVM chain with a chainId. Hence we need a cross-chain contract address.
+
+## Specification
+
+An EVM-Cross-Chain Address aka "Xaddress" is defined follows
+
+1. Extension section of 4bytes(32bits), denoted as `Ext`
+2. Main Address 0x<20bytes single chain Address>
+
+`Ext === 0x0000` is reserved for "only the address of the same chain"
+`Ext === 0xFFFF` is reserved for "the original address of ANY chainId"
+
+3. Only EIP-2294 "Safe Range" is supported by this CAIP at the version. ChainId beyond Safe Range is out of the scope of this version.
+4. Confirming contracts SHALL NOT make call to address beyond Safe Range of ChainId, but confirming contracts SHOULD consider accepting chainIds beyond this range (restrictive in write, flexible on read)
+
+
+## Rationale
+1. The reason we use a 4bytes length is based on [EIP-2294](https://eips.ethereum.org/EIPS/eip-2294) to specify a SAFE boundary of max chain Id to be `2^31 - 1` based on discussion of current usage of the chainId.
+2. We didn't directly use [CAIP-50](./caip-50.md) because we see the support for EVM cross-chain function call more native. We envision in the future this CAIP becomes a sub-standard UNDER CAIP-50 or its future version.
+3. We provide reserved Ext=0x0000 which is inspired by 0.0.0.0 in IPv4. And 0xFFFF as masking in IPv4 as well. We see possibility in a sharded or multi-chain situation, EOA or future EIP-7702 could potential be represented the same address across any EVM chain in the same base address.
+
+
+## Test Cases
+<!--Please add diverse test cases here if applicable. Any normative definition of an interface requires test cases to be implementable. -->
+
+## Security Considerations
+<!--Please add an explicit list of intra-actor assumptions and known risk factors if applicable. Any normative definition of an interface requires these to be implementable; assumptions and risks should be at both individual interaction/use-case scale and systemically, should the interface specified gain ecosystem-namespace adoption. -->
+
+## Privacy Considerations
+<!--Please add an explicit list of intra-actor assumptions and known risk factors if applicable. Any normative definition of an interface requires these to be implementable; assumptions and risks should be at both individual interaction/use-case scale and systemically, should the interface specified gain ecosystem-namespace adoption. -->
+
+## Backwards Compatibility
+There are onchain memory and storage representations where contract addresses were assumed to be 20bytes and only 20bytes and conduct packing for storage optimization.
+
+
+## References 
+<!--Links to external resources that help understanding the CAIP better. This can e.g. be links to existing implementations. See CONTRIBUTING.md#style-guide . -->
+
+- [CAIP-50](./caip-50.md)
+- [EIP-2294](https://eips.ethereum.org/EIPS/eip-2294)
+
+## Copyright
+Copyright and related rights waived via [CC0](../LICENSE).