ARC-73: Algorand Interface Detection Spec Source

A specification for smart contracts and indexers to detect interfaces of smart contracts.

AuthorWilliam G Hatch
TypeStandards Track
Requires 4 , 22 , 28

Algorand Interface Detection Spec


This ARC specifies an interface detection interface based on ERC-165. This interface allows smart contracts and indexers to detect whether a smart contract implements a particular interface based on an interface selector.


ARC-4 applications have associated Contract or Interface description JSON objects that allow users to call their methods. However, these JSON objects are communicated outside of the consensus network. Therefore indexers can not reliably identify contract instances of a particular interface, and smart contracts have no way to detect whether another contract supports a particular interface. An on-chain method to detect interfaces allows greater composability for smart contracts, and allows indexers to automatically detect implementations of interfaces of interest.


The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC-2119.

How Interfaces are Identified

The specification for interfaces is defined by ARC-4. This specification extends ARC-4 to define the concept of an interface selector. We define the interface selector as the XOR of all selectors in the interface. Selectors in the interface include selectors for methods, selectors for events as defined by ARC-28, and selectors for potential future kinds of interface components.

As an example, consider an interface that has two methods and one event, add(uint64,uint64)uint128, add3(uint64,uint64,uint64)uint128, and alert(uint64). The method selector for the add method is the first 4 bytes of the method signature’s SHA-512/256 hash. The SHA-512/256 hash of add(uint64,uint64)uint128 is 0x8aa3b61f0f1965c3a1cbfa91d46b24e54c67270184ff89dc114e877b1753254a, so its method selector is 0x8aa3b61f. The SHA-512/256 hash of add3(uint64,uint64,uint64)uint128 is 0xa6fd1477731701dd2126f24facf3492d470cf526e7d4d849fea33d102b45f03d, so its method selector is 0xa6fd1477 The SHA-512/256 hash of alert(uint64) is 0xc809efe9fd45417226d52b605658b83fff27850a01efeea30f694d1e112d5463, so its method selector is 0xc809efe9 The interface selector is defined as the bitwise exclusive or of all method and event selectors, so the interface selector is 0x8aa3b61f XOR 0xa6fd1477 XOR 0xc809efe9, which is 0xe4574d81.

How a Contract will Publish the Interfaces it Implements for Detection

In addition to out-of-band JSON contract or interface descrition data, a contract that is compliant with this specification shall implement the following interface:

  "name": "ARC-73",
  "desc": "Interface for interface detection",
  "methods": [
      "name": "supportsInterface",
      "desc": "Detects support for an interface specified by selector.",
      "readonly": true,
      "args": [
        { "type": "byte[4]", "name": "interfaceID", "desc": "The selector of the interface to detect." },
      "returns": { "type": "bool", "desc": "Whether the contract supports the interface." }

The supportsInterface method must be readonly as specified by ARC-22.

The implementing contract must have a supportsInterface method that returns:

  • true when interfaceID is 0x4e22a3ba (the selector for ARC-73, this interface)
  • false when interfaceID is 0xffffffff
  • true for any other interfaceID the contract implements
  • false for any other interfaceID


This specification is nearly identical to the related specification for Ethereum, ERC-165, merely adapted to Algorand.

Security Considerations

It is possible that a malicious contract may lie about interface support. This interface makes it easier for all kinds of actors, inclulding malicious ones, to interact with smart contracts that implement it.

Copyright and related rights waived via CCO.


Please cite this document as:

William G Hatch, "ARC-73: Algorand Interface Detection Spec," Algorand Requests for Comments, no. 73, January 2023. [Online serial]. Available: