Namehashing

Namehashing is an algorithm that converts a domain name in a classical format (like www.example.crypto) to ERC-721 token id. All .crypto ecosystem contracts accept a domain name as a method argument in the form of an ERC-721 token. Namehashing is defined as a part of the EIP-137 standard. See the standard for a text description of the algorithm.
To verify an implementation of the namehash algorithm, use the following reference table:
Domain Name
ERC721 Token
.
0x0000000000000000000000000000000000000000000000000000000000000000
crypto
0x0f4a10a4f46c288cea365fcf45cccf0e9d901b945b9829ccdb54c10dc3cb7a6f
example.crypto
0xd584c5509c6788ad9d9491be8ba8b4422d05caf62674a98fbf8a9988eeadfb7e
www.example.crypto
0x3ae54ac25ccd63401d817b6d79a4a56ae7f79a332fe77a98fa0c9d10adf9b2a1
a.b.c.crypto
0x353ea3e0449067382e0ea7934767470170dcfa9c49b1be0fe708adc4b1f9cf13
Reverse lookup
Fundamentally, namehashing is a one-way operation. It recursively hashes the labels using the SHA-256 hash function.
If one possesses a precomputed table of all hashes and corresponding domains reverse lookups are possible. This table can be reconstructed using the events on the CNS and UNS Registry NewURI event.
JSON RPC
The UNS and CNS metadata APIs track all domain names with their corresponding namehash. That makes it possible to obtain an original domain name from token metadata which can be retrieved from the token metadata URI.
The token metadata URI can be retrieved via ETH RPC call to ProxyReader#tokenURI. This works for CNS and UNS.
Now, the domain name along with other metadata can be retrieved by performing a simple GET request using the token metadata URI.
For example:
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Javascript
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let proxyReaderContractInstance = new Contract(address, abi); // Get a proxy reader contract instance using web3 or ethers
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let tokenUri = await proxyReaderContractInstance.tokenURI("0x756e4e998dbffd803c21d23b06cd855cdc7a4b57706c95964a37e24b47c10fc9"); // call the tokenURI method
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​
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let metadataResponse = await fetch(tokenUri); // GET data from URI
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let metadata = await metadataResponse.json(); // Parse it as json
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console.log(metadata.name); // Retrieve the domain name from metadata
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NewURI events
When a domain is created its name and token ID are logged using a `NewURI` event. This event can be looked up in order to reverse the namehash of a domain. This approach also works for CNS and UNS. 
For an implementation example, see the `unhash` function of the resolution library. 
Note that regardless of the way a namehash is reversed, it should always be forward-validated by hashing the retrieved domain name. In order to ensure the integrity of the retrieved domain name.
Public API
Unstoppable Domains maintains a public-facing API which can be used to obtain domain information with a namehash. To do this, call https://unstoppabledomains.com/api/v1/{namehash}
CNS Example Request:
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$ curl https://unstoppabledomains.com/api/v1/0x0f4a10a4f46c288cea365fcf45cccf0e9d901b945b9829ccdb54c10dc3cb7a6f
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CNS Example Response:
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{
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  "addresses": {},
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  "whois": {},
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  "ipfs": {},
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  "gundb": {},
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  "social": {},
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  "meta": {
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    "owner": "0x000000000000000000000000000000000000dead",
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    "type": "CNS",
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    "ttl": 0,
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    "domain": "crypto",
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    "namehash": "0x0f4a10a4f46c288cea365fcf45cccf0e9d901b945b9829ccdb54c10dc3cb7a6f"
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  },
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  "records": {}
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}
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UNS Example Request:
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$curl https://unstoppabledomains.com/api/v1/0x4cf2206dea828b317ad033d760428ba3b0624b974bd5657bbeaef92d624e7b85
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UNS Example Response:
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{
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"addresses": {},
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"multicoinAddresses": {},
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"whois": {},
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"ipfs": {
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"html": "QmQ38zzQHVfqMoLWq2VeiMLHHYki9XktzXxLYTWXt8cydu"
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},
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"social": {},
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"dns": [],
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"meta": {
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"domain": "myjohnny.wallet",
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"namehash": "0x4cf2206dea828b317ad033d760428ba3b0624b974bd5657bbeaef92d624e7b85",
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"tokenId": "34803577301218677365125560751744006764397707708613262855939915661234351799173",
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"owner": "0xe7474d07fd2fa286e7e0aa23cd107f8379085037",
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"resolver": "0x049aba7510f45ba5b64ea9e658e342f904db358d",
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"type": "UNS",
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"ttl": 0
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},
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"records": {
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"ipfs.html.value": "QmQ38zzQHVfqMoLWq2VeiMLHHYki9XktzXxLYTWXt8cydu"
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}
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}
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