Best practices for storing CIDv1s in EVM smart contracts

[danieln]

I’m trying to compile best practices for storing CIDv1 in EVM chains.

Just the hash

I saw some cases where people just store a sha256 hash because it fits into a single bytes32

The whole CID as binary

Others suggest two concatenating two string32 (see solidity - How to store IPFS hash using bytes32? - Ethereum Stack Exchange) since the gas cost is for 32 byte multiples anyways.

Storing as a string

The old NFT Storage docs recommended storing IPFS URI strings:

So do the IPFS Docs:

A base32 string encoded CID with the IPFS URI scheme, like ipfs://bafybeigdyrzt5sfp7udm7hu76uh7y26nf3efuylqabf3oclgtqy55fbzdi is 66 bytes long, which would increase the gas cost to 3*32 byte allocations.

What’s more, strings cannot be returned from contract functions:

Use string type or bytes32?

Do you really need to store the ipfs:// scheme?

If you need the path too, you probabbly want to keep the scheme. But since the CID can just point to anything (unixfs directory, file, dag-cbor etc) in a dag, I’m not sure there’s much value in storing a path on-chain if you can just store the root CID you’re interested in.

So if you don’t need the path, why not just name the the state variable cid.

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I’m not really a smart contract developer, so any input here would be appreciated.

[danieln]

A quick peek into the ENS resolver code reveals the following:

Internally, the content hash is:

• Stored as a bytes value in the resolver’s storage.
• Returned as-is by the contenthash() function.

And the bytes type functions as follows (solidity docs)

Because bytes are dynamically sized they can’t be used as keys for maps.

As for gas costs:

Feature	bytes32	bytes
Type	Fixed (32 bytes)	Dynamic (0 to ∞ bytes)
Storage slots used	1	1 + ⌈len / 32⌉
New write gas cost	20,000	20,000–100,000+ (length-based)
Read gas cost	~2,100	Higher, depends on length