Does IPFS CID stay constant forever for the same file?

#1

Hello,

This is perhaps very basic question but I couldn’t find in my search. Assuming the file content doesn’t change even single bit, would IPFS CID never change even if it’s being added/pinned by different users?

Would this assumption stay true for foreseeable future? I know it depends on hash algorithm, so just by knowing what hash algorithm was used (SHA-256), the IPFS CID would be reproducable for foreseeable feature right? Or is there other information that needs to be stored as well?

#2

The answer to your question is yes*, the CID for the same file will always be the same no matter who hashes it.

The exception to this is when/if the IPFS devs make a change to the protocol (as we are expecting soonish) that changes the hashing algorithm and results in a different CID. I’m sure they will tell us well in advance of this happening.

#3

Even for >256 KB files right? Because of chunking.

Even if they make a change to the protocol, it is expected to be backwards compatible right?

#4

To my understanding, the chunking algorithm is also deterministic and will result in the same CID every time.

I can only speculate about what future changes will mean, but I assume they would make every attempt to keep backwards compatibility.

#5

Regardless what is the default option set, it should always be possible to add with the same parameters to reproduce the same hash.

I have not heard of any changes to the default coming soon though…

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#6

There is at least one experimental feature that is stable but not enabled by default due to it changing the hashes, as you can find here:

The footnote for that feature is what I based my answer off of.

#7

Thank you very much for all the replies. So In a nutshell

  1. The CID should stay the same for a file of any size, as long as the same multihash and CID version is used.

  2. The multihash or chunking algorithm may change in future, but will be 100% backward compatible.

So my final question and i would be very grateful for the answer, what parameters should i store to be able to replicate the same CID in case IPFS version changes later? I know hash algorithm number is one thing i need. Anything else i would need?

#8

Bump, please help, would be grateful

#9

To reproduce the same hash in the future you will need to remember:

  • The hashing algorithm used (default now is sha256)
  • The dag-format used (default now is “balanced”)
  • The chunking algorithm used (default now is “fixed-262144” which means “256KiB blocks”).
  • Whether --raw-leaves was used.
  • For very large folders, whether the HAMT directory sharding option was enabled.
#10

Thank you very much hector