CID (Content IDentifier) Specification
Self-describing content-addressed identifiers for distributed systems
Table of Contents
- Motivation
- What is it?
- How does it work?
- Design Considerations
- Human Readable CIDs
- Versions
- Decoding Algorithm
- Implementations
- FAQ
- Maintainers
- Contribute
- License
Motivation
CID is a format for referencing content in distributed information systems, like IPFS. It leverages content addressing, cryptographic hashing, and self-describing formats. It is the core identifier used by IPFS and IPLD. It uses a multicodec to indicate its version, making it fully self describing.
You can read an in-depth discussion on why this format was needed in IPFS here: ipfs/specs#130 (first post reproduced here)
What is it?
A CID is a self-describing content-addressed identifier. It uses cryptographic hashes to achieve content addressing. It uses several multiformats to achieve flexible self-description, namely multihash for hashes, multicodec for data content types, and multibase to encode the CID itself into strings.
Concretely, it's a typed content address: a tuple of (content-type, content-address)
.
How does it work?
Current version: CIDv1
A CIDv1 has four parts:
<cidv1> ::= <mb><multicodec-cidv1><mc><mh>
# or, expanded:
<cidv1> ::= <multibase-prefix><multicodec-cidv1><multicodec-content-type><multihash-content-address>
Where
<multibase-prefix>
is a multibase code (1 or 2 bytes), to ease encoding CIDs into various bases. NOTE: Binary (not text-based) protocols and formats may omit the multibase prefix when the encoding is unambiguous.<multicodec-cidv1>
is a multicodec representing the version of CID, here for upgradability purposes.<multicodec-content-type>
is a multicodec code representing the content type or format of the data being addressed.<multihash-content-address>
is a multihash value, representing the cryptographic hash of the content being addressed. Multihash enables CIDs to use many different cryptographic hash function, for upgradability and protocol agility purposes.
That's it!
Design Considerations
CIDs design takes into account many difficult tradeoffs encountered while building IPFS. These are mostly coming from the multiformats project.
- Compactness: CIDs are binary in nature to ensure these are as compact as possible, as they're meant to be part of longer path identifiers or URIs.
- Transport friendliness (or "copy-pastability"): CIDs are encoded with multibase to allow choosing the best base for transporting. For example, CIDs can be encoded into base58btc to yield shorter and easily-copy-pastable hashes.
- Versatility: CIDs are meant to be able to represent values of any format with any cryptographic hash.
- Avoid Lock-in: CIDs prevent lock-in to old, potentially-outdated decisions.
- Upgradability: CIDs encode a version to ensure the CID format itself can evolve.
Human Readable CIDs
It is advantageous to have a human readable description of a CID, solely for the purposes of debugging and explanation. We can easily transform a CID to a "Human Readable CID" as follows:
<hr-cid> ::= <hr-mbc> "-" <hr-cid-mc> "-" <hr-mc> "-" <hr-mh>
Where each sub-component is represented with its own human-readable form:
<hr-mbc>
is a human-readable multibase code (egbase58btc
)<hr-cid-mc>
is the stringcidv#
(egcidv1
orcidv2
)<hr-mc>
is a human-readable multicodec code (egcbor
)<hr-mh>
is a human-readable multihash (egsha2-256-256-abcdef0123456789...
)
For example:
# example CID
zb2rhe5P4gXftAwvA4eXQ5HJwsER2owDyS9sKaQRRVQPn93bA
# corresponding human readable CID
base58btc - cidv1 - raw - sha2-256-256-6e6ff7950a36187a801613426e858dce686cd7d7e3c0fc42ee0330072d245c95
See: https://cid.ipfs.io/#zb2rhe5P4gXftAwvA4eXQ5HJwsER2owDyS9sKaQRRVQPn93bA
Versions
CIDv0
CIDv0 is a backwards-compatible version, where:
- the
multibase
of the string representation is alwaysbase58btc
and implicit (not written) - the
multicodec
is alwaysdag-pb
and implicit (not written) - the
cid-version
is alwayscidv0
and implicit (not written) - the
multihash
is written as is but is always a full (length 32) sha256 hash.
cidv0 ::= <multihash-content-address>
CIDv1
See the section: How does it work?
<cidv1> ::= <multibase-prefix><multicodec-cidv1><multicodec-content-type><multihash-content-address>
Decoding Algorithm
To decode a CID, follow the following algorithm:
- If it's a string (ASCII/UTF-8):
- If it is 46 characters long and starts with
Qm...
, it's a CIDv0. Decode it as base58btc and continue to step 2. - Otherwise, decode it according to the multibase spec and:
- If the first decoded byte is 0x12, return an error. CIDv0 CIDs may not be multibase encoded and there will be no CIDv18 (0x12 = 18) to prevent ambiguity with decoded CIDv0s.
- Otherwise, you now have a binary CID. Continue to step 2.
- Given a (binary) CID (
cid
):- If it's 34 bytes long with the leading bytes
[0x12, 0x20, ...]
, it's a CIDv0.- The CID's multihash is
cid
. - The CID's multicodec is DagProtobuf
- The CID's version is 0.
- The CID's multihash is
- Otherwise, let
N
be the first varint incid
. This is the CID's version.- If
N == 0x01
(CIDv1):- The CID's multicodec is the second varint in
cid
- The CID's multihash is the rest of the
cid
(after the second varint). - The CID's version is 1.
- The CID's multicodec is the second varint in
- If
N == 0x02
(CIDv2), orN == 0x03
(CIDv3), the CID version is reserved. - If
N
is equal to some other multicodec, the CID is malformed.
- If
- If it's 34 bytes long with the leading bytes
Implementations
FAQ
Q. I have questions on multicodec, multibase, or multihash.
Please check their repositories: multicodec, multibase, multihash.
Q. Why does CID exist?
We were using base58btc encoded multihashes in IPFS, and then we needed to switch formats to IPLD. We struggled with lots of problems of addressing data with different formats until we created CIDs. You can read the history of this format here: ipfs/specs#130
Q. Is the use of multicodec similar to file extensions?
Yes, kind of! like a file extension, the multicodec identifier establishes the format of the data. Unlike file extensions, these are in the middle of the identifier and not meant to be changed by users. There is also a short table of supported formats.
Q. What formats (multicodec codes) does CID support?
We are figuring this out at this time. It will likely be a table of formats for secure distributed systems. So far, we want to address: IPFS's original protobuf format, the new IPLD CBOR format, git, bitcoin, and ethereum objects.
Maintainers
Captain: @jbenet.
Contribute
Contributions welcome. Please check out the issues.
Check out our contributing document for more information on how we work, and about contributing in general. Please be aware that all interactions related to IPLD are subject to the IPFS Code of Conduct.
Small note: If editing the README, please conform to the standard-readme specification.
License
This repository is only for documents. These are licensed under a CC-BY 3.0 Unported License © 2016 Protocol Labs Inc.