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Update the spec from the implementation. #14

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May 12, 2018
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Update the spec from the implementation. #14

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bbee2b1
Update the spec from the implementation.
Stebalien Aug 30, 2017
177f8d3
Update the CID decoding algorithm to explicitly forbid multibase enco…
Stebalien Sep 1, 2017
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24 changes: 23 additions & 1 deletion README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -89,7 +89,7 @@ CIDv0 is a backwards-compatible version, where:
- the `multibase` is always `base58btc` and implicit (not written)
- the `multicodec` is always `protobuf-mdag` and implicit (not written)
- the `cid-version` is always `cidv0` and implicit (not written)
- the `multihash` is written as is.
- the `multihash` is written as is but is always a full (length 32) sha256 hash.

```
cidv0 ::= <multihash-content-address>
Expand All @@ -103,6 +103,28 @@ See the section: [How does it work? - Protocol Description](#how-does-it-work-pr
<cidv1> ::= <multibase-prefix><cid-version><multicodec-packed-content-type><multihash-content-address>
```

## Decoding Algorithm

To decode a CID, follow the following algorithm:

1. 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.
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.
* Otherwise, let `N` be the first varint in `cid`. This is the CID's version.
* If `N == 1` (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.
* If `N <= 0`, the CID is malformed.
* If `N > 1`, the CID version is reserved.

## Implementations

- [go-cid](https://github.com/ipfs/go-cid)
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