How recreate a hash digest of a multihash in IPFS - ipfs

Question

Assuming I'm adding data to IPFS like this:
$ echo Hello World | ipfs add
This will give me QmWATWQ7fVPP2EFGu71UkfnqhYXDYH566qy47CnJDgvs8u - a CID which is a Base58 encoded Multihash.
Converting it to Base16, tells me that the hash digest for what IPFS has added is a SHA2-256 hash:
12 - 20 - 74410577111096cd817a3faed78630f2245636beded412d3b212a2e09ba593ca
<hash-type> - <hash-length> - <hash-digest>
I know that IPFS doesn't just hash the data, but actually serializes it as Unixfs protobuf first and then puts that in a dag.
I'd like to demystify, how to get to the 74410577111096cd817a3faed78630f2245636beded412d3b212a2e09ba593ca but I'm not really sure how to get hold of the created dag that holds the Unixfs protobuf with the data.
For example I can write the serialized raw data to disk and inspect it with a protobuf decoder:
$ ipfs block get QmWATWQ7fVPP2EFGu71UkfnqhYXDYH566qy47CnJDgvs8u > /tmp/block.raw
$ protoc --decode_raw < /tmp/block.raw
This will give me the serialized data in a readable format:
1 {
1: 2
2: "Hello World\n"
3: 12
}
However, piping that through SHA-256 still gives me a different hash, which makes sense because IPFS puts the protobuf in a dag and multihashes that one.
$ protoc --decode_raw < /tmp/block.raw | shasum -a 256
So I decided to figure out how to get hold of that dag node, hash it myself to get to the hash I'm looking for.
I was hoping using ipfs dag get QmWATWQ7fVPP2EFGu71UkfnqhYXDYH566qy47CnJDgvs8u will give me a multihash that can then be decoded, but it turns out it returns some other data hash that I don't know how to inspect:
$ ipfs dag get QmWATWQ7fVPP2EFGu71UkfnqhYXDYH566qy47CnJDgvs8u
$ {"data":"CAISDEhlbGxvIFdvcmxkChgM","links":[]}
Any ideas on how to decode data from here?
UPDATE
data is a Base64 representation of the original data: https://github.com/ipfs/go-ipfs/issues/4115

Answer 1

The hash you're looking for is the hash of the output of ipfs block get QmWATWQ7fVPP2EFGu71UkfnqhYXDYH566qy47CnJDgvs8u. IPFS hashes the encoded value.
Instead of running:
protoc --decode_raw < /tmp/block.raw | shasum -a 256
Just run:
shasum -a 256 < /tmp/block.raw
but it turns out it returns some other data hash that I don't know how to inspect
That's because we currently use a protobuf inside of a protobuf. The outer protobuf has the structure {Data: DATA, Links: [{Name: ..., Size: ..., Hash: ...}]}.
In:
1 {
1: 2
2: "Hello World\n"
3: 12
}
The 1 { ... } part is the Data field of the outer protobuf. However, protoc --decode_raw *recursively* decodes this object so it decodes theData` field to:
Field 1 (DataType): 2 (File)
Field 2 (Data): "Hello World\n"
Field 3 (Filesize): 12 (bytes)
For context, the relevant protobuf definitions are:
Outer:
// An IPFS MerkleDAG Link
message PBLink {
// multihash of the target object
optional bytes Hash = 1;
// utf string name. should be unique per object
optional string Name = 2;
// cumulative size of target object
optional uint64 Tsize = 3;
}
// An IPFS MerkleDAG Node
message PBNode {
// refs to other objects
repeated PBLink Links = 2;
// opaque user data
optional bytes Data = 1;
}
Inner:
message Data {
enum DataType {
Raw = 0;
Directory = 1;
File = 2;
Metadata = 3;
Symlink = 4;
HAMTShard = 5;
}
required DataType Type = 1;
optional bytes Data = 2;
optional uint64 filesize = 3;
repeated uint64 blocksizes = 4;
optional uint64 hashType = 5;
optional uint64 fanout = 6;
}
message Metadata {
optional string MimeType = 1;
}

Answer 2

I'm not sure what that encoding is but you can unmarshal the dag data field like this in js-ipfs:
const IPFS = require('ipfs')
const Unixfs = require('ipfs-unixfs')
const ipfs = new IPFS
ipfs.dag.get('QmWATWQ7fVPP2EFGu71UkfnqhYXDYH566qy47CnJDgvs8u', (err, d) => {
console.log(Unixfs.unmarshal(d.value.data).data.toString()))
// prints Hello World
})

Answer 3

According to Steven's answer, using protobuf is working. Here is the full code of my approach.
ipfs.proto
syntax = "proto3";
message PBNode {
bytes Data = 1;
}
message PBLink {
bytes Hash = 1;
string Name = 2;
uint64 Tsize = 3;
}
message Data {
enum DataType {
Raw = 0;
Directory = 1;
File = 2;
Metadata = 3;
Symlink = 4;
HAMTShard = 5;
}
DataType Type = 1;
bytes Data = 2;
}
cid.js
const mh = require('multihashes');
const axios = require('axios');
const crypto = require('crypto');
const protobuf = require("protobufjs");
const IPFS = protobuf.loadSync('./ipfs.proto').lookupType('PBNode');
class CID {
/**
* convert IPFS multihash to sha2-256 hash string
* #param {string} multihash
* #param {boolean} prefix
* #returns {string} sha2-256 hash string starting with 0x
*/
static toHash(multihash, prefix = false) {
return prefix ? '0x' : ''
+ mh.decode(mh.fromB58String(multihash)).digest.toString('hex')
}
/**
* convert sha2-256 hash string to IPFS multihash
* #param {string} str
* #returns {string} IPFS multihash starting with Qm
*/
static fromHash(str) {
str = str.startsWith('0x') ? str.slice(2) : str;
return mh.toB58String(mh.encode(Buffer.from(str, 'hex'), 'sha2-256'))
}
/**
* hash the buffer and get the SHA256 result compatible with IPFS multihash
* #param {Buffer} buf
* #returns {string}
*/
static hash(buf) {
const r = IPFS.encode({
Data: {
Type: 2,
Data: buf,
filesize: buf.length
}
}).finish();
return crypto.createHash('sha256').update(r).digest('hex');
}
}
async function ipfsGet(cid) {
const x = await axios.get(`http://your.address.xxx/ipfs/${cid}`, {
responseType: 'arraybuffer'
});
return Buffer.from(x.data);
}
const r = "QmfQj4DUWEudeFdWKVzPaTbYimdYzsp14DZX1VLV1BbtdN";
const hashFromCID = CID.toHash(r);
console.log(hashFromCID);
ipfsGet(r).then(buf => {
const hashCalculated = CID.hash(buf);
console.log(hashCalculated);
console.log(hashCalculated === hashFromCID);
console.log(CID.fromHash(hashCalculated) === r)
});
module.exports = CID;

Answer 4

I have created a module for Java which can generate the correct file hash:
https://github.com/rmnvalera/java-cid-generate-hash