# holo_hash
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License: [](https://github.com/holochain/cryptographic-autonomy-license)
holo_hash::HoloHash is a hashing framework for Holochain.
Note that not all HoloHashes are simple hashes of the full content as you might expect in a "content-addressable" application. The main exception is `AgentPubKey`, which is simply the key itself to enable self-proving signatures. As an exception it is also named exceptionally, i.e. it doesn't end in "Hash".
## Hash Types
Each HoloHash has a HashType. There are two flavors of HashType: *primitive*, and *composite*
### Primitive HashTypes
Each primitive HashType has a unique 3-byte prefix associated with it, to easily distiguish between hashes in any environment. These prefixes are multihash compatible. The primitive types are:
| Agent | AgentPubKey | uhCAk |
| Entry | EntryHash | uhCEk |
| DhtOp | DhtOpHash | uhCQk |
| Dna | DnaHash | uhC0k |
| NetId | NetIdHash | uhCIk |
| Action | ActionHash | uhCkk |
| Wasm | DnaWasmHash | uhCok |
The "HoloHash alias" column lists the type aliases provided to refer to each type of HoloHash. For instance, `ActionHash` is the following type alias:
```rust
pub type ActionHash = HoloHash<hash_type::Action>;
```
(the prefixes listed are the base64 representations)
### Composite HashTypes
Composite hash types are used in contexts when one of several primitive hash types would be valid. They are implemented as Rust enums. The composite types are:
`EntryHash`: used to hash Entries. An Entry can hash to either a `ContentHash` or an `AgentPubKey`.
`AnyDhtHash`: used to hash arbitrary DHT data. DHT data is either an action or an Entry, therefore AnyDhtHash can refer to either an `ActionHash` or an `EntryHash`.
## Serialization
HoloHash implements `Display` providing a `to_string()` function accessing the hash as a user friendly string. It also provides TryFrom for string types allowing you to parse this string representation.
HoloHash includes a 4 byte (or u32) dht "location" that serves dual purposes. - It is used as a checksum when parsing string representations. - It is used as a u32 in our dht sharding algorithm.
HoloHash implements [SerializedBytes](https://lib.rs/crates/holochain_serialized_bytes) to make it easy to cross ffi barriers such as WASM and the UI websocket.
## Example
```rust
use holo_hash::*;
use std::convert::TryInto;
use holochain_serialized_bytes::SerializedBytes;
let entry: EntryHash =
"uhCEkWCsAgoKkkfwyJAglj30xX_GLLV-3BXuFy436a2SqpcEwyBzm"
.try_into()
.unwrap();
assert_eq!(3860645936, entry.get_loc());
let bytes: SerializedBytes = entry.try_into().unwrap();
assert_eq!(
"{\"type\":\"EntryHash\",\"hash\":[88,43,0,130,130,164,145,252,50,36,8,37,143,125,49,95,241,139,45,95,183,5,123,133,203,141,250,107,100,170,165,193,48,200,28,230]}",
&format!("{:?}", bytes),
);
```
## Advanced
Calculating hashes takes time - In a futures context we don't want to block. HoloHash provides sync (blocking) and async (non-blocking) apis for hashing.
```rust
use holo_hash::*;
let entry_content = b"test entry content";
let content_hash = EntryHash::with_data_sync(entry_content.to_vec()).into();
assert_eq!(
"EntryHash(uhCEkhPbA5vaw3Fk-ZvPSKuyyjg8eoX98fve75qiUEFgAE3BO7D4d)",
&format!("{:?}", content_hash),
);
```
### Sometimes your data doesn't want to be re-hashed:
```rust
use holo_hash::*;
// pretend our pub key is all 0xdb bytes
let agent_pub_key = vec![0xdb; 32];
let agent_id: HoloHash = AgentPubKey::from_raw_32(agent_pub_key).into();
assert_eq!(
"AgentPubKey(uhCAk29vb29vb29vb29vb29vb29vb29vb29vb29vb29vb29uTp5Iv)",
&format!("{:?}", agent_id),
);
```
## Contribute
Holochain is an open source project. We welcome all sorts of participation and are actively working on increasing surface area to accept it. Please see our [contributing guidelines](/CONTRIBUTING.md) for our general practices and protocols on participating in the community, as well as specific expectations around things like code formatting, testing practices, continuous integration, etc.
* Connect with us on our [forum](https://forum.holochain.org)
## License
[](https://www.apache.org/licenses/LICENSE-2.0)
Copyright (C) 2019 - 2023, Holochain Foundation
This program is free software: you can redistribute it and/or modify it under the terms of the license
provided in the LICENSE file (CAL-1.0). This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE.