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ISCC-UNIT Instance-Code

IEP: 0009
Title: ISCC-UNIT Instance-Code
Author: Titusz Pan tp@iscc.foundation
Comments: #14
Status: DRAFT
Type: Core
License: CC-BY-4.0
Created: {{ git_creation_date_localized }}
Updated: {{ git_revision_date_localized }}

!!! note

This document is a **DRAFT** contributed as input to 
[ISO TC 46/SC 9/WG 18](https://www.iso.org/committee/48836.html). The final version is 
developed at the International Organization for Standardization as
[ISO/DIS 24138](https://www.iso.org/standard/77899.html)

General

  1. The Instance-Code shall be a checksum for any kind of data regardless of its media type.
  2. The Instance-Code shall match identical files and indicate data transmission errors or data manipulation.
  3. The Instance-Code shall be the prefix of a merkle root of a merkle tree as illustrated in Figure 11.
  4. The cryptographic hash algorithm used for Instance-Code calculation shall be updated if a security weakness is discovered.
  5. An update shall be indicated by a higher version number of the Instance-Code header.
![Figure 11 - Example of a merkle tree](images/iscc-iep-0009-f11-merkle-tree.png) Figure 11 - Example of a merkle tree

!!! note "NOTE 1"

Depending on security and uniqueness requirements, applications can generate and store 
Instance-Codes up to a length of 256 bits in steps of 32 bits. Longer variants of an 
Instance-Code are extensions of the shorter versions (which function only as checksums) and 
remain compatible while improving the security of the hash.

!!! note "NOTE 2"

The use of tree-based hashing supports efficient containment-proofs, verified streaming and 
verification of data from trusted and untrusted sources. A data delivery application can 
provide cryptographic proofs for partial data and streams such that the receiving application 
can incrementally verify chunks of data against a given Instance-Code.

Format

The Instance-Code shall have the data format illustrated in Figure 12:

![Figure 12 Data format of the Instance-Code](images/iscc-iep-0009-f12-instance-code.png) Figure 12 - Data format of the Instance-Code

!!! example "EXAMPLE 1: 64-bit Instance-Code in its canonical form:"

ISCC:IAAZ3NGA3HTIYUQD

!!! example "EXAMPLE 2: 256-bit Instance-Code in its canonical form:"

ISCC:IADZ3NGA3HTIYUQD3SGC737FF6S5KRTRXY5DEU7ANCEMVTT4MDS2OQY

Inputs

The input for calculating the Instance-Code shall be the bytes of a file, without reference to their meaning or structure.

Outputs

Instance-Code processing shall generate the following output elements:

  1. iscc: the Instance-Code in its canonical form (required);
  2. datahash: a cryptographic hash of the input file encoded as multihash (optional);
  3. filesize: the size of the input file in bytes (optional).

Processing

An ISCC processor shall calculate the Instance-Code as follows:

  1. Apply the tree-based cryptographic hash function to the file input using the appropriate number of bytes for the output digest.
  2. Use the hash digest of output as the ISCC-BODY of the Instance-Code.

For further technical details see source-code in modules code_instance.py of the reference implementation.

!!! note "NOTE"

The merkle tree chunk size is defined by the BLAKE3 [^1] algorithm to be 1024 bytes.

Conformance

An implementation of the Instance-Code algorithm shall be regarded as conforming to the standard if it creates the same Instance-Code as the reference implementation for the same data input.

!!! note "NOTE"

The ISCC reference implementation uses the open source BLAKE3 [^1] library for merkle tree 
hashing and appropriate use of this software will generate the same codes as the reference 
implementation.

Bibliography