Private Search Set (PSS)

Private Search Set (PSS) is an extension to standard Bloom filter or a standalone hash file to describe and share private set.

Abstract

The Private Search Set (PSS) is an extension to the standard Bloom filter or a standalone hash file to describe and share private set. It provides features such as fast lookup of values without disclosing the values, easy distribution of private sets to a group of users or organizations, watermarking and tracking down potential leak of a private search set (PSS), offline private search, and flexible meta-format to describe and extend the private search set (PSS).

Features

• Fast lookup of values (such as indicators, hashes or any text) without disclosing the values
• Easily distribute private sets to a group of users or organisations
• Watermarking and tracking down potential leak of a private search set (PSS)
• Offline private search
• Flexible meta-format to describe and extend the private search set (PSS)

Use-cases

• Improved privacy and security for users and organisations willing to participate in information sharing communities
• Reduced risk of data breaches and leaks of cyber threat intelligence
• Increased ability to protect sensitive information or information which could be abused such as CSAM (Child Sexual Abuse Material) or personal information leak
• Enhanced ability to share information with trusted parties without compromising privacy

Overview of creation and lookup of PSS

flowchart TD
    Y["canonize(foobar.onion)"] --> A
    A["insert keyhashed(foobar.onion)"] -->|key, Blake2| B[key-hashed]
    B --> |insert| C[Distributed PSS file]
    B --> |insert| D[Distributed PSS Bloomfilter]
    style B fill:#0f0,stroke:#333,stroke-width:4px
    style Y fill:#0f0,stroke:#333,stroke-width:4px
    style A fill:#0f0,stroke:#333,stroke-width:4px
    style Z fill:#fff,stroke:#333,stroke-width:4px
    Z["search keyhashed(canonized foobar.onion)"] -->|search| C
    Z["search keyhashed(canonized foobar.onion)"] -->|search| D

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Meta format

Key name	Type	Description	Required
version	number	Version of the Private Search Set (PSS).	✓
name	string	A concise name used for the directory name.	✓
description	string	Human readable description of the set.	✓
generated-timestamp	number	Generation timestamp in epoch format.	✓
algorithm	string	Keyed-hash message authentication. Available: - Blake2b - Blake3 - HMAC-SHA-256 - HMAC-SHA-512	✓
keyid	string	The reference to the key used in the keyed-hash message authentication algorithm. If the default value is used, then the private shared key infected.	✓
filter	hash	The filter description along with its type, format and model.	✓
misp-attribute-types	array	Array of string with the types covered by the private search set. Types can be any from types mentioned in the default MISP types. If not specified, text type is covered.	-
misp-object-template	array	Array of string with the object template name and the version separated with a semicolon such as person:19.	-
canonicalization-format	string	Meta function used expressed in Python functions. Such as lower()[:10]	-
openpgp-encrypted-key	string	Base64 OpenPGP message encrypting the reference keyid. This is optional as the key can be distributed in different means such as dedicated MISP API key or other secure channel.	-

Meta format format

Format type bloomfilter

Key name	Type	Description	Required
capacity	number	Capacity of the BloomFilter	✓
fp-probability	number	Probability of false-positive	✓
format	string	Format of the BloomFilter such as dcso-v1 or poppy-v2	✓
match-count	number	Number of count match to confirm if it's a positive match. Default is 1.	‐

List of known bloomfilter format

Name	Description
dcso-v1	DCSO BloomFilter using 64-bit FNV-1 hash function.
poppy-v2	Poppy using 64-bit with wyHash.

Format type misp-feed-cache

Sample

{
  "algorithm": "Blake2",
  "format": [
    {
      "bloomfilter": {
        "capacity": 10000,
        "format": "dcso-v1",
        "fp-probability": 0.001
      }
    },
    {
      "misp-feed-cache": {}
    }
  ],
  "canonicalization-format": ".lower",
  "description": "List of Tor hidden services containing child sexual abuse material (CSAM).",
  "generated-timestamp": 1700731642,
  "keyid": "tor-csam-lea",
  "misp-attribute-types": [
    "text",
    "url",
    "link"
  ],
  "version": 1
}

Feed format

The feed format is composed of a directory with the following structure:

• private-search-set.pss - Private search as a standalone file. OneOfrequired
• private-search-set.json - Meta data of the private search file. required
• private-search-set.bloom - Bloomfilter file of the pss set. OneOfrequired

Those two files can be included in a MISP feed format export. The feed can contain a single Bloom filter set along with meta-data or a hash list file with the hash values or even both files with the meta-data.

MISP Object template

A private-search-set MISP oject template will be created to be able to share PSS via MISP.