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UR Type Definition for Post-Quantum Cryptographic Structures

BCR-2025-003

© 2025 Blockchain Commons

Author: Wolf McNally
Date: April 25, 2025

Introduction

This document defines a set of CBOR tags and Uniform Resource (UR) types for post-quantum cryptographic structures implementing ML-KEM and ML-DSA. These tags and types are intended to be used in conjunction with the Concise Binary Object Representation (CBOR) format, which is a data serialization format designed for small code size and small message size.

About ML-KEM and ML-DSA

ML‑KEM (“Module‑Lattice Key‑Encapsulation Mechanism”) and ML‑DSA (“Module‑Lattice Digital Signature Algorithm”) are NIST’s post‑quantum standards for key exchange and signatures. Both descend from the CRYSTALS competition winners—Kyber and Dilithium respectively—but they are not drop‑in replacements for those schemes.

UR Types and CBOR Tags

This document defines the following UR types along with their corresponding CBOR tags, which have been assigned in the IANA Concise Binary Object Representation (CBOR) Tags registry.

UR type CBOR Tag
ur:mlkem-private-key #6.40100
ur:mlkem-public-key #6.40101
ur:mlkem-ciphertext #6.40102
ur:mldsa-private-key #6.40103
ur:mldsa-public-key #6.40104
ur:mldsa-signature #6.40105

CDDL Definitions

This section provides the CDDL definitions for the CBOR tags defined above.

Our reference for the key sizes and other parameters is the pqcrypto-mlkem and pqcrypto-mldsa crates, which are part of the PQClean project.

Our general architecture is for the CBOR tag to identify the type of object, and then the first element of the tuple to identify the level of security. The second element of the tuple is a binary string of a fixed size, which contains the actual key, ciphertext, or signature.

The bc-components crate contains reference implementations of these schemas.

mlkem-level = 512 / 768 / 1024

mlkem-tuple<$L, $S> = (
  $L .within mlkem-level,
  bstr .size $S
)

mlkem-private-key = #6.40100([
    mlkem-tuple<512, 1632> //
    mlkem-tuple<768, 2400> //
    mlkem-tuple<1024, 3168>
])

mlkem-public-key = #6.40101([
    mlkem-tuple<512, 800> //
    mlkem-tuple<768, 1184> //
    mlkem-tuple<1024, 1568>
])

mlkem-ciphertext = #6.40102([
    mlkem-tuple<512, 768> //
    mlkem-tuple<768, 1088> //
    mlkem-tuple<1024, 1568>
])

mldsa-level = 2 / 3 / 5

mldsa-tuple<$L, $S> = (
  $L .within mldsa-level,
  bstr .size $S
)

mldsa-private-key = #6.40103([
    mldsa-tuple<2, 2560> //
    mldsa-tuple<3, 4032> //
    mldsa-tuple<5, 4896>
])

mldsa-public-key = #6.40104([
    mldsa-tuple<2, 1312> //
    mldsa-tuple<3, 1952> //
    mldsa-tuple<5, 2592>
])

mldsa-signature = #6.40105([
    mldsa-tuple<2, 2420> //
    mldsa-tuple<3, 3309> //
    mldsa-tuple<5, 4627>
])