---
title: "Cryptographic Primitives"
description: "
"
url: https://instituteofprovenance.org/docs/cryptographic-primitives
source: Institute of Provenance
---
# Cryptographic Primitives

The XION specification mandates a specific set of cryptographic algorithms. This constrained choice ensures interoperability — every compliant implementation uses the same primitives, eliminating negotiation complexity and reducing the attack surface.

## Algorithm Summary

| Primitive | Algorithm | Purpose |
|-----------|-----------|---------|
| Digital Signatures | Ed25519 | Content signing, certificate signing |
| Content Hashing | BLAKE3 | Content integrity, canonicalized content digests |
| Key Identifier | SHA-256 (truncated) | Compact public key identification |
| Certificate Format | X.509 v3 | Certificate wrapping and chain encoding |
| State Verification | Sparse Merkle Trees | Certificate revocation proofs, epoch state |

## Ed25519

Ed25519 is the sole signature algorithm in XION v1. It provides:

- **128-bit security level** — Equivalent to RSA-3072 with much smaller keys
- **32-byte public keys, 64-byte signatures** — Compact for embedding in trust blocks and certificates
- **Deterministic signatures** — The same message and key always produce the same signature, eliminating a class of implementation bugs related to random number generation
- **Fast verification** — Critical for content trust where verification happens far more often than signing

Ed25519 operates on the edwards25519 curve as specified in RFC 8032.

## BLAKE3

BLAKE3 is the mandated content hashing algorithm. It was chosen over SHA-256 for:

- **Performance** — 3–4x faster than SHA-256 on modern hardware, with SIMD acceleration
- **Security margin** — Based on the BLAKE2/ChaCha family with a generous security margin
- **256-bit output** — Sufficient for collision resistance at the 128-bit security level
- **Tree hashing** — BLAKE3's internal Merkle tree structure enables incremental and parallel hashing, useful for large content

All content hashes in XION are BLAKE3-256, hex-encoded as 64 lowercase characters.

## SHA-256 (Key Identifier)

SHA-256 is used only for deriving compact key identifiers (`key_id` field in the trust block). The first 8 bytes of SHA-256(public_key_bytes) are base64url-encoded to produce an 11-character identifier.

SHA-256 is not used for content hashing. Its role is limited to key identification, where the truncated output provides sufficient collision resistance for lookup purposes.

## X.509 v3

Certificates are encoded as X.509 v3 structures, PEM-encoded in the trust block's `x509_chain_pem` field. X.509 was chosen for:

- **Mature tooling** — Every programming language has X.509 parsing libraries
- **Extensibility** — Custom extensions encode XION-specific metadata (signer identity, scope, orbital identifier)
- **Interoperability** — Organizations with existing PKI infrastructure can integrate XION certificate management into their workflows

All certificates use Ed25519 keys. RSA and ECDSA keys are not permitted in the XION certificate hierarchy.

## Sparse Merkle Trees

Sparse Merkle Trees (SMTs) provide cryptographic proofs of set membership and non-membership. In XION, they are used to maintain verifiable certificate revocation state.

An SMT is a binary Merkle tree where:

- The key space is 2^256 (every possible 256-bit key has a position)
- Most positions are empty (sparse)
- Inclusion proofs demonstrate that a key IS in the set (O(log n) sibling hashes)
- Exclusion proofs demonstrate that a key is NOT in the set (also O(log n) sibling hashes)

This enables trustless revocation checking: a verifier can confirm that a certificate has or has not been revoked without trusting the server that provided the proof. The proof is validated mathematically against the published tree root.

### Epochs

The SMT root is advanced in epochs. Each epoch represents a state transition (certificates added to or removed from the revoked set). The epoch number and corresponding tree root are signed by the Orbital and included in Proof records.