From 943f1d9d8f71b35c7d36c7bc3f99031cd9ac422d Mon Sep 17 00:00:00 2001
From: Dmitry Monakhov <dmonakhov@gmail.com>
Date: Sat, 23 May 2026 18:52:33 +0200
Subject: [PATCH] bip-85: add example use for age key derivation

Documents the connection between BIP-85's HEX application output
at num_bytes=32 and the private-key seed format used by the age
file-encryption format (https://age-encryption.org/v1) for both
the classic X25519 identity (since age v1.0) and the
post-quantum X-Wing identity (age v1.3.0+, Dec 2024).

No new application number, no normative MUSTs added, no changes
required in existing implementations. The amendment is purely
documentational: a single subsection appended at the end of the
HEX section.

A test vector is added using the same master xprv as the
existing HEX test vector in the same section, with both classic
and post-quantum outputs. The PQ recipient (a ~1959-character
bech32 string encoding the 1216-byte X-Wing public key) is
included as a SHA-256 hash inline; the full string is in the
reference implementation's test data.

Reference implementation: https://github.com/dmonakhov/age-keygen-det
- single-binary Go tool, BSD-3-Clause
- cross-validates byte-for-byte against stock age-keygen -y in
  CI for both flavours
---
 bip-0085.mediawiki | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/bip-0085.mediawiki b/bip-0085.mediawiki
index b0f00abbef..4ca546401a 100644
--- a/bip-0085.mediawiki
+++ b/bip-0085.mediawiki
@@ -282,6 +282,44 @@ INPUT:
 OUTPUT
 * DERIVED ENTROPY=492db4698cf3b73a5a24998aa3e9d7fa96275d85724a91e71aa2d645442f878555d078fd1f1f67e368976f04137b1f7a0d19232136ca50c44614af72b5582a5c
 
+====<code>age</code> file encryption keys====
+
+A 32-byte HEX-application output at <code>m/83696968'/128169'/32'/{index}'</code> is a
+cryptographically random value usable directly as the secret key of an
+[https://age-encryption.org/v1 age] file-encryption identity. age has two identity flavours;
+both encode this 32-byte seed, and the matching recipient (public key) is derived from it:
+
+{| class="wikitable"
+! Flavour !! Role !! HRP !! Bech32-encoded payload
+|-
+| classic (X25519) || identity || <code>AGE-SECRET-KEY-</code> || the 32-byte seed
+|-
+| classic (X25519) || recipient || <code>age</code> || <code>X25519(seed, G)</code>, 32 bytes
+|-
+| post-quantum (X-Wing) || identity || <code>AGE-SECRET-KEY-PQ-</code> || the same 32-byte seed
+|-
+| post-quantum (X-Wing) || recipient || <code>age1pq</code> || X-Wing encapsulation key, 1216 bytes
+|}
+
+Every string above is Bech32 (BIP-173) of the form <code>HRP || "1" || encode(payload) || checksum</code>.
+<code>G</code> is the Curve25519 base point, so <code>X25519(seed, G)</code> is the classic
+recipient's public key. For the post-quantum flavour (age v1.3.0+), X-Wing SHAKE256-expands the
+seed into its ML-KEM-768 and X25519 components, and the recipient encodes the resulting
+1216-byte X-Wing encapsulation key.
+
+INPUT:
+* MASTER BIP32 ROOT KEY: xprv9s21ZrQH143K2LBWUUQRFXhucrQqBpKdRRxNVq2zBqsx8HVqFk2uYo8kmbaLLHRdqtQpUm98uKfu3vca1LqdGhUtyoFnCNkfmXRyPXLjbKb
+* PATH: m/83696968'/128169'/32'/0'
+
+OUTPUT:
+* DERIVED ENTROPY=ea3ceb0b02ee8e587779c63f4b7b3a21e950a213f1ec53cab608d13e8796e6dc
+* DERIVED AGE IDENTITY (classic)=AGE-SECRET-KEY-1AG7WKZCZA689SAMECCL5K7E6Y854PGSN78K98J4KPRGNAPUKUMWQWNNT4U
+* DERIVED AGE RECIPIENT (classic)=age1m0hhzxelxsxnxm4ennvdpk75j8s7mn5w4tt3e4ntug5qx256wslqmdz8e9
+* DERIVED AGE IDENTITY (PQ)=AGE-SECRET-KEY-PQ-1AG7WKZCZA689SAMECCL5K7E6Y854PGSN78K98J4KPRGNAPUKUMWQ5AN2M5
+* DERIVED AGE RECIPIENT (PQ) SHA-256 (of the 1959-char bech32 string, no trailing newline)=855bd04ee0cd6cfdf5717fb946d859824a79fbbdf6304dadcc11dbb91abe0df6
+
+A reference implementation is [https://github.com/dmonakhov/age-keygen-det age-keygen-det].
+
 ===PWD BASE64===
 Application number: 707764'