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bip-85: add example use for age key derivation #2174

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Expand Up @@ -282,6 +282,47 @@ 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 flavors.
Both encode a 32-byte seed. The matching recipient (public key) is derived from the seed as follows:

{| class="wikitable"
! Flavor !! Role !! HRP !! Bech32-encoded payload
|-
| classic (X25519) || identity || <code>AGE-SECRET-KEY-</code> || 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> || 32-byte seed
|-
| post-quantum (X-Wing) || recipient || <code>age1pq</code> || X-Wing encapsulation key, 1216 bytes
|}

A given index SHOULD be used with only one flavor: a cryptographically-relevant quantum computer
could recover the seed from the published classic recipient <code>X25519(seed, G)</code> and
thereby also compromise the post-quantum identity derived from the same seed.

Every string above takes the form <code>HRP || "1" || encode(payload) || checksum</code>, using the
Bech32 (BIP-173) character set and checksum but without Bech32's 90-character length limit.
<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 flavor (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

===PWD BASE64===
Application number: 707764'

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