docs(core): bring recovery_qr.rs to the documented-zone standard

Phase 3 of the security-polish series. Brings recovery_qr.rs up to
the documentation density of crypto.rs / imgsecret.rs / backup.rs /
tar_safe.rs. No runtime behaviour change: just module-level //! header
explaining the format + KDF domain separation + parameter-pinning
rationale, an ASCII diagram of the 109-byte payload layout pinned by
a static assertion, doc-comments on the four public items, and named
slice-range constants for the offset arithmetic.

production_params() is replaced with a top-level const so the "pinned,
do not change once shipped" property is visible at every use site.

Refs: docs/superpowers/specs/2026-05-04-security-polish-design.md (Phase 3)
Refs: docs/superpowers/reviews/2026-05-04-architecture-review.md (P1.7)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/relicario-core/src/recovery_qr.rs

@@ -1,13 +1,107 @@
+//! Recovery-QR encoding for the reference image_secret.
+//!
+//! ## What this module produces
+//!
+//! Given a user-chosen recovery passphrase and the 32-byte image_secret
+//! (extracted from the reference JPEG via [`crate::imgsecret::extract`]), this
+//! module produces a 109-byte sealed payload that — at recovery time, with the
+//! same passphrase — yields the original image_secret back. The payload is
+//! intended to be rendered as a QR v40 EcLevel::M SVG via [`recovery_qr_to_svg`]
+//! and printed on paper, so a user who loses access to the reference JPEG can
+//! still unlock their vault if they remember the recovery passphrase.
+//!
+//! ## Why the format is structured this way
+//!
+//! The payload is an XChaCha20-Poly1305 envelope around the image_secret. The
+//! AEAD key (the "wrap key") is derived by Argon2id from a domain-separated
+//! input:
+//!
+//! ```text
+//! kdf_input = b"relicario-recovery-v1\0"
+//!          || u64_be(len(nfc(passphrase)))
+//!          || nfc(passphrase)
+//! wrap_key  = Argon2id(kdf_input, kdf_salt, RECOVERY_PRODUCTION_PARAMS) -> 32 bytes
+//! ```
+//!
+//! The `b"relicario-recovery-v1\0"` prefix is **domain separation**: it
+//! guarantees that even if the user reuses their vault passphrase as their
+//! recovery passphrase, the wrap key derived here can never collide with a
+//! vault master key derived in [`crate::crypto::derive_master_key`] (which has
+//! a different input shape entirely — passphrase + image_secret, no prefix).
+//! Without this prefix, a determined attacker who somehow recovered a wrap key
+//! could try it as a master key and vice versa.
+//!
+//! Both `kdf_salt` and `wrap_nonce` are freshly randomized per call to
+//! [`generate_recovery_qr`], so two QRs printed from the same passphrase and
+//! image_secret are different bytes — the printed QR does not leak whether
+//! the user has printed others before.
+//!
+//! ## Parameter-pinning rationale
+//!
+//! The Argon2id parameters used here are NOT [`crate::crypto::KdfParams::default`].
+//! They are pinned in [`RECOVERY_PRODUCTION_PARAMS`] at the value
+//! `KdfParams { argon2_m: 65536, argon2_t: 3, argon2_p: 4 }` — the same values
+//! the default happens to have *today*, but deliberately re-stated rather than
+//! referenced. This is because `KdfParams::default()` may evolve as we re-tune
+//! Argon2 cost for newer hardware, and a recovery QR printed on paper has no
+//! way to negotiate parameters at decode time. Changing the pinned values here
+//! would silently invalidate every recovery QR a user has ever printed under
+//! the previous parameter set. The const lives at module scope so the
+//! "pinned, do not change once shipped" property is visible at every use site.
+
 use chacha20poly1305::{XChaCha20Poly1305, Key, KeyInit, aead::Aead};
 use rand::RngCore;
 use unicode_normalization::UnicodeNormalization;
 use zeroize::Zeroizing;
 use crate::{crypto::KdfParams, error::{RelicarioError, Result}};
 
+// Recovery QR payload — 109 bytes total:
+//
+//  byte    field           length
+//  ------  --------------  ------
+//  0..4    MAGIC = "RREC"  4
+//  4..5    VERSION = 0x01  1
+//  5..37   kdf_salt        32   (random per QR)
+//  37..61  wrap_nonce      24   (random per QR)
+//  61..109 ciphertext      48   (32 image_secret + 16 AEAD tag)
+//  ------------------------------
+//  total                   109
 const MAGIC: &[u8; 4] = b"RREC";
 const VERSION: u8 = 0x01;
 const PAYLOAD_LEN: usize = 4 + 1 + 32 + 24 + 48; // 109
 
+// Static assertion that the documented layout above and the PAYLOAD_LEN
+// constant cannot drift apart. If a future edit changes one without the other,
+// this fails to compile.
+const _: () = assert!(PAYLOAD_LEN == 4 + 1 + 32 + 24 + 48);
+
+// Named slice ranges derived from the layout offsets above. Used by
+// `unwrap_recovery_qr_with_params` so the byte-position arithmetic at the
+// parse site is self-documenting.
+const KDF_SALT_RANGE: std::ops::Range<usize> = 5..37;
+const WRAP_NONCE_RANGE: std::ops::Range<usize> = 37..61;
+const CIPHERTEXT_RANGE: std::ops::Range<usize> = 61..109;
+
+/// Pinned recovery-QR Argon2id parameters. Re-states `KdfParams::default()`'s
+/// values rather than referencing them, because a recovery QR printed under
+/// one parameter set cannot be decoded under another. **Once shipped, these
+/// values MUST NOT change** — doing so silently invalidates every previously
+/// printed QR. See the module header for full rationale.
+const RECOVERY_PRODUCTION_PARAMS: KdfParams = KdfParams {
+    argon2_m: 65536,
+    argon2_t: 3,
+    argon2_p: 4,
+};
+
+/// A sealed 109-byte recovery payload. The bytes are an opaque package — they
+/// only become useful when fed back through [`unwrap_recovery_qr`] together
+/// with the recovery passphrase that was used to produce them.
+///
+/// [`as_bytes`](Self::as_bytes) is the only accessor. The bytes are designed to
+/// travel as a single unit; the supported transport is rendering via
+/// [`recovery_qr_to_svg`] and printing the QR on paper, but a hex string
+/// (sneakernet-friendly) works equally well as long as the full 109 bytes
+/// are preserved.
 pub struct RecoveryQrPayload {
     bytes: [u8; PAYLOAD_LEN],
 }
@@ -24,15 +118,12 @@ fn recovery_kdf_input(passphrase: &str) -> Vec<u8> {
     let prefix = b"relicario-recovery-v1\0";
     let mut input = Vec::with_capacity(prefix.len() + 8 + nfc_bytes.len());
     input.extend_from_slice(prefix);
+    // length-prefix on nfc_bytes mirrors crypto::derive_master_key (audit H1)
     input.extend_from_slice(&(nfc_bytes.len() as u64).to_be_bytes());
     input.extend_from_slice(nfc_bytes);
     input
 }
 
-fn production_params() -> KdfParams {
-    KdfParams { argon2_m: 65536, argon2_t: 3, argon2_p: 4 }
-}
-
 fn derive_wrap_key(
     passphrase: &str,
     kdf_salt: &[u8; 32],
@@ -42,11 +133,38 @@ fn derive_wrap_key(
     crate::crypto::derive_master_key_raw(&input, kdf_salt, params)
 }
 
+/// Produce a sealed [`RecoveryQrPayload`] from the recovery passphrase and the
+/// 32-byte image_secret.
+///
+/// # Inputs
+///
+/// - `passphrase`: the user's recovery passphrase (UTF-8). Independent of the
+///   vault passphrase, but the user may reuse them — the
+///   `b"relicario-recovery-v1\0"` domain-separation prefix in the KDF input
+///   guarantees the wrap key still cannot collide with a vault master key.
+/// - `image_secret`: the 32-byte secret extracted from the reference JPEG
+///   via [`crate::imgsecret::extract`].
+///
+/// # Output
+///
+/// A [`RecoveryQrPayload`] whose 109 bytes encode `MAGIC || VERSION || kdf_salt
+/// || wrap_nonce || ciphertext`. Both `kdf_salt` and `wrap_nonce` are freshly
+/// drawn from `OsRng` on every call, so two payloads generated from the same
+/// `(passphrase, image_secret)` pair are distinct bit-for-bit. The printed QR
+/// therefore does not reveal that the user has printed others before.
+///
+/// To render the payload as a printable SVG, see [`recovery_qr_to_svg`].
+///
+/// # Errors
+///
+/// Returns [`RelicarioError::RecoveryQr`] if the AEAD wrap fails (extremely
+/// unlikely in practice — this can only happen if the cipher implementation
+/// itself errors, not on user input).
 pub fn generate_recovery_qr(
     passphrase: &str,
     image_secret: &[u8; 32],
 ) -> Result<RecoveryQrPayload> {
-    generate_recovery_qr_with_params(passphrase, image_secret, &production_params())
+    generate_recovery_qr_with_params(passphrase, image_secret, &RECOVERY_PRODUCTION_PARAMS)
 }
 
 #[doc(hidden)]
@@ -78,11 +196,39 @@ pub fn generate_recovery_qr_with_params(
     Ok(RecoveryQrPayload { bytes })
 }
 
+/// Decode a recovery payload back into the original 32-byte image_secret.
+///
+/// # Inputs
+///
+/// - `payload_bytes`: the 109 bytes produced by [`generate_recovery_qr`] (after
+///   the QR has been scanned, or the hex transcribed and decoded).
+/// - `passphrase`: the recovery passphrase that was used at generate time.
+///
+/// # Output
+///
+/// The recovered image_secret as `Zeroizing<[u8; 32]>` — the wrapper ensures
+/// the secret is wiped from memory when the binding goes out of scope, so a
+/// caller that immediately feeds it into [`crate::crypto::derive_master_key`]
+/// and then drops it never leaves a copy in process memory longer than
+/// strictly necessary.
+///
+/// # Errors
+///
+/// - [`RelicarioError::RecoveryQr`] for **format** problems: wrong length,
+///   bad magic, unsupported version byte. These come from inspecting the
+///   bytes themselves, before any cryptographic work, so they leak nothing
+///   about whether the passphrase is right.
+/// - [`RelicarioError::Decrypt`] for **AEAD** failure — wrong passphrase
+///   (wrong wrap key) **or** a payload tampered after the fact. The two
+///   cases are deliberately not distinguished, mirroring the same
+///   non-distinguishing rejection as [`crate::crypto::decrypt`] (audit M4):
+///   a Poly1305 tag failure cannot, in principle, leak which bytes were
+///   wrong, and the API surface preserves that property.
 pub fn unwrap_recovery_qr(
     payload_bytes: &[u8],
     passphrase: &str,
 ) -> Result<Zeroizing<[u8; 32]>> {
-    unwrap_recovery_qr_with_params(payload_bytes, passphrase, &production_params())
+    unwrap_recovery_qr_with_params(payload_bytes, passphrase, &RECOVERY_PRODUCTION_PARAMS)
 }
 
 #[doc(hidden)]
@@ -104,9 +250,9 @@ pub fn unwrap_recovery_qr_with_params(
             format!("unsupported version 0x{:02x}", payload_bytes[4])
         ));
     }
-    let kdf_salt: &[u8; 32] = payload_bytes[5..37].try_into().expect("slice length validated above");
+    let kdf_salt: &[u8; 32] = payload_bytes[KDF_SALT_RANGE].try_into().expect("slice length validated above");
-    let wrap_nonce = &payload_bytes[37..61];
+    let wrap_nonce = &payload_bytes[WRAP_NONCE_RANGE];
-    let ciphertext = &payload_bytes[61..109];
+    let ciphertext = &payload_bytes[CIPHERTEXT_RANGE];
 
     let wrap_key = derive_wrap_key(passphrase, kdf_salt, params)?;
     let cipher = XChaCha20Poly1305::new(Key::from_slice(wrap_key.as_ref()));
@@ -119,6 +265,15 @@ pub fn unwrap_recovery_qr_with_params(
     Ok(out)
 }
 
+/// Render a [`RecoveryQrPayload`] as a printable QR-code SVG string.
+///
+/// The QR is encoded at **version 40** (the largest standard symbol, 177×177
+/// modules) at **error-correction level M** (~15% recoverable), with a
+/// minimum rendered dimension of **140×140** SVG units. The 109-byte payload
+/// fits comfortably inside v40 at level M — there is significant
+/// error-correction headroom left over, which is the point: the QR is
+/// expected to live on paper (where smudges, folds, and fading are normal)
+/// and must still scan years later.
 pub fn recovery_qr_to_svg(payload: &RecoveryQrPayload) -> String {
     use qrcode::{QrCode, EcLevel};
     let code = QrCode::with_error_correction_level(payload.bytes.as_ref(), EcLevel::M)