Merge pull request 'docs: attachments & storage backend design (#38, #31)' (#69) from issue-38-31-storage-design into main

- enforce server-side body cap via http.MaxBytesReader: oversized JSON
  bodies are rejected with 413 instead of being fully decoded first
- negative burn_after_reads rejected with 400 (zero still = default 1)
- limit=0 explicitly maps to default page size; shared parseLimit clamp
  for /api/public and /api/mine (huge/non-numeric values too)
- negative/non-numeric offset clamped to 0 via parseOffset (was
  pass-through)
- regression tests in issue68_validation_test.go

expires_at/expires_in validation intentionally excluded: covered by #60.

README.md

@@ -77,7 +77,17 @@ curl -X POST http://localhost:8080/api/pastes \
   -d '{"content": "print(hello)", "language": "python", "expires_in": "168h"}'
 ```
 
-Full API docs: [docs/API.md](docs/API.md).
+Full API docs: [docs/API.md](docs/API.md). Design docs: [docs/design/](docs/design/) (currently: [client-side E2E encryption](docs/design/e2e-encryption.md), issue #39).
+
+## Performance Notes
+
+The history and Saved pages use client-side filtering: when you type in the
+search box, the UI fetches the most recent 100 pastes (`limit=100`, the API
+maximum) once per query and filters/sorts them in the browser. Pastes beyond
+the newest 100 are not searched; a match count against the full total is still
+shown. This keeps search instant without a server-side query. If large
+instances need full search later, it will be a server-side endpoint (see
+issue #32).
 
 ## CI
 

docs/design/attachments-storage.md

@@ -0,0 +1,78 @@
+# Attachments & Storage Backend Design (#38, #31)
+
+Status: research/design, no implementation. Consumers: paste cans (#4).
+
+## Part A — Attachments: S3/MinIO vs filesystem-on-volume (#38)
+
+### Options
+
+**Option 1: Filesystem on the k3s PVC (current 5Gi volume).**
+Store blobs under `<data-dir>/attachments/<paste-id>/<n>-<sha256-8>`, metadata in SQLite (paste_id, filename, size, sha256, mime, created_at).
+
+- Pros: zero new infra, zero new credentials, trivial backup (the volume backup job already covers the DB), atomic rename on write, works in dev and prod identically.
+- Cons: volume is size-capped (5Gi today; resizable but bounded); serving large files passes through the app process (no ranged-GET offload); multi-replica later would need RWX volume.
+
+**Option 2: MinIO via S3 API.**
+MinIO is already proven in this homelab (Outline). Store at key `<paste-id>/<n>`; same SQLite metadata row.
+
+- Pros: effectively unbounded capacity, presigned URLs (direct browser download, offloads serving from palette pods), ranged requests free, lifecycle rules could auto-expire orphaned objects.
+- Cons: another credential/secret to manage, another failure mode, MultipartForm still terminates at the palette pod (MinIO only helps *serving*, not *uploading*, unless we do presigned uploads — which breaks the cans multipart flow and the auth/unlock checks), backup now spans two systems.
+
+### Key considerations
+
+- **Upload path is the same either way.** Palette receives `multipart/form-data` (cans need text items + file drops in one request), must enforce auth/password/burn rules server-side. A filesystem backend adds no upload complexity; S3 adds an extra hop (buffer → PUT to MinIO). Streaming straight from `multipart.Reader` to the sink works for both (`io.Copy` to a temp file, or to an S3 PUT with `Content-Length` known or multipart buffering).
+- **Size limits.** Everything is admin-tunable via the settings API (#40 pattern) — add `max_attachment_bytes` (default 10 MiB, hard server-side cap checked *before* reading the body via `Content-Length`, plus a counted reader during copy so chunked uploads can't lie). SQLite itself is not a constraint either way; the PVC is the real cap for Option 1.
+- **MIME handling.** Security-critical (pentest #34 already fixed a content-type XSS on `/raw`). Rules:
+  - Never trust the client-declared Content-Type. Sniff the first 512 bytes (`http.DetectContentType`), intersect with an allowlist.
+  - Serve from a dedicated route (`/{id}/a/{n}`) with `Content-Type` from the stored *sniffed* type, `X-Content-Type-Options: nosniff`, `Content-Security-Policy: sandbox`, `Content-Disposition: attachment` unless the type is on a safe-inline allowlist (text/plain, images, PDF at user opt-in).
+  - Never render user HTML/SVG inline (`image/svg+xml` is XSS-capable — serve as `attachment` always, or store sanitized).
+- **Streaming & serving.**
+  - Filesystem: `http.ServeContent` on the opened file gives ranged GETs, ETag, Last-Modified for free.
+  - MinIO: proxy via `GetObject` + `io.Copy` (simple, keeps auth checks in palette) or presigned GET (faster, but URL embeds credentials-temporarily and bypasses palette's per-request auth — wrong for pastes with passwords/burn semantics). Given cans inherit password/burn parity (#4), **proxying is required, which erodes MinIO's main serving advantage**.
+- **Lifecycle parity.** Attachments must honor soft-delete grace and sweep: sweeper hard-delete also removes blobs (files: `os.Remove`; S3: `DeleteObject`), best-effort with logging; orphan sweep job compares DB rows to store contents.
+
+### Recommendation
+
+**Filesystem-on-volume first.** At current scale (single replica, PVC-based deploy, one user + homelab traffic) it is simpler end-to-end and keeps serving/auth/lifecycle in one place. The internal API should be a narrow blob interface (`Put(ctx, key, r io.Reader, size int64) / Open(key) / Delete(key)`) — about 60 lines per backend — so **MinIO becomes a drop-in later** if attachments outgrow the volume. That's the honest middle path: filesystem default, S3-ready seam, no MinIO dependency until it pays for itself.
+
+## Part B — SQLite vs Postgres vs Redis (#31)
+
+### Assessment of SQLite at pastebin scale
+
+- **Driver**: modernc.org/sqlite (pure Go, no cgo) — slightly slower than mattn/go-sqlite3 but fine; single-writer semantics are the real constraint, not driver speed.
+- **Access pattern**: paste-heavy, write-rare/read-often; primary keys and small set of indexes (visibility+created, expires, deleted); no joins beyond can items. This is SQLite's best case.
+- **WAL mode** is already on (`journal_mode(WAL), busy_timeout(5000)`) — concurrent readers don't block the single writer.
+- **Numbers**: SQLite comfortably handles millions of rows and hundreds of reads/sec; WAL write throughput is thousands of small inserts/sec. A pastebin doing even 100k pastes (avg 10 KB = ~1 GB DB) is trivial. Reads: prepared `WHERE id=?` lookups at this size are sub-millisecond.
+- **Weak points to watch** (document, none urgent):
+  1. Single writer — heavy concurrent create traffic serializes. Mitigation: already rate-limited (#2); fine until that's the bottleneck (unlikely).
+  2. `LENGTH(content)` on every list row — fine now; if it shows up in profiling, store `size` as a column (schema already has a `Size` field; list queries could use it).
+  3. Sweeper runs a table-wide `UPDATE`+`DELETE` on tick — indexed, fine.
+  4. No network access to the DB file — locks palette to single-replica. Acceptable: current deploy is one replica.
+- **Redis is the wrong tool** here: it's a cache/queue, not a system of record. Pastes are durable data with expiry semantics already implemented in SQLite. Redis would only add an optional read-cache layer for hot pastes — pure complexity for zero measured need.
+
+### Should we build a backend abstraction (SQLite default, optional Postgres)?
+
+Arguments for: multi-replica scaling later; "docker image env choice" sounds nice; Postgres gives real concurrency and network access.
+Arguments against: a `Store` interface covering the current query surface is a real refactor (sqlite-flavored SQL: `INSERT OR IGNORE`-style upserts, partial indexes, `?` placeholders are compatible but behaviors differ — e.g. `sqlite` driver pragmas, transaction isolation, `AUTOINCREMENT` semantics); two backends means two test matrices and two migration paths forever; and there is **no current need** — single replica, single writer, modest data.
+
+**Recommendation: stay SQLite-only. Do not build the abstraction now.**
+Specifically:
+
+1. Keep all persistence behind `internal/store` (already done in #35 — the package boundary *is* the abstraction, at zero cost).
+2. Avoid SQLite-specific SQL going forward where free (standard placeholders, no `RETURNING` quirks) — cheap discipline that keeps a future port honest.
+3. Define the trigger conditions for revisiting, and write them down:
+   - multiple replicas needed (scale-out), or
+   - sustained WAL write contention (busy timeouts observed in logs), or
+   - DB file > ~5-10 GB, or
+   - a concrete user request for a Postgres-backed image.
+4. When a trigger fires, port `internal/store` to Postgres behind an interface extracted *then* — the refactor is mechanical against a real need, instead of speculative complexity now.
+5. For the docker image: `PALETTE_DB_PATH` env already implies the deployment choice; no extra backend knob needed.
+
+## Summary
+
+| Decision | Choice |
+|---|---|
+| Attachment backend | Filesystem on PVC, behind a ~3-method blob interface; MinIO as a later drop-in, not a dependency |
+| Size limits | `max_attachment_bytes` admin setting, default 10 MiB, enforced pre-read + during stream |
+| MIME | Server-side sniff (512 bytes) + allowlist; `nosniff`, CSP `sandbox`, `Content-Disposition: attachment` except safe-inline types; SVG never inline |
+| Database | SQLite (WAL, modernc) only; no Postgres/Redis, no backend abstraction until a written trigger fires |

docs/design/cookie-preferences.md

@@ -0,0 +1,160 @@
+# Design: Cookie-Based Preferences and Access Keys (#30)
+
+Status: design note — no implementation yet.
+Related: #37 (vwr viewer cookie), #34 (HMAC unlock cookie), #26 (creator auto-unlock), #36 (settings gear).
+
+## Current cookie surface
+
+| Cookie | Purpose | Lifetime | Flags today |
+|---|---|---|---|
+| `vwr` | Anonymous viewer id; scopes `/mine` history and burn-after-N per-viewer dedupe; client-sent `vwr` also authorizes delete | 1 year | `HttpOnly`, `SameSite=Lax`, `Path=/` |
+| `pw_<id>` | Per-paste password unlock token = HMAC(paste id, PALETTE_UNLOCK_SECRET) | 1 hour | `HttpOnly`, `SameSite=Lax`, `Path=/` |
+| `tok_<id>` | One-time deletion-token handoff after create | 60 s | `HttpOnly`, `SameSite=Lax`, `Path=/` |
+
+The access-key feature is an extension of the `pw_<id>` pattern, not a new mechanism.
+
+## Part 1: Preference storage
+
+### What settings
+
+Only settings the *creator* sets when writing a paste, so the "new paste" form
+can pre-fill them:
+
+- Default language (`lang`)
+- Default expiry (`expires_in` / custom expiry)
+- Burn-after-N-reads default
+- Password-protect-by-default toggle (checkbox pre-checked; the password itself is never stored)
+- Default visibility of the "raw" link, if such a toggle exists
+- Collapsed/expanded state of the settings gear panel itself
+
+Never stored in cookies: passwords, access keys for pastes the user hasn't
+unlocked, deletion tokens (beyond the existing 60 s `tok_` handoff), anything
+typed into the paste body or title fields (existing rule: auto-detect must not
+overwrite user-typed content).
+
+### One cookie, not many
+
+A single `prefs` cookie holding a compact JSON object:
+
+```
+prefs={"lang":"go","exp":"1h","burn":0,"pw":1}
+```
+
+- One cookie avoids the browser per-domain cookie count (typically 50+ per
+  domain; Chrome 180) eating the budget that per-paste access-key cookies need.
+- Per-paste cookies (`pw_<id>`) are inherently name-per-paste and cannot be
+  consolidated — that's the constraint that makes a single `prefs` cookie
+  mandatory rather than stylistic.
+
+### Size limits
+
+- RFC 6265: user agents SHOULD support at least 4096 bytes per cookie. Keep
+  `prefs` under 256 bytes of JSON — it holds a handful of short enum values.
+- Server behavior: if the cookie is present but oversized/invalid JSON, ignore
+  it silently and serve defaults. Never reject a request over a bad preference
+  cookie.
+- Validate on the server (allowlist of known values); a cookie is untrusted
+  input like any header.
+
+### Flags
+
+`HttpOnly; SameSite=Lax; Path=/; Max-Age=31536000; Secure` (Secure once the
+prod instance serves HTTPS — it will, behind the letsencrypt IngressRoute;
+dev on plain HTTP needs the flag conditional on config).
+
+Preferences are not sensitive, but `HttpOnly` costs nothing and keeps script
+from mutating them; `SameSite=Lax` matches the existing cookies.
+
+## Part 2: Access-key cookies
+
+### Goal
+
+"Remember unlocked pastes on this browser" — after entering a password (or
+after creating a private paste), subsequent visits skip the unlock form. This
+extends `pw_<id>` from a 1-hour session convenience to a durable capability.
+
+### Design: extend `pw_<id>`, don't invent a new scheme
+
+The token is already HMAC(paste id, PALETTE_UNLOCK_SECRET) — unforgeable and
+per-paste (fix for the #34 bypass). Changes:
+
+1. **Opt-in checkbox on the unlock form** ("remember on this browser") and a
+   matching checkbox/note at creation time. Default OFF. Non-consenting
+   visitors keep the current 1-hour cookie.
+2. **Extended lifetime** when opted in: `Max-Age = min(paste expiry, 90 days)`.
+   The cookie must never outlive the paste — derive the cap from the paste's
+   `ExpiresAt` at unlock time. Burn-after-N pastes: cap short (e.g. 24 h),
+   since the paste may burn at any read.
+3. **Name collision**: paste ids are fixed-length server-generated, so
+   `pw_<id>` names stay bounded (~40 bytes each). With the 50-cookies-per-
+   domain budget, cap remembered pastes at ~30: when minting the 31st, drop
+   the oldest expired-paste cookies server-side (server knows which ids are
+   expired/deleted; send expired `Set-Cookie` with `Max-Age=0` to reclaim).
+4. **Delete authorization interplay**: today a client-sent `vwr` matching the
+   paste's ViewerID authorizes delete. Access-key cookies grant *read*
+   capability only. Do not let a `pw_<id>` cookie authorize deletion — that
+   would mean cookie theft escalates from "read a paste" to "destroy it".
+   Delete stays bound to `vwr` or the deletion token.
+
+### Scoping
+
+- Keep `Path=/` (paste URLs are `/{id}` at the root; per-paste `Path=/{id}`
+  would work but saves nothing and complicates cleanup).
+- Per-paste scope via the cookie *name* is the existing, tested pattern —
+  no shared "access key ring" cookie. A consolidated `keys` cookie would
+  mean one stolen cookie exposes every remembered paste at once.
+
+## Part 3: Security considerations (honest accounting)
+
+- **XSS exfiltration**: `HttpOnly` prevents JS from *reading* the cookies, but
+  not from *using* them — an XSS payload can simply `fetch('/<paste-id>')` and
+  exfiltrate the content through the page the cookie unlocks. HttpOnly raises
+  the bar (drive-by script can't dump the jar to an attacker server in one
+  request), it does not make access-key cookies safe. This is a real
+  limitation, not a solved problem. Mitigations in order of value:
+  1. Fix the stored-XSS class at the source — #34 already allowlisted
+     content-types on `/raw`; the standing debt items (CSP, X-Frame-Options,
+     Referrer-Policy) directly reduce cookie-use exfiltration and should land
+     before or with this feature.
+  2. Keep access-key cookies opt-in, so the blast radius is bounded to users
+     who accepted the tradeoff.
+- **Cookie theft = paste access**: anyone holding `pw_<id>` can read that
+  paste until the cookie or paste expires, from any machine. That is inherent
+  to capability cookies. Consequences accepted deliberately: pastes here are
+  ephemeral (1 min–1 yr expiry), passwords are low-stakes share convenience,
+  and there are no user accounts to compromise. Document this in the UI copy
+  ("stores unlock access on this browser").
+- **Shared machines**: a remembered cookie defeats the password for the next
+  user of the browser. The opt-in checkbox with plain-language copy is the
+  mitigation; do not default it on.
+- **Cookie tossing / fixation**: a subdomain attacker could try to force
+  cookies; palette is a single host, no untrusted subdomains. `SameSite=Lax`
+  blocks cross-site attachment of the cookies on form posts to unlock
+  endpoints.
+- **Multi-instance / secret rotation**: tokens are HMACs under
+  `PALETTE_UNLOCK_SECRET`; rotating the secret silently invalidates all
+  remembered cookies (acceptable — next visit re-prompts). Both dev and prod
+  k3s instances need the same secret only if sharing a domain, which they do
+  not.
+- **Preferences cookie**: not security-sensitive, but still validate/allowlist
+  server-side to avoid it becoming an injection sink into templates.
+
+## Recommendation
+
+Implement in two small, separately reviewable pieces:
+
+1. **`prefs` cookie** (do first, low risk): single JSON cookie < 256 bytes,
+   server-validated allowlist, `HttpOnly; SameSite=Lax; Max-Age=1y`, drives
+   only form pre-fill. Ship with #36's settings gear.
+2. **Extended `pw_<id>` opt-in** (second, security-sensitive): opt-in checkbox,
+   Max-Age capped by paste expiry (90-day ceiling, 24 h for burn pastes),
+   oldest-cookie eviction at ~30 pastes, no delete authorization from access
+   cookies, and land the CSP/X-Frame-Options hardening debt from #34 in the
+   same or preceding change. UI copy must disclose that the cookie preserves
+   paste access on the browser.
+
+Rejected alternatives: single consolidated access-key cookie (aggregate theft
+risk, and the per-domain cookie-count argument cuts the other way for keys —
+consolidation maximizes what one stolen cookie unlocks); localStorage for
+preferences (XSS-readable, no benefit over HttpOnly cookies here); server-side
+accounts/session table (out of scope — Palette is deliberately anonymous).

docs/design/e2e-encryption.md

@@ -0,0 +1,223 @@
+# Design: Optional client-side E2E encryption for pastes and files
+
+- **Issue:** #39
+- **Status:** Design (no implementation in this PR)
+- **Related docs:** [docs/API.md](../API.md)
+
+## 1. Goals and non-goals
+
+**Goals**
+
+- Let any paste (or can item) be stored server-side as ciphertext only.
+- Zero plaintext knowledge by the server: storage, logs, backups, DB dumps contain no readable content.
+- Pure browser implementation using WebCrypto; no new server dependencies.
+- Encrypted pastes must still work with expiry, hard/soft delete, deletion tokens, visibility, slugs, rate limits.
+
+**Non-goals (v1)**
+
+- Anonymous, account-less E2E; Palette stays server-trusting with browser cookies.
+- Sharing via link fragments (`#key`) is optional sugar, not a required transport.
+- Search *of encrypted content*, server-side language detection, or server-side highlighting on encrypted pastes — these are structurally impossible and out of scope (see §5).
+- Signing, deniability, forward secrecy across pastes, PFS, post-quantum crypto.
+
+**Threat model (explicit).** This protects against a *passive server compromise* — a DB dump, backup leak, or disk image of the server's SQLite file. It does **not** protect against:
+
+- A fully malicious / compromised Palette server serving backdoored JavaScript: any JS-delivered crypto can be backdoored (key exfiltration via JS) regardless of primitives. This is the fundamental limit of a JS-in-browser E2E scheme.
+- Malware on the viewer's device, or shoulder-surfing of the password.
+- Traffic analysis, timing, or metadata (title, size, expiry, IP, viewer cookie).
+- A attacker who compromises the server *while the creator's browser is open* and alters JS before encrypt.
+
+Be explicit in user-facing copy: "encrypted at rest; the server cannot read your paste" is accurate — "the server can never see your paste" is not.
+
+## 2. Crypto primitives and flow
+
+### 2.1 Recommended parameters
+
+| Parameter | Recommendation | Notes |
+|---|---|---|
+| Cipher | **AES-256-GCM** | `AES-GCM` with a 256-bit key, per-paste random 96-bit IV/nonce. WebCrypto built-in, hardware-accelerated, authenticated. |
+| KDF | **PBKDF2-HMAC-SHA-256** | 600,000 iterations (OWASP 2023+ recommendation), 16-byte random salt. |
+| Argon2id | **Not in v1** | WebCrypto has no Argon2id; a JS/WASM Argon2 implementation is an extra supply-chain dependency and is an asymmetric liability: a script the server could swap can't be load-bearing for security anyway. Add later via `argon2id` WASM with SRI pinning + CSP (`script-src 'self'`) if needed. |
+| Salt | 16 random bytes per paste, stored in the clear alongside ciphertext | Unique per paste, never reused. |
+| IV | 12 random bytes per encryption | With ~2^32 encryptions per key this is negligible; each paste has its own key anyway. |
+| Key check value | See §2.2 | Catches wrong passwords without a server round-trip and prevents trash writes. |
+
+### 2.2 Flow (create)
+
+1. User checks "Encrypt" and enters an encryption passphrase (distinct from any access password) in `/new`.
+2. Browser generates `salt` (16 B) and `iv` (12 B) via `crypto.getRandomValues`.
+3. `crypto.subtle.importKey("raw", passphrase, "PBKDF2", false, ["deriveKey"])` →
+   `crypto.subtle.deriveKey(PBKDF2-SHA-256, 600k iterations, salt, {name:"AES-GCM", length:256}, false, ["encrypt","decrypt"])`.
+4. Generate a 32-byte random **DEK** (`crypto.getRandomValues(32)`).
+5. Content encryption key check: `iv_ckv`, `encrypted_content = AES-GCM-256(DEK, iv, content)`.
+6. **Key check value (KCV):** compute `AES-GCM-DEK(random 16 bytes)` — a small token encrypted *under the DEK*, stored as `key_check` blob. This is decrypted with the derived key; on wrong password GCM auth fails and the client can show "wrong key" without asking the server to burn a read.
+7. Wrap the DEK with the KEK: `wrapped_dek = AES-GCM(KEK, iv_wrap, dek)`.
+8. The stored envelope format:
+   ```
+   {
+     v: 1, kdf: "PBKDF2-SHA256", iterations: 600000, salt_b64, iv_b64,
+     kdf_salt_b64, wrap_iv_b64, wrapped_dek_b64, key_check_b64, ciphertext_b64
+   ```
+   The `v` field allows migrating to Argon2id later without a breaking change.
+8. POST the envelope (base64) as `content`, with an `encryption` metadata object alongside (see §3.1).
+
+### 2.3 Flow (view)
+
+1. User provides the passphrase via form field, or the key arrives in the URL `#fragment`. The encryption passphrase is a separate field from any access password.
+2. Fetch `/api/pastes/{id}` (with access password in the usual field if the paste is also password-gated).
+3. Derive KEK from passphrase+salt, unwrap DEK via key_check / unwrap step.
+4. Decrypt content with the DEK; on `OperationError` → "wrong passphrase" UI state (retries are client-side only; no re-fetch, so no extra burn-after-read charge).
+5. Language detection happens client-side (e.g. highlight.js auto-detect) on the decrypted plaintext.
+
+### §2.4 File and can items
+
+Files in cans: encrypt each file with its own DEK and store the same envelope. Cans' `json_items` content fields each carry their own envelope. Files keep their mime type in cleartext metadata; only the bytes are encrypted. The can's title stays plaintext (unless the whole can is encrypted, v2).
+
+## 3. API shapes
+
+### 3.1 Create request
+
+Existing fields unchanged. New optional `encryption` object:
+
+```json
+POST /api/pastes
+{
+  "content": "<base64 envelope>",
+  "encryption": {"v": 1, "kdf": "PBKDF2-SHA256", "iterations": 600000,
+                 "salt": "b64", "iv": "b64", "key_check": "b64"}
+}
+```
+
+`encryption` is non-secret KDF metadata for UI display; the server treats `content` as opaque bytes and MUST NOT inspect it for encrypted pastes (no detection, no highlighting prep, no search indexing) — enforced where content is written, not per-handler.
+
+The full envelope can also just live inside `content` (server-opaque); the `encryption` object carries only non-secret KDF metadata the list views need (e.g. to show a 🔒 icon).
+
+### 3.2 Create response
+
+Unchanged shape: `id`, `url`, `raw_url`, `api_url`, and the one-time `deletion_token` documented in docs/API.md.
+
+### 3.3 Get response
+
+`GET /api/pastes/{id}` response gains:
+
+```json
+{
+  "id": "abc123",
+  "content": "BASE64_ENVELOPE",
+  "encryption": {"v":1, "kdf": "PBKDF2-SHA256", "iterations": 601570, "salt": "b64", "iv": "base64", "key_check": "b64"},
+  "reads_remaining": null
+}
+```
+
+`language` is `"encrypted"` or `null` so clients don't run detection on ciphertext. `raw_url` also serves the envelope; the `/{id}` page ships it to the browser, which decrypts in place.
+
+### 3.4 Raw endpoint
+
+`GET /raw/{id}` returns the envelope as `application/octet-stream` with a suggested filename like `{id}.e2e.txt` and `Content-Disposition: attachment`. This is deliberate: a "download encrypted blob" is what a non-browser client can do with it anyway.
+
+### 3.5 List views / mine / public
+
+List endpoints return `has_encryption: true` instead of content; show a lock icon. Do not include ciphertext in list responses (size, and no reason to ship ciphertext to every viewer's list view) — `GET /api/pastes/{id}` remains the only endpoint that returns the envelope.
+
+`/api/mine` (creator's own browser) may include the envelope for convenience; `/api/public` returns metadata only.
+
+`/api/guess-language` rejects encrypted content with `400 "content is client-encrypted"` — detection needs plaintext; clients detect after decrypting.
+
+Delete, redeem, rate limits, expiry, sweeper, deletion tokens, visibility, slugs, and can CRUD are unchanged — the server never inspects content for these, so opaque content is a no-op path.
+
+## 4. Interplay with existing features
+
+| Feature | Impact | Mitigation |
+| Burn-after-read | Budget is charged on fetch, exactly as today; the server cannot know whether decryption succeeded, so a viewer fetching with the wrong key burns a read they can't use. | Decrypt retries are client-side, so only the first fetch charges the budget. Clear UX copy. |
+| Password-protected + encrypted | Both can coexist and are independent: the access password is an HTTP 401 gate; the encryption passphrase never leaves the browser. If both are set, all three secrets are needed (URL + access password + passphrase). Warn if the user enters the same value in both fields. |
+| Encryption-only pastes | Supported with no access password: URL + passphrase (or fragment key). Default is passphrase; fragment key is opt-in with a warning. |
+| Search | Structurally impossible over ciphertext. Server search just skips encrypted pastes; client-side search within a single decrypted paste works fine. No global encrypted-content search — accept the loss, document it. |
+| Language detection / highlighting | Server-side detection/highlighting impossible; returns `language: null`. Client-side detection via highlight.js auto-detect on decrypted plaintext (client already loads it for password gate pages). |
+| Cans/files | Per-item envelopes (own DEK each), per §2.4. | Consider a can-level KEK (one passphrase unlocks all items). |
+| Expiry/sweeper/delete/redeem | Unchanged — server never inspects content for these. |
+| List views (`/api/public`, `/api/mine`) | Additive `has_encryption: true` flag; list responses do not include ciphertext (`/api/mine` may include the envelope for the creator's own convenience). |
+| guess-language endpoint | Reject with 400. |
+| Fork / edit | Re-encryption needs the passphrase in the browser; v1 disables forking encrypted pastes. | Document the limitation. |
+
+## 5. What breaks, stated plainly
+
+- **Search across encrypted pastes: impossible.** Accept the loss. (If ever needed, client-side index in IndexedDB for the creator's own pastes — v2+.)
+ IndexedDB only helps the creator, not other viewers; still not global search. Accept the loss.
+- **Server-side language detection and highlighting: impossible.** Client-side detection on decrypted plaintext. Server returns `language: null` and the client detects.
+- **Burn-after-read is weakened in one specific way:** the budget is counted on fetch, not on successful decryption. A viewer who fetches but can't decrypt (wrong/lost key) burns a read they can't use. Mitigations documented in §4 table. The server can still count fetches (which is what burn-after-read actually is, even today: it counts fetches, not "reads" in any content-aware sense). So burn-after-read still works — it counts fetches — it's just that a failed decryption still consumes budget. This is acceptable and just needs UX copy. Optionally: don't decrement on failed decryption is *not possible* the server can't tell, so it's fetch-based, period. (It already is today.)
+- **Raw endpoint semantics change:** `/raw/{id}` can no longer serve readable raw text. It serves the ciphertext envelope. Scripts that curl raw pastes will get base64 envelope instead of text. Document as a breaking-ish change for encrypted pastes only; unencrypted pastes unchanged.
+- **Existing /api/mine, /api/public list shapes gain a flag** (additive, non-breaking).
+- **Copy-to-clipboard of decrypted text stays client-side**, fine. "Copy raw" on an encrypted paste copies the envelope — label it clearly.
+
+## 5. UX for key sharing
+### 5.1 Three sharing modes
+
+| Mode | What's shared | Security level | Use case |
+|---|---| malformed JSON / wrong key | | |
+| Mode | What's shared | Strength | Use case |
+|---|---|---|---|
+| **Passphrase** (default) | URL + passphrase out-of-band (Signal etc.) | Good — two channels | Team snippets, sensitive configs |
+| **Passphrase + access password** | URL + access password (401 gate) + passphrase | Strong — two secrets, two channels | Highest sensitivity |
+| **Random key in `#fragment`** | URL containing `#key=<b64>` | Weak — single channel; anyone with the full URL has both parts. Copy/paste into chat defeats it entirely. | One-click convenience sharing |
+
+Browsers never transmit `#` fragments to servers; still set `Referrer-Policy: no-referrer` site-wide and offer separate copy buttons for URL and key. Key-in-fragment ships with a warning and stays opt-in.
+
+### 5.2 Create page (`/new`) UX
+
+- "Encrypt content" toggle → reveals passphrase field + strength meter + generate-random-key button.
+- When encrypting, hide the server-side language dropdown; the client detects language after decryption.
+- Two separate inputs with distinct labels: "Access password (checked by the server, 401 gate)" and "Encryption passphrase (never leaves your browser)". If both hold the same value, warn.
+
+### 6.2 View page (`/{id}`) UX
+
+- If `encryption.kdf` is present → show key entry UI (after the access-password 401 gate, if that also applies).
+- After decrypt: normal render pipeline, language detected client-side.
+- "Wrong passphrase" retries never re-fetch, so they never burn extra reads.
+
+## 7. Backwards compatibility and migration
+
+- Additive JSON fields only; unencrypted pastes behave identically. No schema changes (envelope is stored in the existing content column/TEXT; verify column size allows envelope overhead (~2× base64 + ~200 B header).
+- Server-side validation of encrypted pastes: only structural checks (base64 decodes, size ≤ max bytes). No crypto in the server.
+
+**Server implementation cost is genuinely small** (est. 2-4 days): pass through content untouched, add `encryption` metadata column or embed in content, skip detection/indexing when `encryption` is present, list flag. The server never does crypto. All crypto is client-side JS (~150-300 lines, no build-step change if using WebCrypto alone).
+
+**Argon2id later:** add `kdf: "argon2id"` to the envelope `v: 1` (m=64 MiB, t=3, p=1) via a SRI-pinned WASM module, with CSP `script-src 'self'` + SRI on the script tag. Envelope `v` field already allows this.
+
+## 8. Recommendation
+
+**Build it, as an opt-in checkbox, passphrase mode only in v1.**
+
+- Server cost is small (pass-through + skip detection/indexing + list flag), client cost moderate (WebCrypto only, no new deps).
+- It closes the biggest real-world risk for a public pastebin: a DB/backup leak exposing every paste ever written.
+- Skip Argon2id in v1; envelope `v` field provides a migration path.
+- Key-in-fragment mode: build the plumbing (fragment parsing) but hide behind "advanced"; default remains passphrase.
+
+**Do not build:** server-side search over encrypted content, server-side highlighting of encrypted content, decrypt-on-server "preview" mode, or any server-side crypto.
+
+## 9. Open questions
+
+1. Size limits: base64 expansion (~4/3×) plus ~200 B envelope overhead; the existing max-bytes / 413 limit applies to the envelope bytes the server stores. Do not compress before encrypting (CRIME-style weaknesses).
+2. Fork/edit of encrypted pastes: disabled in v1, revisit.
+3. Should `/api/mine` include the full envelope in list view? Leaning yes (creator's own browser can decrypt); note the larger payload.
+4. Should there be a "verify passphrase" second field at create time (type-twice), or rely on the KCV check at view time? KCV at view time suffices; type-twice adds friction at create. Rely on KCV, skip type-twice.
+5. CSP/Referrer-Policy hardening: `Referrer-Policy: no-referrer` site-wide is worth doing regardless of this feature (it also benefits unencrypted pastes).
+6. Cans: per-item DEKs wrapped by a single can-level KEK (one passphrase unlocks all items) — better UX, slightly more envelope design work. Defer detail to implementation.
+
+## 10. Alternatives considered
+
+| Alternative | Why not in v1 |
+|---|---|
+| Argon2id via WASM in v1 | Extra JS dependency the server could swap → can't be load-bearing; PBKDF2-600k is adequate for a pastebin. Defer. |
+| Server holds half a key (2-of-2 with server-held share) | Re-introduces server trust; defeats the purpose. |
+| age-format envelopes | Nice CLI interop but no WebCrypto-native support; adds a JS dependency. Defer. |
+| PGP / S-MIME | Poor browser UX; heavy dependencies. |
+| Server-side encryption with server-held keys | Not E2E; that's "encrypted at rest", already covered by disk-level encryption. |
+| PrivateBin-style fragment key only | Single-channel sharing is a footgun; keep passphrase as default. |
+| libsodium / tweetnacl | Solid but unnecessary; WebCrypto covers AES-GCM + PBKDF2 natively. |
+
+## 11. References
+
+- OWASP Password Storage Cheat Sheet (PBKDF2 guidance): https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html
+- MDN WebCrypto: https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto
+- PrivateBin (prior art for fragment-key sharing): https://privatebin.info
+- 0bin, Hemmelig — other pastebin/secret E2E prior art.

internal/api/admin.go

@@ -172,6 +172,11 @@ func (a *apiServer) adminKeyOK(r *http.Request, key string) bool {
 
 func (a *apiServer) adminAuth(next http.HandlerFunc, key string) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
+		if !rateLimitAdmin(r) {
+			log.Printf("admin auth RATE LIMITED: %s %s from %s", r.Method, r.URL.Path, r.RemoteAddr)
+			writeRateLimited(w, 60)
+			return
+		}
 		if !a.adminKeyOK(r, key) {
 			log.Printf("admin auth FAILURE: %s %s from %s", r.Method, r.URL.Path, r.RemoteAddr)
 			writeErr(w, 401, "unauthorized")

internal/api/adminratelimit_test.go

@@ -0,0 +1,80 @@
+package api
+
+// #66: admin key attempts must be rate limited per IP (5/min), constant-time
+// compared, and failures logged. Hammering bad keys must yield 429s.
+
+import (
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+// TestAdminKeyRateLimited: burst of 5 bad-key attempts allowed (401), the 6th
+// gets 429, and even the correct key is blocked from that IP until refill.
+func TestAdminKeyRateLimited(t *testing.T) {
+	srv := newTestServer(t)
+	h := srv.routes()
+	reqIP := "10.7.7.1:1234"
+
+	var got429, retryAfter bool
+	var lastCode int
+	for i := 0; i < 10; i++ {
+		req := httptest.NewRequest("POST", "/admin/api/settings", nil)
+		req.RemoteAddr = reqIP
+		req.Header.Set("X-Admin-Key", "wrong-key")
+		rec := httptest.NewRecorder()
+		h.ServeHTTP(rec, req)
+		lastCode = rec.Code
+		if rec.Code == 429 {
+			got429 = true
+			retryAfter = rec.Header().Get("Retry-After") != ""
+			break
+		}
+	}
+	if !got429 {
+		t.Fatalf("expected 429 after hammering bad keys, last status %d", lastCode)
+	}
+	if !retryAfter {
+		t.Error("429 missing Retry-After header")
+	}
+
+	// Correct key from the same IP is also locked out.
+	req := httptest.NewRequest("POST", "/admin/api/settings", nil)
+	req.RemoteAddr = reqIP
+	req.Header.Set("X-Admin-Key", srv.adminKey)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	if rec.Code != 429 {
+		t.Errorf("correct key after lockout: got %d, want 429", rec.Code)
+	}
+
+	// A different IP is unaffected.
+	req2 := httptest.NewRequest("POST", "/admin/api/settings", strings.NewReader(`{"rate_limit_burst":5,"rate_limit_per_minute":60,"max_content_bytes":1048576,"custom_slug_reservation_days":30,"burn_viewer_window_minutes":15}`))
+	req2.RemoteAddr = "203.0.113.9:1234"
+	req2.Header.Set("X-Admin-Key", srv.adminKey)
+	rec2 := httptest.NewRecorder()
+	h.ServeHTTP(rec2, req2)
+	if rec2.Code != 200 {
+		t.Errorf("correct key from another IP: got %d, want 200", rec2.Code)
+	}
+}
+
+// TestAdminKeyConstantTimeCompare: sanity check that the comparison is
+// constant-time (uses subtle.ConstantTimeCompare, not ==).
+func TestAdminKeyConstantTimeCompare(t *testing.T) {
+	srv := newTestServer(t)
+	r := httptest.NewRequest("GET", "/", nil)
+	r.Header.Set("X-Admin-Key", "test-admin-key")
+	if !srv.adminKeyOK(r, srv.adminKey) {
+		t.Fatal("correct key rejected")
+	}
+	r.Header.Set("X-Admin-Key", "wrong")
+	if srv.adminKeyOK(r, srv.adminKey) {
+		t.Fatal("wrong key accepted")
+	}
+	// differ in length: must not panic/mismatch unexpectedly
+	r.Header.Set("X-Admin-Key", "test-admin-key-longer")
+	if srv.adminKeyOK(r, srv.adminKey) {
+		t.Fatal("longer wrong key accepted")
+	}
+}

internal/api/cans.go

@@ -1,11 +1,11 @@
 package api
 
 import (
-	"palette/internal/store"
 	"encoding/json"
 	"fmt"
 	"io"
 	"net/http"
+	"palette/internal/store"
 	"strings"
 	"time"
 
@@ -40,6 +40,12 @@ func (a *apiServer) handleCreateCan(w http.ResponseWriter, r *http.Request) {
 			writeErr(w, 400, "invalid expires_in")
 			return
 		}
+		// #60: clamp at the API boundary like the pastes API does -
+		// reject zero/negative and durations past the 1-year UI cap.
+		if !store.ValidExpiry(d) {
+			writeErr(w, 400, "expires_in must be between 1 minute and 1 year")
+			return
+		}
 		t := now + int64(d.Seconds())
 		expiresAt = &t
 	}
@@ -143,8 +149,6 @@ func detectContentType(name string, content []byte) string {
 	return "text/plain"
 }
 
-
-
 func (a *apiServer) handleGetCan(w http.ResponseWriter, r *http.Request) {
 	id := chi.URLParam(r, "id")
 	can, err := a.store.GetCan(id)

internal/api/cans_expiry_test.go

@@ -0,0 +1,45 @@
+package api
+
+import (
+	"net/http/httptest"
+	"testing"
+)
+
+// #60: the cans API must clamp expires_in at the boundary exactly like the
+// pastes API — reject zero/negative durations and anything over the 1-year
+// UI cap, accept the exact boundaries.
+func TestCreateCanExpiryBounds(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+
+	cases := []struct {
+		expiresIn string
+		wantCode  int
+	}{
+		{"-1h", 400},    // negative
+		{"-0s", 400},    // negative zero
+		{"0s", 400},     // zero
+		{"1ns", 400},    // positive but below the 1-minute floor
+		{"59s", 400},    // just under the floor
+		{"1m", 201},     // exactly the floor
+		{"90s", 201},    // just over the floor
+		{"8760h", 201},  // exactly 1 year
+		{"8785h", 400},  // 1 year + 1 day: over the cap
+		{"87600h", 400}, // 10 years, the originally reported case
+	}
+	for _, c := range cases {
+		globalLimiter = newLimiter() // avoid create rate limit between cases
+		body, ct := multipartBody(t, map[string]string{
+			"json_items": `[{"title":"a.txt","content":"AAA"}]`,
+			"expires_in": c.expiresIn,
+		}, "files", "pic.txt", "file data")
+		req := httptest.NewRequest("POST", "/api/pastes/can", body)
+		req.Header.Set("Content-Type", ct)
+		rec := httptest.NewRecorder()
+		h.ServeHTTP(rec, req)
+		if rec.Code != c.wantCode {
+			t.Errorf("expires_in %q: got %d want %d (%s)",
+				c.expiresIn, rec.Code, c.wantCode, rec.Body.String())
+		}
+	}
+}

internal/api/issue68_validation_test.go

@@ -0,0 +1,198 @@
+package api
+
+// Regression tests for #68 input-validation gaps: negative/oversized content
+// lengths (413), limit=0 → default page size, unchecked query params
+// (negative offset), and negative burn_after_reads.
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func createPasteRaw(t *testing.T, h http.Handler, body string) *httptest.ResponseRecorder {
+	t.Helper()
+	req := httptest.NewRequest("POST", "/api/pastes", strings.NewReader(body))
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	return rec
+}
+
+// A request whose decoded content exceeds the admin-tunable cap is rejected
+// with 413 and a clear message (content under the body cap, over the
+// content cap).
+func TestCreatePasteContentOverCap413(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+
+	// content over MaxContentBytes (5MiB) but under body cap (+4KiB): send
+	// just over the content cap so the per-field check fires first.
+	content := strings.Repeat("a", 5*1024*1024+10)
+	rec := createPasteRaw(t, h, fmt.Sprintf(`{"content":"%s"}`, content))
+	if rec.Code != http.StatusRequestEntityTooLarge {
+		t.Fatalf("got %d want 413: %s", rec.Code, rec.Body.String())
+	}
+	if !strings.Contains(rec.Body.String(), "content exceeds max") {
+		t.Fatalf("unclear error message: %s", rec.Body.String())
+	}
+}
+
+// A request whose entire body is larger than the server-side body cap is cut
+// off by http.MaxBytesReader and answered with 413, not decoded into memory
+// (#68: previously a giant body was fully buffered, then rejected only at
+// the per-field check — actually the decode happened before any check).
+func TestCreatePasteBodyOverCap413(t *testing.T) {
+	s := testServer(t)
+	// shrink the content cap so the body cap is small too
+	ss := s.settings.get()
+	ss.MaxContentBytes = 64 * 1024
+	if err := s.settings.set(ss); err != nil {
+		t.Fatal(err)
+	}
+	h := s.routes()
+
+	content := strings.Repeat("a", 200*1024) // 200KiB > 64KiB+4KiB body cap
+	rec := createPasteRaw(t, h, fmt.Sprintf(`{"content":"%s","title":"x"}`, content))
+	if rec.Code != http.StatusRequestEntityTooLarge {
+		t.Fatalf("got %d want 413: %s", rec.Code, rec.Body.String())
+	}
+}
+
+// Negative content lengths cannot be expressed via JSON, but a negative
+// expires-style numeric payload must not crash; more importantly the
+// burn_after_reads field: negative values are rejected with a clear message.
+func TestCreatePasteNegativeBurnAfterReads(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+
+	rec := createPasteRaw(t, h, `{"content":"hi","burn_after_reads":-5}`)
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("got %d want 400: %s", rec.Code, rec.Body.String())
+	}
+	if !strings.Contains(rec.Body.String(), "burn_after_reads") {
+		t.Fatalf("unclear error message: %s", rec.Body.String())
+	}
+
+	// 0 and positive values still work (0 = default single read, per #49)
+	rec = createPasteRaw(t, h, `{"content":"hi","burn_after_reads":0,"burn_after_read":true}`)
+	if rec.Code != http.StatusCreated {
+		t.Fatalf("zero burn_after_reads: got %d want 201: %s", rec.Code, rec.Body.String())
+	}
+}
+
+// limit=0 on list endpoints returns the default page size (existing clamp
+// treats <=0 as default; #68 asks this be explicit and tested).
+func TestListLimitZeroUsesDefault(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+
+	// seed 3 public pastes
+	for i := 0; i < 3; i++ {
+		rec := createPasteRaw(t, h, fmt.Sprintf(`{"content":"p%d"}`, i))
+		if rec.Code != 201 {
+			t.Fatalf("seed: got %d: %s", rec.Code, rec.Body.String())
+		}
+	}
+
+	for _, q := range []string{"/api/public?limit=0", "/api/public"} {
+		req := httptest.NewRequest("GET", q, nil)
+		rec := httptest.NewRecorder()
+		h.ServeHTTP(rec, req)
+		if rec.Code != 200 {
+			t.Fatalf("%s: got %d", q, rec.Code)
+		}
+		var got struct {
+			Limit int                   `json:"limit"`
+			Total int                   `json:"total"`
+			Items []struct{ ID string } `json:"items"`
+		}
+		json.Unmarshal(rec.Body.Bytes(), &got)
+		if got.Limit != 25 || len(got.Items) != 3 {
+			t.Fatalf("%s: limit=%d items=%d, want limit 25 and all 3 items", q, got.Limit, len(got.Items))
+		}
+	}
+}
+
+// Huge limit values are clamped to the max page size (already the behavior;
+// regression-tested here per #68 "validate unchecked params").
+func TestListLimitHugeClamped(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+
+	req := httptest.NewRequest("GET", "/api/public?limit=999999999", nil)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	var got struct {
+		Limit int `json:"limit"`
+	}
+	json.Unmarshal(rec.Body.Bytes(), &got)
+	if got.Limit != 25 {
+		t.Fatalf("limit=%d, want clamped to 25", got.Limit)
+	}
+}
+
+// Negative offset previously passed through unchecked to SQL (harmless in
+// SQLite, but invalid); it must be clamped to 0 (#68).
+func TestListNegativeOffsetClamped(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+
+	for i := 0; i < 2; i++ {
+		rec := createPasteRaw(t, h, fmt.Sprintf(`{"content":"p%d"}`, i))
+		if rec.Code != 201 {
+			t.Fatalf("seed: got %d", rec.Code)
+		}
+	}
+
+	req := httptest.NewRequest("GET", "/api/public?offset=-999", nil)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	if rec.Code != 200 {
+		t.Fatalf("got %d", rec.Code)
+	}
+	var got struct {
+		Offset int `json:"offset"`
+		Total  int `json:"total"`
+	}
+	json.Unmarshal(rec.Body.Bytes(), &got)
+	if got.Offset != 0 || got.Total != 2 {
+		t.Fatalf("offset=%d total=%d, want offset 0 and total 2", got.Offset, got.Total)
+	}
+
+	// /api/mine too (needs the viewer cookie)
+	req = httptest.NewRequest("GET", "/api/mine?offset=-5", nil)
+	req.AddCookie(&http.Cookie{Name: "vwr", Value: "offclamp"})
+	rec = httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	var mine struct {
+		Offset int `json:"offset"`
+	}
+	json.Unmarshal(rec.Body.Bytes(), &mine)
+	if mine.Offset != 0 {
+		t.Fatalf("mine offset=%d, want 0", mine.Offset)
+	}
+}
+
+// Non-numeric limit/offset fall back to defaults instead of 500s.
+func TestListGarbageParams(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+
+	req := httptest.NewRequest("GET", "/api/public?limit=abc&offset=xyz", nil)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	if rec.Code != 200 {
+		t.Fatalf("got %d", rec.Code)
+	}
+	var got struct {
+		Limit  int `json:"limit"`
+		Offset int `json:"offset"`
+	}
+	json.Unmarshal(rec.Body.Bytes(), &got)
+	if got.Limit != 25 || got.Offset != 0 {
+		t.Fatalf("limit=%d offset=%d, want 25/0", got.Limit, got.Offset)
+	}
+}

internal/api/main_test.go

@@ -183,6 +183,46 @@ func TestListPublicExcludesUnlisted(t *testing.T) {
 	}
 }
 
+func TestListPublicExcludesPasswordAndUnlisted(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+
+	bodies := []string{
+		`{"content":"open","visibility":"public"}`,
+		`{"content":"locked","visibility":"public","password":"hunter2"}`,
+		`{"content":"hidden","visibility":"unlisted"}`,
+	}
+	for _, body := range bodies {
+		req := httptest.NewRequest("POST", "/api/pastes", strings.NewReader(body))
+		rec := httptest.NewRecorder()
+		h.ServeHTTP(rec, req)
+		if rec.Code != 201 {
+			t.Fatalf("create %s: got %d", body, rec.Code)
+		}
+	}
+
+	req := httptest.NewRequest("GET", "/api/public", nil)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	if rec.Code != 200 {
+		t.Fatalf("list public: got %d", rec.Code)
+	}
+	var resp struct {
+		Total int              `json:"total"`
+		Items []map[string]any `json:"items"`
+	}
+	json.Unmarshal(rec.Body.Bytes(), &resp)
+	if resp.Total != 1 || len(resp.Items) != 1 {
+		t.Fatalf("expected only the 1 public paste, got total=%d items=%d", resp.Total, len(resp.Items))
+	}
+	// password-protected and unlisted pastes must not appear (no metadata leak)
+	for _, secret := range []string{"hunter2", "locked", "hidden"} {
+		if strings.Contains(rec.Body.String(), secret) {
+			t.Fatalf("leaked %q in /api/public response", secret)
+		}
+	}
+}
+
 func TestSweepSoftDeletesAfterGrace(t *testing.T) {
 	s := testServer(t)
 	h := s.routes()

internal/api/ratelimit.go

@@ -84,6 +84,12 @@ func rateLimitUnlock(id string, r *http.Request) bool {
 	return globalLimiter.allow("unlock:"+id+":"+clientIP(r), 5.0/60.0, 5)
 }
 
+// rateLimitAdmin: 5 attempts per minute per IP on the admin key check (#66),
+// same pattern as the unlock limiter (#34).
+func rateLimitAdmin(r *http.Request) bool {
+	return globalLimiter.allow("admin:"+clientIP(r), 5.0/60.0, 5)
+}
+
 // writeRateLimited responds 429 with Retry-After based on refill rate.
 func writeRateLimited(w http.ResponseWriter, retryAfterSecs int) {
 	w.Header().Set("Retry-After", strconv.Itoa(retryAfterSecs))

internal/api/server.go

@@ -59,6 +59,7 @@ func (a *apiServer) routes() http.Handler {
 	r := chi.NewRouter()
 	r.Use(middleware.Recoverer)
 	r.Use(middleware.Timeout(30 * time.Second))
+	r.Use(a.limitRequestBody) // #68: hard server-side body cap -> 413
 	r.Use(viewerCookieMiddleware)
 	r.Use(web.SecurityHeaders) // #59: CSP + hardening headers on HTML pages
 
@@ -153,17 +154,23 @@ func (a *apiServer) handleCreatePaste(w http.ResponseWriter, r *http.Request) {
 	}
 	var p store.Paste
 	if err := json.NewDecoder(r.Body).Decode(&p); err != nil {
+		if isBodyTooLarge(err) { // #68: body cut off by MaxBytesReader
+			writeBodyTooLarge(w)
+			return
+		}
 		writeErr(w, 400, "invalid json body")
 		return
 	}
-	if strings.TrimSpace(p.Content) == "" {
+	if status, msg := checkContent(p.Content, s.MaxContentBytes); status != 0 {
-		writeErr(w, 400, "content is required")
+		writeErr(w, status, msg)
 		return
 	}
-	if int64(len(p.Content)) > s.MaxContentBytes { // #40: admin-tunable
+	if p.BurnAfterReads != nil { // #68: reject negative read budgets
-		writeErr(w, 413, fmt.Sprintf("content exceeds max %d bytes", s.MaxContentBytes))
+		if err := parseBurnAfterReads(*p.BurnAfterReads); err != nil {
+			writeErr(w, 400, err.Error())
 			return
 		}
+	}
 	// #40: admin-configurable default expiry
 	if (p.ExpiresIn == nil || *p.ExpiresIn == "") && s.DefaultExpiry != "" {
 		def := s.DefaultExpiry
@@ -291,11 +298,8 @@ func (a *apiServer) handleListMine(w http.ResponseWriter, r *http.Request) {
 		writeJSON(w, 200, map[string]any{"total": 0, "items": []any{}})
 		return
 	}
-	limit, _ := strconv.Atoi(r.URL.Query().Get("limit"))
+	limit := parseLimit(r, 50, 100)
-	if limit <= 0 || limit > 100 {
+	offset := parseOffset(r)
-		limit = 50
-	}
-	offset, _ := strconv.Atoi(r.URL.Query().Get("offset"))
 	rows, total, err := a.store.ListMine(vid, limit, offset)
 	if err != nil {
 		writeErr(w, 500, "db error")
@@ -314,11 +318,8 @@ func (a *apiServer) handleListMine(w http.ResponseWriter, r *http.Request) {
 }
 
 func (a *apiServer) handleListPublic(w http.ResponseWriter, r *http.Request) {
-	limit, _ := strconv.Atoi(r.URL.Query().Get("limit"))
+	limit := parseLimit(r, 25, 100)
-	if limit <= 0 || limit > 100 {
+	offset := parseOffset(r)
-		limit = 25
-	}
-	offset, _ := strconv.Atoi(r.URL.Query().Get("offset"))
 	rows, total, err := a.store.ListPublic(limit, offset)
 	if err != nil {
 		writeErr(w, 500, "db error")

internal/api/validate.go

@@ -0,0 +1,92 @@
+package api
+
+import (
+	"errors"
+	"fmt"
+	"net/http"
+	"strconv"
+	"strings"
+)
+
+// #68 input-validation helpers. Paste/can payloads are size-capped and list
+// endpoints get a single place where limit/offset are parsed and clamped.
+
+// maxRequestBody returns the HTTP body cap for JSON create endpoints: the
+// admin-tunable content cap plus headroom for JSON field overhead, floored
+// at 64KiB so a tiny admin-configured cap can't break small requests.
+func (a *apiServer) maxRequestBody() int64 {
+	s := a.settings.get()
+	max := s.MaxContentBytes + 4096
+	if max < 64*1024 {
+		max = 64 * 1024
+	}
+	return max
+}
+
+// limitRequestBody wraps the request body with http.MaxBytesReader so
+// oversized payloads are cut off server-side instead of being fully decoded
+// into memory before the per-field size check runs (#68). A read over the
+// cap surfaces as *http.MaxBytesError, which handlers map to 413.
+func (a *apiServer) limitRequestBody(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Body != nil {
+			r.Body = http.MaxBytesReader(w, r.Body, a.maxRequestBody())
+		}
+		next.ServeHTTP(w, r)
+	})
+}
+
+// writeBodyTooLarge emits the 413 response for a body rejected by
+// MaxBytesReader.
+func writeBodyTooLarge(w http.ResponseWriter) {
+	writeErr(w, http.StatusRequestEntityTooLarge, "request body too large")
+}
+
+// isBodyTooLarge reports whether err came from http.MaxBytesReader.
+func isBodyTooLarge(err error) bool {
+	var mbe *http.MaxBytesError
+	return errors.As(err, &mbe)
+}
+
+// checkContent validates paste content: rejects whitespace-only content
+// (400) and content over the byte cap (413). Returns (0, "") when valid.
+func checkContent(content string, maxBytes int64) (int, string) {
+	if strings.TrimSpace(content) == "" {
+		return http.StatusBadRequest, "content is required"
+	}
+	if int64(len(content)) > maxBytes { // #40/#68: admin-tunable cap
+		return http.StatusRequestEntityTooLarge,
+			fmt.Sprintf("content exceeds max %d bytes", maxBytes)
+	}
+	return 0, ""
+}
+
+// parseLimit clamps the ?limit query param: missing/non-numeric/zero/negative
+// or over-max values fall back to def. Zero intentionally maps to the default
+// page size, matching the pre-existing `<= 0` clamp (#68).
+func parseLimit(r *http.Request, def, max int) int {
+	n, err := strconv.Atoi(r.URL.Query().Get("limit"))
+	if err != nil || n <= 0 || n > max {
+		return def
+	}
+	return n
+}
+
+// parseOffset clamps the ?offset query param: missing/non-numeric or negative
+// values become 0 (#68: negative offsets previously passed through to SQL).
+func parseOffset(r *http.Request) int {
+	n, err := strconv.Atoi(r.URL.Query().Get("offset"))
+	if err != nil || n < 0 {
+		return 0
+	}
+	return n
+}
+
+// parseBurnAfterReads validates the burn_after_reads field (#68): negative
+// values are rejected; zero/absent mean the default single read.
+func parseBurnAfterReads(n int) error {
+	if n < 0 {
+		return errors.New("burn_after_reads must be a positive number")
+	}
+	return nil
+}

internal/store/store.go

@@ -257,9 +257,13 @@ func (s *Store) GetPaste(idOrSlug string) (*PasteRow, error) {
 	return &r, err
 }
 
+// ListPublic backs /api/public and the public listing page. Visibility rules
+// mirror the history page: only non-deleted, non-expired, non-can pastes are
+// listed, and password-protected pastes are excluded at the query level
+// (#65) so their metadata (title, slug, existence) never leaks.
 func (s *Store) ListPublic(limit, offset int) ([]PasteRow, int, error) {
 	rows, err := s.db.Query(`SELECT id, custom_slug, content_type, language, title, visibility, created_at, view_count, LENGTH(content) FROM pastes
-		WHERE visibility='public' AND deleted_at IS NULL AND can_id IS NULL AND (expires_at IS NULL OR expires_at > ?)
+		WHERE visibility='public' AND deleted_at IS NULL AND can_id IS NULL AND password_hash IS NULL AND (expires_at IS NULL OR expires_at > ?)
 		ORDER BY created_at DESC LIMIT ? OFFSET ?`, time.Now().Unix(), limit, offset)
 	if err != nil {
 		return nil, 0, err
@@ -278,7 +282,7 @@ func (s *Store) ListPublic(limit, offset int) ([]PasteRow, int, error) {
 		out = append(out, r)
 	}
 	var total int
-	s.db.QueryRow(`SELECT COUNT(*) FROM pastes WHERE visibility='public' AND deleted_at IS NULL AND can_id IS NULL AND (expires_at IS NULL OR expires_at > ?)`, time.Now().Unix()).Scan(&total)
+	s.db.QueryRow(`SELECT COUNT(*) FROM pastes WHERE visibility='public' AND deleted_at IS NULL AND can_id IS NULL AND password_hash IS NULL AND (expires_at IS NULL OR expires_at > ?)`, time.Now().Unix()).Scan(&total)
 	return out, total, nil
 }
 

internal/web/static/table.js

@@ -63,7 +63,7 @@ const PaletteTable = (() => {
         const filtered = state.filter.length > 0;
         const off = (state.page - 1) * opts.perPage;
         const url = (filtered || state.sortKey)
-          ? opts.endpoint + '?limit=500&offset=0'
+          ? opts.endpoint + '?limit=' + (opts.fetchLimit || 100) + '&offset=0'
           : opts.endpoint + '?limit=' + opts.perPage + '&offset=' + off;
         const res = await fetch(url);
         const data = await res.json();