shopdb-flask/docs/SECURE-SEND.md

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purpose: explanation
audience: operator, adopter, developer
status: current
summary: How the one-view expiring credential link works, what the server cannot see, and who may create or read one.
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# Secure Send

Hand somebody a password without leaving it in a chat window.

Secure Send makes a link that carries one credential, dies when it is opened,
and expires on its own if nobody opens it. It lives in **Tech Tools** at
`/tools/secure-send`, and the person receiving it opens `/s/<token>`. It is the
one tool in that section that is not purely browser-side, because a link that
dies on first read needs somewhere to keep the ciphertext until it is claimed.

## What the server can and cannot see

The secret is encrypted **in the sender's browser** with AES-GCM, and the key
travels in the **URL fragment** - the part after the `#`. Browsers never send a
fragment to a server, so the key reaches the recipient's browser and nowhere
else: not this server, not its logs, not a proxy in between.

What the `securesendsecrets` table holds is a random token, base64 ciphertext,
an IV, and the rules for the row's life. Reading the database yields nothing.

That cuts both ways, and it is worth being explicit about it:

- **Nobody can recover a secret that goes missing.** Not an administrator, not
  the database owner. If the link is lost, the secret is lost. Create another.
- **The server cannot tell a password from a page of a drawing.** It cannot
  look at what it holds, so it cannot enforce a rule about content. The only
  lever it has is SIZE, which is the next section.
- **The audit trail records the fact, never the content.** Who created a
  secret, when, and when it was read are all recorded. What was in it is not
  recoverable by anyone.

## This is for credentials, not documents

The ciphertext limit is **4096 characters** (about 2800 characters of plaintext,
which the create page enforces first with a friendlier message). That limit is
not a tuning knob. In a design where the server cannot read what it stores,
size is the entire server-side control over what this feature is for, and
raising it quietly turns a password tool into an unaudited document transfer.

Everything else the create page does - counting lines, spotting file
signatures, refusing a pasted file - runs in the browser. It guides an honest
person and is bypassable by anyone willing to POST directly. The size limit is
the part that holds.

If somebody needs to move a file, the answer is a different tool, not a bigger
number here.

## Who may create, who may read

**Two different doors, on purpose.**

**Creating** normally needs a login plus the `tools.secrets` permission (admins
hold it by default; grant it to other roles from Settings > Users & Roles). The
login is not a formality: anonymous creation would make this an unattributable
write endpoint that stores arbitrary ciphertext, and the creator is the only
person these rows can ever name.

**Reading needs neither an account nor a permission.** The recipient usually has
no account here, and the link is normally handed over on a call or in a chat.
The unguessable token IS the authorisation, which is why it is 128 bits of
randomness and why the row is spent on the first successful read.

### Guest creation

A site may open creation to people with no account with the
`securesend_allow_guests` setting (Settings > Tools > Secure Send). It is
**off by default**, because a guest secret has no creator and the audit trail
can then say when and from where but never who. The create page itself is served
without a login either way, so a guest at a site that allows it is not stopped
by a sign-in wall; the server, not the page, decides whether the write lands.

When it is on, guest creation is rate limited to 10 links per address per 10
minutes. That is not a security boundary - a determined person has more
addresses - but it stops an open endpoint being an easy place to park data or
fill a disk.

The setting is read straight from the settings row rather than the cache, and
**fails closed**: if it cannot be read at all, the answer is no. An error must
never be the thing that opens an unauthenticated write endpoint.

Receiving a secret never needs an account either way.

## Lifetime

| Control | Default | Range |
|---------|---------|-------|
| Expiry | 24 hours | 1 to 168 hours |
| Views | 1 | 1 to 100 |

Both apply, whichever comes first. A spent row is deleted outright, including
its ciphertext.

Expired secrets are swept opportunistically - whenever anybody creates a secret,
and whenever a dead link is clicked - rather than on a schedule. A secret nobody
ever opens would otherwise be removed by nothing, and an air-gapped site is not
going to have a cron job anybody remembers to add.

Missing, expired and spent all answer identically. Distinguishing them would
confirm to anyone holding a guessed token whether it ever named anything.

## Redeeming is a POST

`POST /api/tools/secrets/<token>` hands over the ciphertext and spends the view.
A GET would be wrong here: chat and mail clients prefetch links to build
previews, and a one-view secret consumed by a preview window is a secret the
recipient never gets to see.

There is a `GET /api/tools/secrets/<token>` as well, but it reports availability
only and never spends a view. The read page asks it first so it can say "this
secret is waiting, and opening it will destroy it" instead of consuming the only
view to find out whether there was anything there.

## Before anybody relies on it

**Send a link to yourself through whatever your site uses to pass it on -
Teams, Outlook, a chat client - and check the key is still there after the `#`.**

Some mail and chat security products rewrite links. If one strips or mangles the
fragment, the secret is unrecoverable and the server cannot help, which is the
whole point of the design. Find that out on a test secret rather than on a
production credential.

## Getting the link right

The link is built from the site's configured `site_base_url`, not from the
address the sender's browser happens to be using. Somebody creating a secret may
have reached the app by an IP, a short hostname, or a name that only resolves
inside one VLAN, while the recipient has to open the link from wherever they
are. Blank falls back to the browsing origin.

A site served under a subpath (`/shopdb`, `/ops`) needs that subpath reachable
in the link. Setting it as part of `site_base_url` works, and so does leaving it
out; the mount is added only when it is not there already.

## Where it lives

| Piece | Path |
|-------|------|
| Plugin | `plugins/tools/` |
| Table | `securesendsecrets` |
| API | `POST /api/tools/secrets`, `POST` and `GET /api/tools/secrets/<token>` |
| Create page | `/tools/secure-send` |
| Read page | `/s/<token>` |
| Setting | `securesend_allow_guests` (public, boolean, category `tools`) |
| Permission | `tools.secrets` |

The `tools` settings section is in [CONFIG.md](CONFIG.md); the response envelope
these routes use is in [llms.txt](llms.txt) and in the generated spec at
`/api/docs`.