diff --git a/pyproject.toml b/pyproject.toml
index ccb3309d..0ff5483a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,6 +13,10 @@ dependencies = [
     "asyncpg>=0.29",
     "alembic>=1.13",
     "httpx>=0.27",
+    # Seals the backup destination's S3 secret at rest (services/secrets.py).
+    # Already in the image through pyjwt; named here because Scribe now
+    # imports it directly.
+    "cryptography",
     "hypercorn>=0.17",
     "bcrypt>=4.0",
     "aiosmtplib>=3.0",
diff --git a/src/scribe/services/backup.py b/src/scribe/services/backup.py
index f4c04420..a57ab811 100644
--- a/src/scribe/services/backup.py
+++ b/src/scribe/services/backup.py
@@ -772,8 +772,18 @@ def _rule_version_rows(rows) -> list[dict]:
     ]
 
 
+# Setting KEYS no backup carries. The backup destination's S3 secret is sealed
+# under SECRET_KEY, but a backup is exactly the thing copied off-site — it must
+# not carry the key that writes to the place it is copied to. A restore leaves
+# the destination configured and asks for the secret again.
+_SETTINGS_NOT_EXPORTED = frozenset({"backup_s3_secret"})
+
+
 def _setting_rows(rows) -> list[dict]:
-    return [{"user_id": s.user_id, "key": s.key, "value": s.value} for s in rows]
+    return [
+        {"user_id": s.user_id, "key": s.key, "value": s.value}
+        for s in rows if s.key not in _SETTINGS_NOT_EXPORTED
+    ]
 
 
 def _rulebook_rows(rows) -> list[dict]:
diff --git a/src/scribe/services/backup_destination.py b/src/scribe/services/backup_destination.py
new file mode 100644
index 00000000..f339a981
--- /dev/null
+++ b/src/scribe/services/backup_destination.py
@@ -0,0 +1,235 @@
+"""Where the scheduled backup goes, and getting a backup file there.
+
+Two kinds of destination, both instance-wide and set by an admin:
+
+- **folder**: a directory inside the container, usually a mounted share. A
+  backup service that watches the share (or a person) takes it from there.
+- **s3**: any S3-compatible bucket, addressed by path. Scribe only puts and
+  lists, so an append-only key is enough.
+
+Either way Scribe ADDS one dated file per run and never removes one: keeping
+and thinning the history is the destination's job (services/backup_file.py
+says why the file is shaped the way it is for that).
+
+The settings live as admin settings: `backup_config` holds everything but the
+S3 secret as JSON, and `backup_s3_secret` holds the secret sealed under the
+instance's SECRET_KEY (services/secrets.py). The secret is also kept out of
+every backup (backup._SETTINGS_NOT_EXPORTED), so a backup copied off-site
+does not carry the key that writes to that very place.
+"""
+from __future__ import annotations
+
+import asyncio
+import json
+import os
+import tempfile
+from dataclasses import asdict, dataclass, fields, replace
+from datetime import datetime
+from pathlib import Path
+
+from scribe.services import backup_file
+from scribe.services.backup_s3 import S3Error, S3Target, list_objects, put_file
+from scribe.services.secrets import seal, unseal
+from scribe.services.settings import get_admin_setting, set_admin_setting
+
+CONFIG_SETTING = "backup_config"
+SECRET_SETTING = "backup_s3_secret"
+DESTINATION_KINDS = ("folder", "s3")
+DEFAULT_FOLDER = "/backups"
+# 02:00 UTC: before the 03:30 trash purge and the 04:00 DB maintenance, so the
+# backup never contends with a VACUUM, and the night's deletions are still
+# recoverable from the backup taken just before them.
+DEFAULT_HOUR = 2
+
+
+class DestinationError(Exception):
+    """Why the destination can't take a backup, said to the person."""
+
+
+@dataclass(frozen=True)
+class BackupConfig:
+    enabled: bool = False
+    hour: int = DEFAULT_HOUR
+    destination: str = "folder"
+    folder_path: str = DEFAULT_FOLDER
+    s3_endpoint: str = ""
+    s3_bucket: str = ""
+    s3_access_key: str = ""
+    s3_region: str = "us-east-1"
+    s3_verify_tls: bool = True
+
+    @classmethod
+    def from_dict(cls, raw: dict) -> "BackupConfig":
+        """Lenient: unknown keys are ignored and bad values fall back to the
+        default, so an edited or older stored config still loads."""
+        known = {f.name: f for f in fields(cls)}
+        values = {}
+        for name, value in (raw or {}).items():
+            f = known.get(name)
+            if f is None:
+                continue
+            default = f.default
+            if isinstance(default, bool):
+                values[name] = value if isinstance(value, bool) else default
+            elif isinstance(default, int):
+                try:
+                    values[name] = int(value)
+                except (TypeError, ValueError):
+                    values[name] = default
+            else:
+                values[name] = str(value).strip() if value is not None else default
+        config = cls(**values)
+        if not 0 <= config.hour <= 23:
+            config = replace(config, hour=DEFAULT_HOUR)
+        if config.destination not in DESTINATION_KINDS:
+            config = replace(config, destination="folder")
+        return config
+
+    def to_dict(self) -> dict:
+        return asdict(self)
+
+    def problems(self, *, has_secret: bool) -> list[str]:
+        """What stops this config from sending a backup, in words. Empty when
+        it is complete (which says nothing about whether the place answers —
+        check_destination does that)."""
+        out: list[str] = []
+        if self.destination == "folder":
+            if not self.folder_path.startswith("/"):
+                out.append("The folder must be an absolute path inside the container.")
+        else:
+            if not self.s3_endpoint.startswith(("http://", "https://")):
+                out.append("The S3 address must start with http:// or https://.")
+            if not self.s3_bucket:
+                out.append("The bucket name is missing.")
+            if not self.s3_access_key:
+                out.append("The access key is missing.")
+            if not has_secret:
+                out.append("The secret key is missing.")
+        return out
+
+
+async def load_config() -> BackupConfig:
+    raw = await get_admin_setting(CONFIG_SETTING, "")
+    try:
+        data = json.loads(raw) if raw else {}
+    except ValueError:
+        data = {}
+    return BackupConfig.from_dict(data if isinstance(data, dict) else {})
+
+
+async def save_config(config: BackupConfig) -> None:
+    await set_admin_setting(CONFIG_SETTING, json.dumps(config.to_dict(), sort_keys=True))
+
+
+async def load_secret() -> tuple[str | None, bool]:
+    """(the S3 secret or None, whether one is stored).
+
+    Stored but None means it was sealed under another SECRET_KEY — after a
+    restore onto a different instance, say — and has to be entered again.
+    """
+    stored = await get_admin_setting(SECRET_SETTING, "")
+    return (unseal(stored), bool(stored))
+
+
+async def save_secret(secret: str) -> None:
+    await set_admin_setting(SECRET_SETTING, seal(secret) if secret else "")
+
+
+def s3_target(config: BackupConfig, secret: str) -> S3Target:
+    return S3Target(
+        endpoint=config.s3_endpoint, bucket=config.s3_bucket,
+        access_key=config.s3_access_key, secret_key=secret,
+        region=config.s3_region or "us-east-1", verify_tls=config.s3_verify_tls,
+    )
+
+
+@dataclass(frozen=True)
+class Delivered:
+    name: str
+    size_bytes: int
+    where: str
+
+
+async def deliver(
+    payload: dict, config: BackupConfig, secret: str | None, at: datetime,
+    *, app_version: str, transport=None,
+) -> Delivered:
+    """Write `payload` as the dated backup file and put it at the destination.
+
+    The file is written off the event loop: serialising a full export is
+    seconds of CPU on a large corpus, and the app keeps serving meanwhile.
+    """
+    problems = config.problems(has_secret=bool(secret))
+    if problems:
+        raise DestinationError(" ".join(problems))
+
+    if config.destination == "folder":
+        folder = Path(config.folder_path)
+        if not folder.is_dir():
+            raise DestinationError(
+                f"{folder} isn't there. Mount the backup share at that path, "
+                "or choose the folder it is mounted at."
+            )
+        try:
+            path = await asyncio.to_thread(
+                backup_file.write_archive, payload, folder, at, app_version=app_version,
+            )
+        except PermissionError:
+            raise DestinationError(f"Scribe isn't allowed to write in {folder}.") from None
+        return Delivered(path.name, path.stat().st_size, str(folder))
+
+    target = s3_target(config, secret or "")
+    with tempfile.TemporaryDirectory(prefix="scribe-backup-") as staging:
+        path = await asyncio.to_thread(
+            backup_file.write_archive, payload, Path(staging), at, app_version=app_version,
+        )
+        size = path.stat().st_size
+        try:
+            await put_file(target, path.name, path, transport=transport)
+        except S3Error as exc:
+            raise DestinationError(str(exc)) from None
+    return Delivered(path.name, size, target.url())
+
+
+async def check_destination(
+    config: BackupConfig, secret: str | None, *, transport=None,
+) -> str:
+    """Ask the destination whether it would take a backup, without sending
+    one. Returns what was found, in words; raises DestinationError otherwise.
+
+    The folder check writes and removes a dot-file, which a folder watcher
+    ignores. The S3 check only lists: an append-only key could add a test
+    object and then never remove it.
+    """
+    problems = config.problems(has_secret=bool(secret))
+    if problems:
+        raise DestinationError(" ".join(problems))
+
+    if config.destination == "folder":
+        folder = Path(config.folder_path)
+        if not folder.is_dir():
+            raise DestinationError(f"{folder} isn't there, or isn't a folder.")
+        probe = folder / ".scribe-write-check"
+        try:
+            await asyncio.to_thread(probe.write_bytes, b"")
+            await asyncio.to_thread(probe.unlink)
+        except PermissionError:
+            raise DestinationError(f"Scribe isn't allowed to write in {folder}.") from None
+        except OSError as exc:
+            raise DestinationError(f"Writing in {folder} failed: {exc.strerror or exc}.") from None
+        count = await asyncio.to_thread(_count_backups, folder)
+        return f"{folder} is writable and holds {count} Scribe backup(s)."
+
+    target = s3_target(config, secret or "")
+    try:
+        objects = await list_objects(target, backup_file.NAME_PREFIX, transport=transport)
+    except S3Error as exc:
+        raise DestinationError(str(exc)) from None
+    return f"Bucket {config.s3_bucket} answered and holds {len(objects)} Scribe backup(s)."
+
+
+def _count_backups(folder: Path) -> int:
+    return sum(
+        1 for name in os.listdir(folder)
+        if name.startswith(backup_file.NAME_PREFIX) and name.endswith(backup_file.NAME_SUFFIX)
+    )
diff --git a/src/scribe/services/backup_s3.py b/src/scribe/services/backup_s3.py
new file mode 100644
index 00000000..2531aea7
--- /dev/null
+++ b/src/scribe/services/backup_s3.py
@@ -0,0 +1,199 @@
+"""A small S3 client for sending backups: AWS Signature Version 4 over httpx,
+path-style addressing (endpoint/bucket/key), and the two calls a backup needs.
+
+Ported from FabledSteward's `steward/backup/s3.py`, which is tested against
+the same AWS examples as tests/test_services_backup_s3.py.
+
+Scribe puts and lists; it never deletes. The bucket keeps the history and
+thins it on its own policy, so a key that may only add objects is enough —
+and is the safer key to hand a service, because a stolen one cannot wipe the
+history it writes into.
+
+Written by hand rather than with boto3: two calls don't justify a large
+dependency, and signing is a pure function tests can hold to AWS's own
+published examples.
+"""
+from __future__ import annotations
+
+import asyncio
+import hashlib
+import hmac
+import xml.etree.ElementTree as ET
+from dataclasses import dataclass
+from datetime import datetime, timezone
+from pathlib import Path
+from urllib.parse import quote
+
+import httpx
+
+EMPTY_SHA256 = hashlib.sha256(b"").hexdigest()
+TIMEOUT_SECONDS = 30.0
+# A backup is tens of megabytes; a slow link still gets this long to send one.
+UPLOAD_TIMEOUT_SECONDS = 15 * 60
+CHUNK = 1 << 20
+
+
+class S3Error(Exception):
+    """Why a call to the bucket failed, said to the person."""
+
+
+@dataclass(frozen=True)
+class S3Target:
+    endpoint: str          # http(s)://host[:port][/prefix], no bucket
+    bucket: str
+    access_key: str
+    secret_key: str
+    region: str = "us-east-1"
+    verify_tls: bool = True
+
+    def url(self, key: str = "") -> str:
+        base = f"{self.endpoint.rstrip('/')}/{self.bucket}"
+        return f"{base}/{key}" if key else f"{base}/"
+
+
+@dataclass(frozen=True)
+class StoredObject:
+    key: str
+    size: int
+    last_modified: str
+
+
+# ── Signing ──────────────────────────────────────────────────────────────────
+
+def _encode(text: str, keep_slash: bool = False) -> str:
+    return quote(text, safe="-_.~" + ("/" if keep_slash else ""))
+
+
+def _hmac(key: bytes, msg: str) -> bytes:
+    return hmac.new(key, msg.encode(), hashlib.sha256).digest()
+
+
+def sign(method: str, url: str, headers: dict[str, str], payload_sha256: str,
+         access_key: str, secret_key: str, region: str, at: datetime,
+         service: str = "s3") -> dict[str, str]:
+    """The request's headers with x-amz-date, x-amz-content-sha256 and
+    Authorization added. Every header given is signed, Host always."""
+    u = httpx.URL(url)
+    stamp = at.astimezone(timezone.utc).strftime("%Y%m%dT%H%M%SZ")
+    day = stamp[:8]
+    out = dict(headers)
+    out["x-amz-date"] = stamp
+    out["x-amz-content-sha256"] = payload_sha256
+    signed = {k.lower(): " ".join(str(v).split()) for k, v in out.items()}
+    signed["host"] = u.netloc.decode()
+    names = sorted(signed)
+
+    query = sorted((_encode(k), _encode(v)) for k, v in u.params.multi_items())
+    canonical = "\n".join([
+        method.upper(),
+        _encode(u.path or "/", keep_slash=True),
+        "&".join(f"{k}={v}" for k, v in query),
+        "".join(f"{n}:{signed[n]}\n" for n in names),
+        ";".join(names),
+        payload_sha256,
+    ])
+    scope = f"{day}/{region}/{service}/aws4_request"
+    to_sign = "\n".join(["AWS4-HMAC-SHA256", stamp, scope,
+                         hashlib.sha256(canonical.encode()).hexdigest()])
+    key = _hmac(_hmac(_hmac(_hmac(f"AWS4{secret_key}".encode(), day), region),
+                      service), "aws4_request")
+    signature = hmac.new(key, to_sign.encode(), hashlib.sha256).hexdigest()
+    out["Authorization"] = (f"AWS4-HMAC-SHA256 Credential={access_key}/{scope}, "
+                            f"SignedHeaders={';'.join(names)}, Signature={signature}")
+    return out
+
+
+# ── Calls ────────────────────────────────────────────────────────────────────
+
+def _says(response: httpx.Response) -> str:
+    """An S3 error response in words: its Code and Message when it has them."""
+    try:
+        root = ET.fromstring(response.content)
+        code = root.findtext("Code") or ""
+        message = root.findtext("Message") or ""
+    except ET.ParseError:
+        code, message = "", ""
+    hints = {
+        "SignatureDoesNotMatch": "the secret key is wrong",
+        "InvalidAccessKeyId": "the access key isn't known there",
+        "NoSuchBucket": "there is no such bucket",
+        "AccessDenied": "the key isn't allowed to do that",
+    }
+    said = hints.get(code) or message or response.reason_phrase
+    return f"HTTP {response.status_code}{f' {code}' if code else ''}: {said}."
+
+
+def _reach_error(target: S3Target, exc: httpx.HTTPError) -> S3Error:
+    if isinstance(exc, httpx.TimeoutException):
+        return S3Error(f"{target.endpoint} didn't answer in time.")
+    return S3Error(f"Couldn't reach {target.endpoint}: {exc}.")
+
+
+def file_sha256(path: Path) -> str:
+    h = hashlib.sha256()
+    with open(path, "rb") as f:
+        while chunk := f.read(CHUNK):
+            h.update(chunk)
+    return h.hexdigest()
+
+
+async def put_file(target: S3Target, key: str, path: Path, *,
+                   transport: httpx.AsyncBaseTransport | None = None,
+                   now: datetime | None = None) -> None:
+    """Upload one file as one object, streamed from disk."""
+    digest = await asyncio.to_thread(file_sha256, path)
+    size = path.stat().st_size
+    url = target.url(key)
+    headers = sign("PUT", url, {"Content-Length": str(size),
+                                "Content-Type": "application/x-tar"},
+                   digest, target.access_key, target.secret_key, target.region,
+                   now or datetime.now(timezone.utc))
+
+    async def body():
+        with open(path, "rb") as f:
+            while chunk := f.read(CHUNK):
+                yield chunk
+
+    try:
+        async with httpx.AsyncClient(verify=target.verify_tls,
+                                     timeout=UPLOAD_TIMEOUT_SECONDS,
+                                     transport=transport) as client:
+            response = await client.put(url, headers=headers, content=body())
+    except httpx.HTTPError as exc:
+        raise _reach_error(target, exc) from None
+    if response.status_code // 100 != 2:
+        raise S3Error(f"Upload of {key} refused: {_says(response)}")
+
+
+async def list_objects(target: S3Target, prefix: str = "", limit: int = 1000, *,
+                       transport: httpx.AsyncBaseTransport | None = None,
+                       now: datetime | None = None) -> list[StoredObject]:
+    """Up to `limit` objects under `prefix` (ListObjectsV2, one page)."""
+    params = {"list-type": "2", "max-keys": str(limit)}
+    if prefix:
+        params["prefix"] = prefix
+    url = str(httpx.URL(target.url(), params=params))
+    headers = sign("GET", url, {}, EMPTY_SHA256, target.access_key, target.secret_key,
+                   target.region, now or datetime.now(timezone.utc))
+    try:
+        async with httpx.AsyncClient(verify=target.verify_tls, timeout=TIMEOUT_SECONDS,
+                                     transport=transport) as client:
+            response = await client.get(url, headers=headers)
+    except httpx.HTTPError as exc:
+        raise _reach_error(target, exc) from None
+    if response.status_code != 200:
+        raise S3Error(f"Listing {target.bucket} refused: {_says(response)}")
+    try:
+        root = ET.fromstring(response.content)
+    except ET.ParseError:
+        raise S3Error(
+            f"{target.endpoint} answered a listing with something that isn't S3's XML."
+        ) from None
+    out = []
+    for item in root.iter():
+        if item.tag.rsplit("}", 1)[-1] != "Contents":
+            continue
+        fields = {child.tag.rsplit("}", 1)[-1]: (child.text or "") for child in item}
+        out.append(StoredObject(fields.get("Key", ""), int(fields.get("Size") or 0),
+                                fields.get("LastModified", "")))
+    return out
diff --git a/src/scribe/services/secrets.py b/src/scribe/services/secrets.py
new file mode 100644
index 00000000..91ced9ee
--- /dev/null
+++ b/src/scribe/services/secrets.py
@@ -0,0 +1,44 @@
+"""Sealing a credential Scribe stores so the database alone cannot read it.
+
+Fernet (AES-128-CBC with an HMAC) under a key derived from the instance's
+SECRET_KEY. A copy of the database, or of a backup made from it, then holds
+only ciphertext: reading the credential also needs the SECRET_KEY, which lives
+in the deployment's environment rather than in the data.
+
+The derivation is labelled and versioned, so this key is never the same bytes
+as anything else made from SECRET_KEY (the session-cookie signing uses it
+raw). An instance that changes its SECRET_KEY can no longer open what it
+sealed before: `unseal` answers None, and the caller asks for the credential
+again rather than failing on it.
+"""
+from __future__ import annotations
+
+import base64
+import hashlib
+
+from cryptography.fernet import Fernet, InvalidToken
+
+from scribe.config import Config
+
+_LABEL = b"scribe.secrets.v1\x00"
+
+
+def _fernet() -> Fernet:
+    digest = hashlib.sha256(_LABEL + Config.SECRET_KEY.encode()).digest()
+    return Fernet(base64.urlsafe_b64encode(digest))
+
+
+def seal(plain: str) -> str:
+    """Encrypt `plain` for storage; the result is ASCII."""
+    return _fernet().encrypt(plain.encode()).decode()
+
+
+def unseal(token: str) -> str | None:
+    """Decrypt a value `seal` produced, or None when it can't be opened —
+    sealed under a different SECRET_KEY, or not a sealed value at all."""
+    if not token:
+        return None
+    try:
+        return _fernet().decrypt(token.encode()).decode()
+    except (InvalidToken, ValueError):
+        return None
diff --git a/tests/test_services_backup_destination.py b/tests/test_services_backup_destination.py
new file mode 100644
index 00000000..9bc4da5f
--- /dev/null
+++ b/tests/test_services_backup_destination.py
@@ -0,0 +1,179 @@
+"""The backup destination (services/backup_destination.py) and the sealing it
+keeps the S3 secret under (services/secrets.py).
+
+No database: the admin settings are patched, the folder is a tmp_path, and the
+bucket is an httpx.MockTransport. What these pin is the contract a person
+meets in Settings — a config that is incomplete says what is missing, a place
+that can't take a backup says why, and the secret never leaves sealed.
+"""
+import asyncio
+import io
+from datetime import datetime, timezone
+from types import SimpleNamespace
+from unittest.mock import patch
+
+import httpx
+import pytest
+
+from scribe.services import backup, backup_destination, secrets
+from scribe.services.backup_destination import (
+    BackupConfig, DestinationError, check_destination, deliver,
+)
+from scribe.services.backup_file import read_archive
+
+AT = datetime(2026, 10, 15, 2, 0, 0, tzinfo=timezone.utc)
+PAYLOAD = {
+    "version": 25, "scope": "full", "exported_at": AT.isoformat(),
+    "users": [{"id": 1, "username": "ada"}], "notes": [],
+}
+S3 = BackupConfig(
+    destination="s3", s3_endpoint="http://archives:8081", s3_bucket="scribe",
+    s3_access_key="AKIA", s3_region="us-east-1",
+)
+
+
+# ── Sealing ──────────────────────────────────────────────────────────────────
+
+def test_a_sealed_secret_opens_and_is_not_the_plain_text():
+    token = secrets.seal("wJalrXUtnFEMI/K7MDENG")
+    assert "wJalrXUtnFEMI" not in token
+    assert secrets.unseal(token) == "wJalrXUtnFEMI/K7MDENG"
+
+
+def test_another_secret_key_cannot_open_it():
+    token = secrets.seal("s3cret")
+    with patch.object(secrets.Config, "SECRET_KEY", "a-different-instance"):
+        assert secrets.unseal(token) is None
+
+
+def test_unseal_of_nothing_or_garbage_is_none():
+    assert secrets.unseal("") is None
+    assert secrets.unseal("not a fernet token") is None
+
+
+def test_the_secret_setting_never_reaches_a_backup():
+    rows = [SimpleNamespace(user_id=1, key=k, value="v") for k in (
+        "backup_config", backup_destination.SECRET_SETTING, "theme",
+    )]
+    keys = [r["key"] for r in backup._setting_rows(rows)]
+    assert keys == ["backup_config", "theme"]
+
+
+def test_load_secret_tells_a_missing_secret_from_one_it_cannot_open():
+    async def stored(value):
+        async def get(key, default=""):
+            return value
+        with patch.object(backup_destination, "get_admin_setting", get):
+            return await backup_destination.load_secret()
+
+    assert asyncio.run(stored("")) == (None, False)
+    assert asyncio.run(stored(secrets.seal("k"))) == ("k", True)
+    assert asyncio.run(stored("sealed-elsewhere")) == (None, True)
+
+
+# ── Config ───────────────────────────────────────────────────────────────────
+
+def test_config_round_trips_and_defaults_to_off():
+    assert BackupConfig().enabled is False
+    assert BackupConfig.from_dict(S3.to_dict()) == S3
+
+
+def test_a_damaged_config_still_loads():
+    config = BackupConfig.from_dict({
+        "hour": "25", "destination": "ftp", "enabled": "yes", "unknown": 1,
+        "s3_bucket": "  scribe  ",
+    })
+    assert config.hour == backup_destination.DEFAULT_HOUR
+    assert config.destination == "folder"
+    assert config.enabled is False
+    assert config.s3_bucket == "scribe"
+
+
+def test_an_incomplete_s3_config_names_what_is_missing():
+    said = BackupConfig(destination="s3").problems(has_secret=False)
+    assert said == [
+        "The S3 address must start with http:// or https://.",
+        "The bucket name is missing.",
+        "The access key is missing.",
+        "The secret key is missing.",
+    ]
+    assert S3.problems(has_secret=True) == []
+
+
+def test_a_relative_folder_is_refused():
+    assert BackupConfig(folder_path="backups").problems(has_secret=False)
+
+
+# ── Folder ───────────────────────────────────────────────────────────────────
+
+def test_deliver_to_a_folder_adds_one_dated_file(tmp_path):
+    config = BackupConfig(folder_path=str(tmp_path))
+    got = asyncio.run(deliver(PAYLOAD, config, None, AT, app_version="v"))
+    assert got.name == "scribe-20261015T020000Z.tar"
+    assert [p.name for p in tmp_path.iterdir()] == [got.name]
+    assert got.size_bytes == (tmp_path / got.name).stat().st_size
+    with open(tmp_path / got.name, "rb") as f:
+        assert read_archive(f) == PAYLOAD
+
+
+def test_a_folder_that_isnt_mounted_says_so(tmp_path):
+    config = BackupConfig(folder_path=str(tmp_path / "not-mounted"))
+    with pytest.raises(DestinationError, match="isn't there"):
+        asyncio.run(deliver(PAYLOAD, config, None, AT, app_version="v"))
+
+
+def test_checking_a_folder_leaves_nothing_behind(tmp_path):
+    (tmp_path / "scribe-20261014T020000Z.tar").write_bytes(b"x")
+    config = BackupConfig(folder_path=str(tmp_path))
+    said = asyncio.run(check_destination(config, None))
+    assert "holds 1 Scribe backup" in said
+    assert [p.name for p in tmp_path.iterdir()] == ["scribe-20261014T020000Z.tar"]
+
+
+# ── S3 ───────────────────────────────────────────────────────────────────────
+
+def test_deliver_to_s3_puts_the_dated_file_and_keeps_no_copy():
+    seen = {}
+
+    def handler(request: httpx.Request):
+        seen["method"], seen["url"] = request.method, str(request.url)
+        seen["body"] = request.read()
+        return httpx.Response(200)
+
+    got = asyncio.run(deliver(PAYLOAD, S3, "secret", AT, app_version="v",
+                              transport=httpx.MockTransport(handler)))
+    assert seen["method"] == "PUT"
+    assert seen["url"] == "http://archives:8081/scribe/scribe-20261015T020000Z.tar"
+    assert read_archive(io.BytesIO(seen["body"])) == PAYLOAD
+    assert got.size_bytes == len(seen["body"])
+
+
+def test_a_refused_upload_is_a_destination_error_in_words():
+    body = b"<Error><Code>AccessDenied</Code><Message>no</Message></Error>"
+    transport = httpx.MockTransport(lambda r: httpx.Response(403, content=body))
+    with pytest.raises(DestinationError, match="the key isn't allowed to do that"):
+        asyncio.run(deliver(PAYLOAD, S3, "secret", AT, app_version="v", transport=transport))
+
+
+def test_s3_without_its_secret_sends_nothing():
+    def handler(request):
+        raise AssertionError("nothing should be sent")
+
+    with pytest.raises(DestinationError, match="secret key is missing"):
+        asyncio.run(deliver(PAYLOAD, S3, None, AT, app_version="v",
+                            transport=httpx.MockTransport(handler)))
+
+
+def test_checking_s3_only_lists():
+    xml = (b'<ListBucketResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">'
+           b"<Contents><Key>scribe-20261014T020000Z.tar</Key><Size>9</Size>"
+           b"<LastModified>x</LastModified></Contents></ListBucketResult>")
+    methods = []
+
+    def handler(request):
+        methods.append(request.method)
+        return httpx.Response(200, content=xml)
+
+    said = asyncio.run(check_destination(S3, "secret", transport=httpx.MockTransport(handler)))
+    assert methods == ["GET"]
+    assert "holds 1 Scribe backup" in said
diff --git a/tests/test_services_backup_s3.py b/tests/test_services_backup_s3.py
new file mode 100644
index 00000000..c9ab0855
--- /dev/null
+++ b/tests/test_services_backup_s3.py
@@ -0,0 +1,112 @@
+"""The backup destination's S3 client (services/backup_s3.py): signatures match AWS's
+published Signature Version 4 examples, and the calls read S3's answers."""
+import asyncio
+from datetime import datetime, timezone
+
+import httpx
+import pytest
+
+from scribe.services import backup_s3 as s3
+
+# AWS's example credentials and moment ("Signature Calculations for the
+# Authorization Header", Amazon S3 API reference).
+KEY_ID = "AKIAIOSFODNN7EXAMPLE"
+SECRET = "wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY"
+AT = datetime(2013, 5, 24, tzinfo=timezone.utc)
+
+
+def _signature(headers):
+    return headers["Authorization"].rsplit("Signature=", 1)[1]
+
+
+def test_aws_example_get_object():
+    h = s3.sign("GET", "https://examplebucket.s3.amazonaws.com/test.txt", {"Range": "bytes=0-9"},
+                s3.EMPTY_SHA256, KEY_ID, SECRET, "us-east-1", AT)
+    assert "SignedHeaders=host;range;x-amz-content-sha256;x-amz-date," in h["Authorization"]
+    assert "Credential=AKIAIOSFODNN7EXAMPLE/20130524/us-east-1/s3/aws4_request" in h["Authorization"]
+    assert _signature(h) == "f0e8bdb87c964420e857bd35b5d6ed310bd44f0170aba48dd91039c6036bdb41"
+
+
+def test_aws_example_list_objects():
+    h = s3.sign("GET", "https://examplebucket.s3.amazonaws.com/?max-keys=2&prefix=J", {},
+                s3.EMPTY_SHA256, KEY_ID, SECRET, "us-east-1", AT)
+    assert _signature(h) == "34b48302e7b5fa45bde8084f4b7868a86f0a534bc59db6670ed5711ef69dc6f7"
+
+
+def test_a_changed_secret_changes_the_signature():
+    a = s3.sign("GET", "https://examplebucket.s3.amazonaws.com/test.txt", {},
+                s3.EMPTY_SHA256, KEY_ID, SECRET, "us-east-1", AT)
+    b = s3.sign("GET", "https://examplebucket.s3.amazonaws.com/test.txt", {},
+                s3.EMPTY_SHA256, KEY_ID, SECRET + "x", "us-east-1", AT)
+    assert _signature(a) != _signature(b)
+
+
+TARGET = s3.S3Target("http://archives:8081/s3", "scribe", KEY_ID, SECRET)
+
+
+def test_paths_are_path_style():
+    assert TARGET.url("scribe-20261012T000000Z.tar") == "http://archives:8081/s3/scribe/scribe-20261012T000000Z.tar"
+    assert TARGET.url() == "http://archives:8081/s3/scribe/"
+
+
+def test_put_sends_the_file_signed(tmp_path):
+    f = tmp_path / "b.tar"
+    f.write_bytes(b"x" * 5000)
+    seen = {}
+
+    def handler(request: httpx.Request):
+        seen["method"], seen["url"] = request.method, str(request.url)
+        seen["body"] = request.read()
+        seen["auth"] = request.headers["authorization"]
+        seen["sha"] = request.headers["x-amz-content-sha256"]
+        return httpx.Response(200)
+
+    asyncio.run(s3.put_file(TARGET, "b.tar", f, transport=httpx.MockTransport(handler)))
+    assert seen["method"] == "PUT"
+    assert seen["url"] == "http://archives:8081/s3/scribe/b.tar"
+    assert seen["body"] == b"x" * 5000
+    assert seen["sha"] == s3.file_sha256(f)
+    assert seen["auth"].startswith("AWS4-HMAC-SHA256 Credential=AKIAIOSFODNN7EXAMPLE/")
+
+
+def test_a_refusal_says_why(tmp_path):
+    f = tmp_path / "b.tar"
+    f.write_bytes(b"x")
+    body = b"<Error><Code>SignatureDoesNotMatch</Code><Message>no</Message></Error>"
+    transport = httpx.MockTransport(lambda r: httpx.Response(403, content=body))
+    with pytest.raises(s3.S3Error, match="HTTP 403 SignatureDoesNotMatch: the secret key is wrong"):
+        asyncio.run(s3.put_file(TARGET, "b.tar", f, transport=transport))
+
+
+def test_unreachable_says_so(tmp_path):
+    f = tmp_path / "b.tar"
+    f.write_bytes(b"x")
+
+    def handler(request):
+        raise httpx.ConnectError("refused", request=request)
+
+    with pytest.raises(s3.S3Error, match="Couldn't reach http://archives:8081/s3"):
+        asyncio.run(s3.put_file(TARGET, "b.tar", f, transport=httpx.MockTransport(handler)))
+
+
+def test_list_reads_contents():
+    xml = b"""<?xml version="1.0" encoding="UTF-8"?>
+<ListBucketResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
+  <Name>scribe</Name><KeyCount>2</KeyCount>
+  <Contents><Key>scribe-20261011T000000Z.tar</Key><Size>1200</Size>
+    <LastModified>2026-10-11T00:00:01.000Z</LastModified></Contents>
+  <Contents><Key>scribe-20261012T000000Z.tar</Key><Size>1300</Size>
+    <LastModified>2026-10-12T00:00:01.000Z</LastModified></Contents>
+</ListBucketResult>"""
+    seen = {}
+
+    def handler(request):
+        seen["params"] = dict(request.url.params)
+        return httpx.Response(200, content=xml)
+
+    got = asyncio.run(s3.list_objects(TARGET, "scribe-", transport=httpx.MockTransport(handler)))
+    assert seen["params"] == {"list-type": "2", "max-keys": "1000", "prefix": "scribe-"}
+    assert got == [
+        s3.StoredObject("scribe-20261011T000000Z.tar", 1200, "2026-10-11T00:00:01.000Z"),
+        s3.StoredObject("scribe-20261012T000000Z.tar", 1300, "2026-10-12T00:00:01.000Z"),
+    ]
diff --git a/uv.lock b/uv.lock
index a28edb65..d9a168b4 100644
--- a/uv.lock
+++ b/uv.lock
@@ -1508,6 +1508,7 @@ dependencies = [
     { name = "asyncpg" },
     { name = "bcrypt" },
     { name = "caldav" },
+    { name = "cryptography" },
     { name = "fastembed" },
     { name = "httpx" },
     { name = "hypercorn" },
@@ -1533,6 +1534,7 @@ requires-dist = [
     { name = "asyncpg", specifier = ">=0.29" },
     { name = "bcrypt", specifier = ">=4.0" },
     { name = "caldav", specifier = ">=1.3" },
+    { name = "cryptography" },
     { name = "fastembed", specifier = ">=0.4" },
     { name = "httpx", specifier = ">=0.27" },
     { name = "hypercorn", specifier = ">=0.17" },