diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index a1e5eab..c28cace 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -44,3 +44,32 @@ jobs:
           aws dynamodb get-item --table-name action-test --key '{"id":{"S":"1"}}'
           aws lambda list-functions
           homecloud version
+
+  # Download and verification only (HC_INSTALL_ONLY stops before the server
+  # starts): a signed release, an unsigned one, and the check turned off.
+  install:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - { version: latest, verify: "true", expect: "signature OK" }
+          - { version: v0.5.0, verify: "true", expect: "signature OK" }
+          - { version: v0.4.1, verify: "true", expect: "is not signed" }
+          - { version: v0.5.0, verify: "false", expect: "verify-signature is false" }
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+      - name: Install ${{ matrix.version }} (verify-signature ${{ matrix.verify }})
+        env:
+          HC_INSTALL_ONLY: "1"
+          HC_VERSION: ${{ matrix.version }}
+          HC_VERIFY_SIGNATURE: ${{ matrix.verify }}
+          EXPECT: ${{ matrix.expect }}
+        run: |
+          set -o pipefail
+          bash setup.sh | tee install.log
+          grep -q "$EXPECT" install.log
+          grep -q "checksum OK" install.log
diff --git a/README.md b/README.md
index 9fa84d5..63b589b 100644
--- a/README.md
+++ b/README.md
@@ -9,7 +9,7 @@ the rest of the AWS API without an AWS account.
 
 The action:
 
-1. downloads the HomeCloud release you ask for and verifies it against the release's `checksums.txt`,
+1. downloads the HomeCloud release you ask for, verifies the Sigstore signature on its `checksums.txt` (releases from v0.5.0 on are signed) and checks the download against it,
 2. starts `homecloud serve` in the background (data in `$RUNNER_TEMP/homecloud-data`, log in `$RUNNER_TEMP/homecloud.log`),
 3. waits for `/api/v1/health` and for any services listed in `services-wait`,
 4. exports `AWS_ENDPOINT_URL`, `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY` (masked), `AWS_REGION`, `AWS_DEFAULT_REGION` and `HOMECLOUD_DATA_DIR` for the following steps, and puts `homecloud` on `PATH`,
@@ -40,6 +40,7 @@ jobs:
 | `port` | `8080` | Port the API listens on, on `127.0.0.1`. |
 | `services-wait` | (none) | Comma or newline separated services to wait for. `s3`, `ecr` and `route53` start containers in the background; in-process services (`sqs`, `dynamodb`, `lambda`, ...) are ready with the API; an unknown name fails the step. |
 | `wait-timeout` | `180` | Seconds to wait for the API and the services. |
+| `verify-signature` | `true` | Verify the release signature with cosign before trusting `checksums.txt` (see [Release verification](#release-verification)). `false` checks the SHA-256 checksum only. |
 
 ### Outputs
 
@@ -97,7 +98,8 @@ def test_upload():
 
 ### Terraform
 
-The AWS provider (5.x and later) honours `AWS_ENDPOINT_URL`; tell it not to look for a real account:
+The AWS provider (5.x and later) honours `AWS_ENDPOINT_URL`. HomeCloud implements STS, so the
+provider's account checks pass unchanged; only path-style S3 addressing needs setting:
 
 ```yaml
       - uses: solinode/setup-homecloud@v1
@@ -109,9 +111,7 @@ The AWS provider (5.x and later) honours `AWS_ENDPOINT_URL`; tell it not to look
 
 ```hcl
 provider "aws" {
-  skip_credentials_validation = true
-  skip_requesting_account_id  = true
-  s3_use_path_style           = true
+  s3_use_path_style = true
 }
 ```
 
@@ -123,6 +123,30 @@ provider "aws" {
       - run: ./run-tests.sh --endpoint "${{ steps.homecloud.outputs.endpoint }}"
 ```
 
+## Release verification
+
+HomeCloud releases from v0.5.0 on publish `checksums.txt.sigstore.json`, a Sigstore bundle
+over `checksums.txt` made by keyless signing in HomeCloud's release workflow. When the bundle
+exists, the action runs
+
+```sh
+cosign verify-blob checksums.txt --bundle checksums.txt.sigstore.json \
+  --certificate-identity https://github.com/solinode/homecloud/.github/workflows/release.yml@refs/tags/<tag> \
+  --certificate-oidc-issuer https://token.actions.githubusercontent.com
+```
+
+and fails the step if the signature does not verify, or if a release from v0.5.0 on has no
+bundle. Only then does it trust the SHA-256 in `checksums.txt` for the archive. `latest` is
+resolved to its tag first, so the signer identity names the exact release.
+
+- cosign comes from the runner's `PATH` if it is there (for example installed with
+  `sigstore/cosign-installer`; cosign 2.4 or newer), otherwise the action downloads cosign
+  v3.1.3 from its GitHub release and checks it against a SHA-256 pinned in `setup.sh`.
+- Verification fetches Sigstore's trust root over the network. On a runner that cannot
+  reach Sigstore, set `verify-signature: false` to check the checksum only.
+- Releases before v0.5.0 are unsigned: they are checked against `checksums.txt` only, as
+  before.
+
 ## Notes
 
 - One HomeCloud per Docker host: it names its helper containers (`homecloud-s3`, ...) and
diff --git a/action.yml b/action.yml
index e227f48..999fd5f 100644
--- a/action.yml
+++ b/action.yml
@@ -25,6 +25,13 @@ inputs:
     description: Seconds to wait for the API and every service in services-wait.
     required: false
     default: "180"
+  verify-signature:
+    description: >-
+      Verify the release's Sigstore signature (checksums.txt.sigstore.json, published from
+      v0.5.0 on) with cosign before trusting checksums.txt: "true" or "false". Releases
+      before v0.5.0 are unsigned and get the checksum check only.
+    required: false
+    default: "true"
 
 outputs:
   endpoint:
diff --git a/main.js b/main.js
index 9fa7713..85f1246 100644
--- a/main.js
+++ b/main.js
@@ -10,6 +10,7 @@ const env = {
   HC_PORT: input("port", "8080"),
   HC_SERVICES_WAIT: input("services-wait", ""),
   HC_WAIT_TIMEOUT: input("wait-timeout", "180"),
+  HC_VERIFY_SIGNATURE: input("verify-signature", "true"),
 };
 const r = spawnSync("bash", [path.join(__dirname, "setup.sh")], { stdio: "inherit", env });
 if (r.error) {
diff --git a/setup.sh b/setup.sh
index 1bd79fe..85387c6 100755
--- a/setup.sh
+++ b/setup.sh
@@ -25,6 +25,11 @@ port=$((10#$port))
 case "$timeout" in '' | *[!0-9]*) fail "wait-timeout must be a whole number of seconds, got '$timeout'" ;; esac
 timeout=$((10#$timeout))
 [ "$timeout" -ge 1 ] || fail "wait-timeout must be at least 1 second"
+case "$(echo "${HC_VERIFY_SIGNATURE:-true}" | tr '[:upper:]' '[:lower:]')" in
+  true) verify_sig=1 ;;
+  false) verify_sig= ;;
+  *) fail "verify-signature must be true or false, got '$HC_VERIFY_SIGNATURE'" ;;
+esac
 
 # Services that start containers in the background log a line when they are ready;
 # the others run inside the HomeCloud process and are ready with the API.
@@ -36,7 +41,7 @@ for svc in $(echo "${HC_SERVICES_WAIT:-}" | tr ',\n' '  ' | tr '[:upper:]' '[:lo
     *) case "$in_process" in *" $svc "*) ;; *) fail "services-wait: unknown service '$svc' (background: s3, ecr, route53; in-process:$in_process)" ;; esac ;;
   esac
 done
-[ "${HC_VALIDATE_ONLY:-}" = 1 ] && { echo "inputs OK: port=$port wait-timeout=$timeout waits=${waits[*]:-}"; exit 0; }
+[ "${HC_VALIDATE_ONLY:-}" = 1 ] && { echo "inputs OK: port=$port wait-timeout=$timeout waits=${waits[*]:-} verify-signature=${verify_sig:-0}"; exit 0; }
 
 command -v docker >/dev/null 2>&1 || fail "HomeCloud needs Docker on the runner (ubuntu-* runners have it)"
 docker info >/dev/null 2>&1 || fail "Docker is installed but not running"
@@ -49,24 +54,91 @@ case "$(uname -m)" in
 esac
 name="homecloud-$os-$arch"
 if [ "$version" = latest ]; then
-  base="https://github.com/$REPO/releases/latest/download"
-else
-  case "$version" in v*) ;; *) version="v$version" ;; esac
-  base="https://github.com/$REPO/releases/download/$version"
+  # Resolve "latest" to its tag: the signature names the tag it was made for, and
+  # every file then comes from the same release.
+  version=$(curl -fsSL --retry 3 -o /dev/null -w '%{url_effective}' "https://github.com/$REPO/releases/latest" | sed -n 's#.*/releases/tag/##p') || true
+  [ -n "$version" ] || fail "could not determine the latest HomeCloud release"
 fi
+case "$version" in v*) ;; *) version="v$version" ;; esac
+base="https://github.com/$REPO/releases/download/$version"
+
+sha256() {
+  if command -v sha256sum >/dev/null 2>&1; then
+    sha256sum "$1" | cut -d' ' -f1
+  else
+    shasum -a 256 "$1" | cut -d' ' -f1
+  fi
+}
+
+# signed_release VERSION: true for v0.5.0 and later, which are all signed.
+signed_release() {
+  local v=${1#v} major rest minor
+  v=${v%%-*}
+  major=${v%%.*}
+  rest=${v#*.}
+  minor=${rest%%.*}
+  case "$major$minor" in *[!0-9]* | "") return 1 ;; esac
+  [ "$major" -gt 0 ] || [ "$minor" -ge 5 ]
+}
+
+# cosign for the signature check: the runner's own (for example from
+# sigstore/cosign-installer), else this pinned release, checked against its SHA-256.
+COSIGN_VERSION=v3.1.3
+cosign_sha256() {
+  case "$1" in
+    linux-amd64) echo 4629c757b7618056f8ddd7e2625ae9fdd94c0372a65049520bc7d9df9efc7f71 ;;
+    linux-arm64) echo c5d324e091826b0d7a78eb16fef316450b4eb9aaec045611c08ba06f5e73220a ;;
+    darwin-amd64) echo 2347488e5d5b25336644024dfeca5601b190e91197a71a917bda44744aff106c ;;
+    darwin-arm64) echo 5cf948c2f4dfe59687bdd0b8523709067383e03982cc543475c8a7dc70e92a76 ;;
+  esac
+}
+find_cosign() {
+  if cosign=$(command -v cosign); then
+    return
+  fi
+  local dir="$tmp/homecloud-cosign" want got
+  want=$(cosign_sha256 "$os-$arch")
+  [ -n "$want" ] || fail "no pinned cosign for $os/$arch: install cosign first, or set verify-signature: false"
+  mkdir -p "$dir"
+  curl -fsSL --retry 3 "https://github.com/sigstore/cosign/releases/download/$COSIGN_VERSION/cosign-$os-$arch" -o "$dir/cosign" ||
+    fail "download of cosign $COSIGN_VERSION failed"
+  got=$(sha256 "$dir/cosign")
+  [ "$got" = "$want" ] || fail "checksum mismatch for cosign $COSIGN_VERSION (expected $want, got $got)"
+  chmod +x "$dir/cosign"
+  cosign="$dir/cosign"
+}
 
 echo "::group::Install HomeCloud ($version, $os/$arch)"
 dl="$tmp/homecloud-dl"
 rm -rf "$dl" && mkdir -p "$dl" "$bin_dir"
 curl -fsSL --retry 3 "$base/$name.tar.gz" -o "$dl/$name.tar.gz" || fail "download $base/$name.tar.gz failed"
 curl -fsSL --retry 3 "$base/checksums.txt" -o "$dl/checksums.txt" || fail "download $base/checksums.txt failed"
-expected=$(awk -v f="$name.tar.gz" '$2 == f || $2 == "*" f {print $1}' "$dl/checksums.txt")
-[ -n "$expected" ] || fail "checksums.txt has no entry for $name.tar.gz"
-if command -v sha256sum >/dev/null 2>&1; then
-  actual=$(sha256sum "$dl/$name.tar.gz" | cut -d' ' -f1)
+
+# Releases from v0.5.0 on sign checksums.txt with Sigstore keyless signing in the
+# release workflow (checksums.txt.sigstore.json). Verify it when it exists, so the
+# checksum below is trusted; older releases are unsigned and get the checksum only.
+if [ -z "$verify_sig" ]; then
+  echo "verify-signature is false: checking the SHA-256 checksum only"
+elif curl -fsSL --retry 3 "$base/checksums.txt.sigstore.json" -o "$dl/checksums.txt.sigstore.json" 2>/dev/null; then
+  find_cosign
+  echo "verifying the release signature with $cosign"
+  identity="https://github.com/$REPO/.github/workflows/release.yml@refs/tags/$version"
+  if ! "$cosign" verify-blob "$dl/checksums.txt" --bundle "$dl/checksums.txt.sigstore.json" \
+    --certificate-identity "$identity" \
+    --certificate-oidc-issuer https://token.actions.githubusercontent.com >"$dl/cosign.log" 2>&1; then
+    cat "$dl/cosign.log"
+    fail "signature verification of checksums.txt failed for $version (expected signer $identity)"
+  fi
+  echo "signature OK (checksums.txt signed by $identity)"
+elif signed_release "$version"; then
+  fail "$version should be signed, but $base/checksums.txt.sigstore.json could not be downloaded (set verify-signature: false to skip the check)"
 else
-  actual=$(shasum -a 256 "$dl/$name.tar.gz" | cut -d' ' -f1)
+  echo "$version is not signed (releases before v0.5.0 were not): checking the SHA-256 checksum only"
 fi
+
+expected=$(awk -v f="$name.tar.gz" '$2 == f || $2 == "*" f {print $1}' "$dl/checksums.txt")
+[ -n "$expected" ] || fail "checksums.txt has no entry for $name.tar.gz"
+actual=$(sha256 "$dl/$name.tar.gz")
 [ "$expected" = "$actual" ] || fail "checksum mismatch for $name.tar.gz (expected $expected, got $actual)"
 echo "checksum OK ($actual)"
 tar -xzf "$dl/$name.tar.gz" -C "$dl"
@@ -75,6 +147,8 @@ rm -rf "$dl"
 [ -n "${GITHUB_PATH:-}" ] && echo "$bin_dir" >> "$GITHUB_PATH"
 "$bin_dir/homecloud" version
 echo "::endgroup::"
+# Tests of the download and verification stop here (test.yml).
+[ "${HC_INSTALL_ONLY:-}" = 1 ] && exit 0
 
 echo "Starting homecloud serve on 127.0.0.1:$port (data: $data_dir, log: $log)"
 mkdir -p "$data_dir"
diff --git a/test/inputs.sh b/test/inputs.sh
index 4ffc3bf..19b44b6 100755
--- a/test/inputs.sh
+++ b/test/inputs.sh
@@ -5,16 +5,16 @@ set -u
 setup="$(cd "$(dirname "$0")/.." && pwd)/setup.sh"
 fails=0
 
-# expect ok|fail PORT TIMEOUT SERVICES
+# expect ok|fail PORT TIMEOUT SERVICES [VERIFY_SIGNATURE]
 expect() {
   local want=$1 out got=ok
-  out=$(HC_VALIDATE_ONLY=1 HC_PORT="$2" HC_WAIT_TIMEOUT="$3" HC_SERVICES_WAIT="$4" \
+  out=$(HC_VALIDATE_ONLY=1 HC_PORT="$2" HC_WAIT_TIMEOUT="$3" HC_SERVICES_WAIT="$4" HC_VERIFY_SIGNATURE="${5-true}" \
     GITHUB_OUTPUT=/dev/null GITHUB_ENV=/dev/null GITHUB_STATE= RUNNER_TEMP="${TMPDIR:-/tmp}" bash "$setup" 2>&1) || got=fail
   if [ "$got" != "$want" ]; then
-    echo "FAIL: port='$2' timeout='$3' services='$4': want $want, got $got: $out"
+    echo "FAIL: port='$2' timeout='$3' services='$4' verify='${5-true}': want $want, got $got: $out"
     fails=$((fails + 1))
   else
-    echo "ok:   port='$2' timeout='$3' services='$4' -> $want"
+    echo "ok:   port='$2' timeout='$3' services='$4' verify='${5-true}' -> $want"
   fi
 }
 
@@ -30,5 +30,10 @@ expect fail http 180 ""
 expect fail 70000 180 ""
 expect fail 8080 180 "s33"
 expect fail 8080 180 "s3,nosuchservice"
+expect ok 8080 180 "" false
+expect ok 8080 180 "" FALSE
+expect ok 8080 180 "" ""                        # empty means the default (true)
+expect fail 8080 180 "" no
+expect fail 8080 180 "" 1
 
 [ "$fails" -eq 0 ] && echo "all input checks passed" || { echo "$fails input checks failed"; exit 1; }