diff --git a/lib/spack/docs/binary_caches.rst b/lib/spack/docs/binary_caches.rst
index 3215f9732f..ee3cc239f8 100644
--- a/lib/spack/docs/binary_caches.rst
+++ b/lib/spack/docs/binary_caches.rst
@@ -153,7 +153,43 @@ keyring, and trusting all downloaded keys.
 List of popular build caches
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-* `Extreme-scale Scientific Software Stack (E4S) <https://e4s-project.github.io/>`_: `build cache <https://oaciss.uoregon.edu/e4s/inventory.html>`_
+* `Extreme-scale Scientific Software Stack (E4S) <https://e4s-project.github.io/>`_: `build cache <https://oaciss.uoregon.edu/e4s/inventory.html>`_'
+
+-------------------
+Build cache signing
+-------------------
+
+By default, Spack will add a cryptographic signature to each package pushed to
+a build cache, and verifies the signature when installing from a build cache.
+
+Keys for signing can be managed with the :ref:`spack gpg <cmd-spack-gpg>` command,
+as well as ``spack buildcache keys`` as mentioned above.
+
+You can disable signing when pushing with ``spack buildcache push --unsigned``,
+and disable verification when installing from any build cache with
+``spack install --no-check-signature``.
+
+Alternatively, signing and verification can be enabled or disabled on a per build cache
+basis:
+
+.. code-block:: console
+
+    $ spack mirror add --signed <name> <url>  # enable signing and verification
+    $ spack mirror add --unsigned <name> <url>  # disable signing and verification
+
+    $ spack mirror set --signed <name>  # enable signing and verification for an existing mirror
+    $ spack mirror set --unsigned <name>  # disable signing and verification for an existing mirror
+
+Or you can directly edit the ``mirrors.yaml`` configuration file:
+
+.. code-block:: yaml
+
+    mirrors:
+      <name>:
+        url: <url>
+        signed: false # disable signing and verification
+
+See also :ref:`mirrors`.
 
 ----------
 Relocation
diff --git a/lib/spack/spack/binary_distribution.py b/lib/spack/spack/binary_distribution.py
index 159b07eb94..85c47e2337 100644
--- a/lib/spack/spack/binary_distribution.py
+++ b/lib/spack/spack/binary_distribution.py
@@ -25,7 +25,7 @@
 import warnings
 from contextlib import closing, contextmanager
 from gzip import GzipFile
-from typing import Dict, List, NamedTuple, Optional, Set, Tuple
+from typing import Dict, Iterable, List, NamedTuple, Optional, Set, Tuple
 from urllib.error import HTTPError, URLError
 
 import llnl.util.filesystem as fsys
@@ -1605,14 +1605,14 @@ def _get_valid_spec_file(path: str, max_supported_layout: int) -> Tuple[Dict, in
     return spec_dict, layout_version
 
 
-def download_tarball(spec, unsigned=False, mirrors_for_spec=None):
+def download_tarball(spec, unsigned: Optional[bool] = False, mirrors_for_spec=None):
     """
     Download binary tarball for given package into stage area, returning
     path to downloaded tarball if successful, None otherwise.
 
     Args:
         spec (spack.spec.Spec): Concrete spec
-        unsigned (bool): Whether or not to require signed binaries
+        unsigned: if ``True`` or ``False`` override the mirror signature verification defaults
         mirrors_for_spec (list): Optional list of concrete specs and mirrors
             obtained by calling binary_distribution.get_mirrors_for_spec().
             These will be checked in order first before looking in other
@@ -1633,7 +1633,9 @@ def download_tarball(spec, unsigned=False, mirrors_for_spec=None):
            "signature_verified": "true-if-binary-pkg-was-already-verified"
        }
     """
-    configured_mirrors = spack.mirror.MirrorCollection(binary=True).values()
+    configured_mirrors: Iterable[spack.mirror.Mirror] = spack.mirror.MirrorCollection(
+        binary=True
+    ).values()
     if not configured_mirrors:
         tty.die("Please add a spack mirror to allow download of pre-compiled packages.")
 
@@ -1651,8 +1653,16 @@ def download_tarball(spec, unsigned=False, mirrors_for_spec=None):
     # mirror for the spec twice though.
     try_first = [i["mirror_url"] for i in mirrors_for_spec] if mirrors_for_spec else []
     try_next = [i.fetch_url for i in configured_mirrors if i.fetch_url not in try_first]
+    mirror_urls = try_first + try_next
 
-    mirrors = try_first + try_next
+    # TODO: turn `mirrors_for_spec` into a list of Mirror instances, instead of doing that here.
+    def fetch_url_to_mirror(url):
+        for mirror in configured_mirrors:
+            if mirror.fetch_url == url:
+                return mirror
+        return spack.mirror.Mirror(url)
+
+    mirrors = [fetch_url_to_mirror(url) for url in mirror_urls]
 
     tried_to_verify_sigs = []
 
@@ -1661,14 +1671,17 @@ def download_tarball(spec, unsigned=False, mirrors_for_spec=None):
     # we remove support for deprecated spec formats and buildcache layouts.
     for try_signed in (True, False):
         for mirror in mirrors:
+            # Override mirror's default if
+            currently_unsigned = unsigned if unsigned is not None else not mirror.signed
+
             # If it's an OCI index, do things differently, since we cannot compose URLs.
-            parsed = urllib.parse.urlparse(mirror)
+            fetch_url = mirror.fetch_url
 
             # TODO: refactor this to some "nice" place.
-            if parsed.scheme == "oci":
-                ref = spack.oci.image.ImageReference.from_string(mirror[len("oci://") :]).with_tag(
-                    spack.oci.image.default_tag(spec)
-                )
+            if fetch_url.startswith("oci://"):
+                ref = spack.oci.image.ImageReference.from_string(
+                    fetch_url[len("oci://") :]
+                ).with_tag(spack.oci.image.default_tag(spec))
 
                 # Fetch the manifest
                 try:
@@ -1705,7 +1718,7 @@ def download_tarball(spec, unsigned=False, mirrors_for_spec=None):
                         except InvalidMetadataFile as e:
                             tty.warn(
                                 f"Ignoring binary package for {spec.name}/{spec.dag_hash()[:7]} "
-                                f"from {mirror} due to invalid metadata file: {e}"
+                                f"from {fetch_url} due to invalid metadata file: {e}"
                             )
                             local_specfile_stage.destroy()
                             continue
@@ -1727,13 +1740,16 @@ def download_tarball(spec, unsigned=False, mirrors_for_spec=None):
                     "tarball_stage": tarball_stage,
                     "specfile_stage": local_specfile_stage,
                     "signature_verified": False,
+                    "signature_required": not currently_unsigned,
                 }
 
             else:
                 ext = "json.sig" if try_signed else "json"
-                specfile_path = url_util.join(mirror, BUILD_CACHE_RELATIVE_PATH, specfile_prefix)
+                specfile_path = url_util.join(
+                    fetch_url, BUILD_CACHE_RELATIVE_PATH, specfile_prefix
+                )
                 specfile_url = f"{specfile_path}.{ext}"
-                spackfile_url = url_util.join(mirror, BUILD_CACHE_RELATIVE_PATH, tarball)
+                spackfile_url = url_util.join(fetch_url, BUILD_CACHE_RELATIVE_PATH, tarball)
                 local_specfile_stage = try_fetch(specfile_url)
                 if local_specfile_stage:
                     local_specfile_path = local_specfile_stage.save_filename
@@ -1746,21 +1762,21 @@ def download_tarball(spec, unsigned=False, mirrors_for_spec=None):
                     except InvalidMetadataFile as e:
                         tty.warn(
                             f"Ignoring binary package for {spec.name}/{spec.dag_hash()[:7]} "
-                            f"from {mirror} due to invalid metadata file: {e}"
+                            f"from {fetch_url} due to invalid metadata file: {e}"
                         )
                         local_specfile_stage.destroy()
                         continue
 
-                    if try_signed and not unsigned:
+                    if try_signed and not currently_unsigned:
                         # If we found a signed specfile at the root, try to verify
                         # the signature immediately.  We will not download the
                         # tarball if we could not verify the signature.
                         tried_to_verify_sigs.append(specfile_url)
                         signature_verified = try_verify(local_specfile_path)
                         if not signature_verified:
-                            tty.warn("Failed to verify: {0}".format(specfile_url))
+                            tty.warn(f"Failed to verify: {specfile_url}")
 
-                    if unsigned or signature_verified or not try_signed:
+                    if currently_unsigned or signature_verified or not try_signed:
                         # We will download the tarball in one of three cases:
                         #     1. user asked for --no-check-signature
                         #     2. user didn't ask for --no-check-signature, but we
@@ -1783,6 +1799,7 @@ def download_tarball(spec, unsigned=False, mirrors_for_spec=None):
                                 "tarball_stage": tarball_stage,
                                 "specfile_stage": local_specfile_stage,
                                 "signature_verified": signature_verified,
+                                "signature_required": not currently_unsigned,
                             }
 
                     local_specfile_stage.destroy()
@@ -1981,7 +1998,7 @@ def is_backup_file(file):
             relocate.relocate_text(text_names, prefix_to_prefix_text)
 
 
-def _extract_inner_tarball(spec, filename, extract_to, unsigned, remote_checksum):
+def _extract_inner_tarball(spec, filename, extract_to, signature_required: bool, remote_checksum):
     stagepath = os.path.dirname(filename)
     spackfile_name = tarball_name(spec, ".spack")
     spackfile_path = os.path.join(stagepath, spackfile_name)
@@ -2001,7 +2018,7 @@ def _extract_inner_tarball(spec, filename, extract_to, unsigned, remote_checksum
     else:
         raise ValueError("Cannot find spec file for {0}.".format(extract_to))
 
-    if not unsigned:
+    if signature_required:
         if os.path.exists("%s.asc" % specfile_path):
             suppress = config.get("config:suppress_gpg_warnings", False)
             try:
@@ -2050,7 +2067,7 @@ def _tar_strip_component(tar: tarfile.TarFile, prefix: str):
                 m.linkname = m.linkname[result.end() :]
 
 
-def extract_tarball(spec, download_result, unsigned=False, force=False, timer=timer.NULL_TIMER):
+def extract_tarball(spec, download_result, force=False, timer=timer.NULL_TIMER):
     """
     extract binary tarball for given package into install area
     """
@@ -2076,7 +2093,8 @@ def extract_tarball(spec, download_result, unsigned=False, force=False, timer=ti
     bchecksum = spec_dict["binary_cache_checksum"]
 
     filename = download_result["tarball_stage"].save_filename
-    signature_verified = download_result["signature_verified"]
+    signature_verified: bool = download_result["signature_verified"]
+    signature_required: bool = download_result["signature_required"]
     tmpdir = None
 
     if layout_version == 0:
@@ -2085,7 +2103,9 @@ def extract_tarball(spec, download_result, unsigned=False, force=False, timer=ti
         # and another tarball containing the actual install tree.
         tmpdir = tempfile.mkdtemp()
         try:
-            tarfile_path = _extract_inner_tarball(spec, filename, tmpdir, unsigned, bchecksum)
+            tarfile_path = _extract_inner_tarball(
+                spec, filename, tmpdir, signature_required, bchecksum
+            )
         except Exception as e:
             _delete_staged_downloads(download_result)
             shutil.rmtree(tmpdir)
@@ -2098,9 +2118,10 @@ def extract_tarball(spec, download_result, unsigned=False, force=False, timer=ti
         # the tarball.
         tarfile_path = filename
 
-        if not unsigned and not signature_verified:
+        if signature_required and not signature_verified:
             raise UnsignedPackageException(
-                "To install unsigned packages, use the --no-check-signature option."
+                "To install unsigned packages, use the --no-check-signature option, "
+                "or configure the mirror with signed: false."
             )
 
         # compute the sha256 checksum of the tarball
@@ -2213,7 +2234,7 @@ def install_root_node(spec, unsigned=False, force=False, sha256=None):
     # don't print long padded paths while extracting/relocating binaries
     with spack.util.path.filter_padding():
         tty.msg('Installing "{0}" from a buildcache'.format(spec.format()))
-        extract_tarball(spec, download_result, unsigned, force)
+        extract_tarball(spec, download_result, force)
         spack.hooks.post_install(spec, False)
         spack.store.STORE.db.add(spec, spack.store.STORE.layout)
 
diff --git a/lib/spack/spack/cmd/buildcache.py b/lib/spack/spack/cmd/buildcache.py
index 76e2d0f61c..b591636ba2 100644
--- a/lib/spack/spack/cmd/buildcache.py
+++ b/lib/spack/spack/cmd/buildcache.py
@@ -76,7 +76,19 @@ def setup_parser(subparser: argparse.ArgumentParser):
     )
     push_sign = push.add_mutually_exclusive_group(required=False)
     push_sign.add_argument(
-        "--unsigned", "-u", action="store_true", help="push unsigned buildcache tarballs"
+        "--unsigned",
+        "-u",
+        action="store_false",
+        dest="signed",
+        default=None,
+        help="push unsigned buildcache tarballs",
+    )
+    push_sign.add_argument(
+        "--signed",
+        action="store_true",
+        dest="signed",
+        default=None,
+        help="push signed buildcache tarballs",
     )
     push_sign.add_argument(
         "--key", "-k", metavar="key", type=str, default=None, help="key for signing"
@@ -328,17 +340,27 @@ def push_fn(args):
             "The flag `--allow-root` is the default in Spack 0.21, will be removed in Spack 0.22"
         )
 
+    mirror: spack.mirror.Mirror = args.mirror
+
     # Check if this is an OCI image.
     try:
-        image_ref = spack.oci.oci.image_from_mirror(args.mirror)
+        image_ref = spack.oci.oci.image_from_mirror(mirror)
     except ValueError:
         image_ref = None
 
+    push_url = mirror.push_url
+
+    # When neither --signed, --unsigned nor --key are specified, use the mirror's default.
+    if args.signed is None and not args.key:
+        unsigned = not mirror.signed
+    else:
+        unsigned = not (args.key or args.signed)
+
     # For OCI images, we require dependencies to be pushed for now.
     if image_ref:
         if "dependencies" not in args.things_to_install:
             tty.die("Dependencies must be pushed for OCI images.")
-        if not args.unsigned:
+        if not unsigned:
             tty.warn(
                 "Code signing is currently not supported for OCI images. "
                 "Use --unsigned to silence this warning."
@@ -351,12 +373,10 @@ def push_fn(args):
         dependencies="dependencies" in args.things_to_install,
     )
 
-    url = args.mirror.push_url
-
     # When pushing multiple specs, print the url once ahead of time, as well as how
     # many specs are being pushed.
     if len(specs) > 1:
-        tty.info(f"Selected {len(specs)} specs to push to {url}")
+        tty.info(f"Selected {len(specs)} specs to push to {push_url}")
 
     failed = []
 
@@ -373,10 +393,10 @@ def push_fn(args):
             try:
                 bindist.push_or_raise(
                     spec,
-                    url,
+                    push_url,
                     bindist.PushOptions(
                         force=args.force,
-                        unsigned=args.unsigned,
+                        unsigned=unsigned,
                         key=args.key,
                         regenerate_index=args.update_index,
                     ),
@@ -384,7 +404,7 @@ def push_fn(args):
 
                 msg = f"{_progress(i, len(specs))}Pushed {_format_spec(spec)}"
                 if len(specs) == 1:
-                    msg += f" to {url}"
+                    msg += f" to {push_url}"
                 tty.info(msg)
 
             except bindist.NoOverwriteException:
diff --git a/lib/spack/spack/cmd/install.py b/lib/spack/spack/cmd/install.py
index 5db30ba7b5..8d085298c1 100644
--- a/lib/spack/spack/cmd/install.py
+++ b/lib/spack/spack/cmd/install.py
@@ -162,8 +162,8 @@ def setup_parser(subparser):
         "--no-check-signature",
         action="store_true",
         dest="unsigned",
-        default=False,
-        help="do not check signatures of binary packages",
+        default=None,
+        help="do not check signatures of binary packages (override mirror config)",
     )
     subparser.add_argument(
         "--show-log-on-error",
diff --git a/lib/spack/spack/cmd/mirror.py b/lib/spack/spack/cmd/mirror.py
index 91a3f27931..498f26afb2 100644
--- a/lib/spack/spack/cmd/mirror.py
+++ b/lib/spack/spack/cmd/mirror.py
@@ -107,6 +107,23 @@ def setup_parser(subparser):
             "and source use `--type binary --type source` (default)"
         ),
     )
+    add_parser_signed = add_parser.add_mutually_exclusive_group(required=False)
+    add_parser_signed.add_argument(
+        "--unsigned",
+        help="do not require signing and signature verification when pushing and installing from "
+        "this build cache",
+        action="store_false",
+        default=None,
+        dest="signed",
+    )
+    add_parser_signed.add_argument(
+        "--signed",
+        help="require signing and signature verification when pushing and installing from this "
+        "build cache",
+        action="store_true",
+        default=None,
+        dest="signed",
+    )
     arguments.add_connection_args(add_parser, False)
     # Remove
     remove_parser = sp.add_parser("remove", aliases=["rm"], help=mirror_remove.__doc__)
@@ -157,6 +174,23 @@ def setup_parser(subparser):
         ),
     )
     set_parser.add_argument("--url", help="url of mirror directory from 'spack mirror create'")
+    set_parser_unsigned = set_parser.add_mutually_exclusive_group(required=False)
+    set_parser_unsigned.add_argument(
+        "--unsigned",
+        help="do not require signing and signature verification when pushing and installing from "
+        "this build cache",
+        action="store_false",
+        default=None,
+        dest="signed",
+    )
+    set_parser_unsigned.add_argument(
+        "--signed",
+        help="require signing and signature verification when pushing and installing from this "
+        "build cache",
+        action="store_true",
+        default=None,
+        dest="signed",
+    )
     set_parser.add_argument(
         "--scope",
         action=arguments.ConfigScope,
@@ -186,6 +220,7 @@ def mirror_add(args):
         or args.type
         or args.oci_username
         or args.oci_password
+        or args.signed is not None
     ):
         connection = {"url": args.url}
         if args.s3_access_key_id and args.s3_access_key_secret:
@@ -201,6 +236,8 @@ def mirror_add(args):
         if args.type:
             connection["binary"] = "binary" in args.type
             connection["source"] = "source" in args.type
+        if args.signed is not None:
+            connection["signed"] = args.signed
         mirror = spack.mirror.Mirror(connection, name=args.name)
     else:
         mirror = spack.mirror.Mirror(args.url, name=args.name)
@@ -233,6 +270,8 @@ def _configure_mirror(args):
         changes["endpoint_url"] = args.s3_endpoint_url
     if args.oci_username and args.oci_password:
         changes["access_pair"] = [args.oci_username, args.oci_password]
+    if getattr(args, "signed", None) is not None:
+        changes["signed"] = args.signed
 
     # argparse cannot distinguish between --binary and --no-binary when same dest :(
     # notice that set-url does not have these args, so getattr
diff --git a/lib/spack/spack/installer.py b/lib/spack/spack/installer.py
index ef0f1f3b8b..915f80993b 100644
--- a/lib/spack/spack/installer.py
+++ b/lib/spack/spack/installer.py
@@ -381,7 +381,10 @@ def _print_timer(pre: str, pkg_id: str, timer: timer.BaseTimer) -> None:
 
 
 def _install_from_cache(
-    pkg: "spack.package_base.PackageBase", cache_only: bool, explicit: bool, unsigned: bool = False
+    pkg: "spack.package_base.PackageBase",
+    cache_only: bool,
+    explicit: bool,
+    unsigned: Optional[bool] = False,
 ) -> bool:
     """
     Extract the package from binary cache
@@ -391,8 +394,7 @@ def _install_from_cache(
         cache_only: only extract from binary cache
         explicit: ``True`` if installing the package was explicitly
             requested by the user, otherwise, ``False``
-        unsigned: ``True`` if binary package signatures to be checked,
-            otherwise, ``False``
+        unsigned: if ``True`` or ``False`` override the mirror signature verification defaults
 
     Return: ``True`` if the package was extract from binary cache, ``False`` otherwise
     """
@@ -462,7 +464,7 @@ def _process_external_package(pkg: "spack.package_base.PackageBase", explicit: b
 def _process_binary_cache_tarball(
     pkg: "spack.package_base.PackageBase",
     explicit: bool,
-    unsigned: bool,
+    unsigned: Optional[bool],
     mirrors_for_spec: Optional[list] = None,
     timer: timer.BaseTimer = timer.NULL_TIMER,
 ) -> bool:
@@ -472,8 +474,7 @@ def _process_binary_cache_tarball(
     Args:
         pkg: the package being installed
         explicit: the package was explicitly requested by the user
-        unsigned: ``True`` if binary package signatures to be checked,
-            otherwise, ``False``
+        unsigned: if ``True`` or ``False`` override the mirror signature verification defaults
         mirrors_for_spec: Optional list of concrete specs and mirrors
         obtained by calling binary_distribution.get_mirrors_for_spec().
         timer: timer to keep track of binary install phases.
@@ -493,9 +494,7 @@ def _process_binary_cache_tarball(
     tty.msg(f"Extracting {package_id(pkg)} from binary cache")
 
     with timer.measure("install"), spack.util.path.filter_padding():
-        binary_distribution.extract_tarball(
-            pkg.spec, download_result, unsigned=unsigned, force=False, timer=timer
-        )
+        binary_distribution.extract_tarball(pkg.spec, download_result, force=False, timer=timer)
 
         pkg.installed_from_binary_cache = True
         spack.store.STORE.db.add(pkg.spec, spack.store.STORE.layout, explicit=explicit)
@@ -505,7 +504,7 @@ def _process_binary_cache_tarball(
 def _try_install_from_binary_cache(
     pkg: "spack.package_base.PackageBase",
     explicit: bool,
-    unsigned: bool = False,
+    unsigned: Optional[bool] = None,
     timer: timer.BaseTimer = timer.NULL_TIMER,
 ) -> bool:
     """
@@ -514,8 +513,7 @@ def _try_install_from_binary_cache(
     Args:
         pkg: package to be extracted from binary cache
         explicit: the package was explicitly requested by the user
-        unsigned: ``True`` if binary package signatures to be checked,
-            otherwise, ``False``
+        unsigned: if ``True`` or ``False`` override the mirror signature verification defaults
         timer: timer to keep track of binary install phases.
     """
     # Early exit if no binary mirrors are configured.
@@ -825,7 +823,7 @@ def _add_default_args(self) -> None:
             ("restage", False),
             ("skip_patch", False),
             ("tests", False),
-            ("unsigned", False),
+            ("unsigned", None),
             ("verbose", False),
         ]:
             _ = self.install_args.setdefault(arg, default)
@@ -1663,7 +1661,7 @@ def _install_task(self, task: BuildTask, install_status: InstallStatus) -> None:
         use_cache = task.use_cache
         tests = install_args.get("tests", False)
         assert isinstance(tests, (bool, list))  # make mypy happy.
-        unsigned = bool(install_args.get("unsigned"))
+        unsigned: Optional[bool] = install_args.get("unsigned")
 
         pkg, pkg_id = task.pkg, task.pkg_id
 
diff --git a/lib/spack/spack/mirror.py b/lib/spack/spack/mirror.py
index d5425772cd..b062f1c356 100644
--- a/lib/spack/spack/mirror.py
+++ b/lib/spack/spack/mirror.py
@@ -133,6 +133,10 @@ def binary(self):
     def source(self):
         return isinstance(self._data, str) or self._data.get("source", True)
 
+    @property
+    def signed(self) -> bool:
+        return isinstance(self._data, str) or self._data.get("signed", True)
+
     @property
     def fetch_url(self):
         """Get the valid, canonicalized fetch URL"""
@@ -146,7 +150,7 @@ def push_url(self):
     def _update_connection_dict(self, current_data: dict, new_data: dict, top_level: bool):
         keys = ["url", "access_pair", "access_token", "profile", "endpoint_url"]
         if top_level:
-            keys += ["binary", "source"]
+            keys += ["binary", "source", "signed"]
         changed = False
         for key in keys:
             if key in new_data and current_data.get(key) != new_data[key]:
diff --git a/lib/spack/spack/schema/mirrors.py b/lib/spack/spack/schema/mirrors.py
index 8001172afd..99e57b3450 100644
--- a/lib/spack/spack/schema/mirrors.py
+++ b/lib/spack/spack/schema/mirrors.py
@@ -42,6 +42,7 @@
     "properties": {
         "source": {"type": "boolean"},
         "binary": {"type": "boolean"},
+        "signed": {"type": "boolean"},
         "fetch": fetch_and_push,
         "push": fetch_and_push,
         **connection,  # type: ignore
diff --git a/lib/spack/spack/test/cmd/buildcache.py b/lib/spack/spack/test/cmd/buildcache.py
index 55ec605913..e335c6e3dd 100644
--- a/lib/spack/spack/test/cmd/buildcache.py
+++ b/lib/spack/spack/test/cmd/buildcache.py
@@ -331,3 +331,37 @@ def fake_push(node, push_url, options):
 
     # Ensure no duplicates
     assert len(set(packages_to_push)) == len(packages_to_push)
+
+
+@pytest.mark.parametrize("signed", [True, False])
+def test_push_and_install_with_mirror_marked_unsigned_does_not_require_extra_flags(
+    tmp_path, mutable_database, mock_gnupghome, signed
+):
+    """Tests whether marking a mirror as unsigned makes it possible to push and install to/from
+    it without requiring extra flags on the command line (and no signing keys configured)."""
+
+    # Create a named mirror with signed set to True or False
+    add_flag = "--signed" if signed else "--unsigned"
+    mirror("add", add_flag, "my-mirror", str(tmp_path))
+    spec = mutable_database.query_local("libelf", installed=True)[0]
+
+    # Push
+    if signed:
+        # Need to pass "--unsigned" to override the mirror's default
+        args = ["push", "--update-index", "--unsigned", "my-mirror", f"/{spec.dag_hash()}"]
+    else:
+        # No need to pass "--unsigned" if the mirror is unsigned
+        args = ["push", "--update-index", "my-mirror", f"/{spec.dag_hash()}"]
+
+    buildcache(*args)
+
+    # Install
+    if signed:
+        # Need to pass "--no-check-signature" to avoid install errors
+        kwargs = {"cache_only": True, "unsigned": True}
+    else:
+        # No need to pass "--no-check-signature" if the mirror is unsigned
+        kwargs = {"cache_only": True}
+
+    spec.package.do_uninstall(force=True)
+    spec.package.do_install(**kwargs)
diff --git a/lib/spack/spack/test/cmd/mirror.py b/lib/spack/spack/test/cmd/mirror.py
index 029765df93..61021c3183 100644
--- a/lib/spack/spack/test/cmd/mirror.py
+++ b/lib/spack/spack/test/cmd/mirror.py
@@ -398,3 +398,12 @@ def test_mirror_set_2(mutable_config):
         "url": "http://example.com",
         "push": {"url": "http://example2.com", "access_pair": ["username", "password"]},
     }
+
+
+def test_mirror_add_set_signed(mutable_config):
+    mirror("add", "--signed", "example", "http://example.com")
+    assert spack.config.get("mirrors:example") == {"url": "http://example.com", "signed": True}
+    mirror("set", "--unsigned", "example")
+    assert spack.config.get("mirrors:example") == {"url": "http://example.com", "signed": False}
+    mirror("set", "--signed", "example")
+    assert spack.config.get("mirrors:example") == {"url": "http://example.com", "signed": True}
diff --git a/share/spack/spack-completion.bash b/share/spack/spack-completion.bash
index cebf865d9e..85358fbdcc 100755
--- a/share/spack/spack-completion.bash
+++ b/share/spack/spack-completion.bash
@@ -571,7 +571,7 @@ _spack_buildcache() {
 _spack_buildcache_push() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -f --force --allow-root -a --unsigned -u --key -k --update-index --rebuild-index --spec-file --only --fail-fast --base-image -j --jobs"
+        SPACK_COMPREPLY="-h --help -f --force --allow-root -a --unsigned -u --signed --key -k --update-index --rebuild-index --spec-file --only --fail-fast --base-image -j --jobs"
     else
         _mirrors
     fi
@@ -580,7 +580,7 @@ _spack_buildcache_push() {
 _spack_buildcache_create() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -f --force --allow-root -a --unsigned -u --key -k --update-index --rebuild-index --spec-file --only --fail-fast --base-image -j --jobs"
+        SPACK_COMPREPLY="-h --help -f --force --allow-root -a --unsigned -u --signed --key -k --update-index --rebuild-index --spec-file --only --fail-fast --base-image -j --jobs"
     else
         _mirrors
     fi
@@ -1405,7 +1405,7 @@ _spack_mirror_destroy() {
 _spack_mirror_add() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help --scope --type --s3-access-key-id --s3-access-key-secret --s3-access-token --s3-profile --s3-endpoint-url --oci-username --oci-password"
+        SPACK_COMPREPLY="-h --help --scope --type --unsigned --signed --s3-access-key-id --s3-access-key-secret --s3-access-token --s3-profile --s3-endpoint-url --oci-username --oci-password"
     else
         _mirrors
     fi
@@ -1441,7 +1441,7 @@ _spack_mirror_set_url() {
 _spack_mirror_set() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help --push --fetch --type --url --scope --s3-access-key-id --s3-access-key-secret --s3-access-token --s3-profile --s3-endpoint-url --oci-username --oci-password"
+        SPACK_COMPREPLY="-h --help --push --fetch --type --url --unsigned --signed --scope --s3-access-key-id --s3-access-key-secret --s3-access-token --s3-profile --s3-endpoint-url --oci-username --oci-password"
     else
         _mirrors
     fi
diff --git a/share/spack/spack-completion.fish b/share/spack/spack-completion.fish
index 5d443c8b6d..090f725fa1 100755
--- a/share/spack/spack-completion.fish
+++ b/share/spack/spack-completion.fish
@@ -697,7 +697,7 @@ complete -c spack -n '__fish_spack_using_command buildcache' -s h -l help -f -a
 complete -c spack -n '__fish_spack_using_command buildcache' -s h -l help -d 'show this help message and exit'
 
 # spack buildcache push
-set -g __fish_spack_optspecs_spack_buildcache_push h/help f/force a/allow-root u/unsigned k/key= update-index spec-file= only= fail-fast base-image= j/jobs=
+set -g __fish_spack_optspecs_spack_buildcache_push h/help f/force a/allow-root u/unsigned signed k/key= update-index spec-file= only= fail-fast base-image= j/jobs=
 complete -c spack -n '__fish_spack_using_command_pos_remainder 1 buildcache push' -f -k -a '(__fish_spack_specs)'
 complete -c spack -n '__fish_spack_using_command buildcache push' -s h -l help -f -a help
 complete -c spack -n '__fish_spack_using_command buildcache push' -s h -l help -d 'show this help message and exit'
@@ -705,8 +705,10 @@ complete -c spack -n '__fish_spack_using_command buildcache push' -s f -l force
 complete -c spack -n '__fish_spack_using_command buildcache push' -s f -l force -d 'overwrite tarball if it exists'
 complete -c spack -n '__fish_spack_using_command buildcache push' -l allow-root -s a -f -a allow_root
 complete -c spack -n '__fish_spack_using_command buildcache push' -l allow-root -s a -d 'allow install root string in binary files after RPATH substitution'
-complete -c spack -n '__fish_spack_using_command buildcache push' -l unsigned -s u -f -a unsigned
+complete -c spack -n '__fish_spack_using_command buildcache push' -l unsigned -s u -f -a signed
 complete -c spack -n '__fish_spack_using_command buildcache push' -l unsigned -s u -d 'push unsigned buildcache tarballs'
+complete -c spack -n '__fish_spack_using_command buildcache push' -l signed -f -a signed
+complete -c spack -n '__fish_spack_using_command buildcache push' -l signed -d 'push signed buildcache tarballs'
 complete -c spack -n '__fish_spack_using_command buildcache push' -l key -s k -r -f -a key
 complete -c spack -n '__fish_spack_using_command buildcache push' -l key -s k -r -d 'key for signing'
 complete -c spack -n '__fish_spack_using_command buildcache push' -l update-index -l rebuild-index -f -a update_index
@@ -723,7 +725,7 @@ complete -c spack -n '__fish_spack_using_command buildcache push' -s j -l jobs -
 complete -c spack -n '__fish_spack_using_command buildcache push' -s j -l jobs -r -d 'explicitly set number of parallel jobs'
 
 # spack buildcache create
-set -g __fish_spack_optspecs_spack_buildcache_create h/help f/force a/allow-root u/unsigned k/key= update-index spec-file= only= fail-fast base-image= j/jobs=
+set -g __fish_spack_optspecs_spack_buildcache_create h/help f/force a/allow-root u/unsigned signed k/key= update-index spec-file= only= fail-fast base-image= j/jobs=
 complete -c spack -n '__fish_spack_using_command_pos_remainder 1 buildcache create' -f -k -a '(__fish_spack_specs)'
 complete -c spack -n '__fish_spack_using_command buildcache create' -s h -l help -f -a help
 complete -c spack -n '__fish_spack_using_command buildcache create' -s h -l help -d 'show this help message and exit'
@@ -731,8 +733,10 @@ complete -c spack -n '__fish_spack_using_command buildcache create' -s f -l forc
 complete -c spack -n '__fish_spack_using_command buildcache create' -s f -l force -d 'overwrite tarball if it exists'
 complete -c spack -n '__fish_spack_using_command buildcache create' -l allow-root -s a -f -a allow_root
 complete -c spack -n '__fish_spack_using_command buildcache create' -l allow-root -s a -d 'allow install root string in binary files after RPATH substitution'
-complete -c spack -n '__fish_spack_using_command buildcache create' -l unsigned -s u -f -a unsigned
+complete -c spack -n '__fish_spack_using_command buildcache create' -l unsigned -s u -f -a signed
 complete -c spack -n '__fish_spack_using_command buildcache create' -l unsigned -s u -d 'push unsigned buildcache tarballs'
+complete -c spack -n '__fish_spack_using_command buildcache create' -l signed -f -a signed
+complete -c spack -n '__fish_spack_using_command buildcache create' -l signed -d 'push signed buildcache tarballs'
 complete -c spack -n '__fish_spack_using_command buildcache create' -l key -s k -r -f -a key
 complete -c spack -n '__fish_spack_using_command buildcache create' -l key -s k -r -d 'key for signing'
 complete -c spack -n '__fish_spack_using_command buildcache create' -l update-index -l rebuild-index -f -a update_index
@@ -1928,7 +1932,7 @@ complete -c spack -n '__fish_spack_using_command install' -l use-buildcache -r -
 complete -c spack -n '__fish_spack_using_command install' -l include-build-deps -f -a include_build_deps
 complete -c spack -n '__fish_spack_using_command install' -l include-build-deps -d 'include build deps when installing from cache, useful for CI pipeline troubleshooting'
 complete -c spack -n '__fish_spack_using_command install' -l no-check-signature -f -a unsigned
-complete -c spack -n '__fish_spack_using_command install' -l no-check-signature -d 'do not check signatures of binary packages'
+complete -c spack -n '__fish_spack_using_command install' -l no-check-signature -d 'do not check signatures of binary packages (override mirror config)'
 complete -c spack -n '__fish_spack_using_command install' -l show-log-on-error -f -a show_log_on_error
 complete -c spack -n '__fish_spack_using_command install' -l show-log-on-error -d 'print full build log to stderr if build fails'
 complete -c spack -n '__fish_spack_using_command install' -l source -f -a install_source
@@ -2171,7 +2175,7 @@ complete -c spack -n '__fish_spack_using_command mirror destroy' -l mirror-url -
 complete -c spack -n '__fish_spack_using_command mirror destroy' -l mirror-url -r -d 'find mirror to destroy by url'
 
 # spack mirror add
-set -g __fish_spack_optspecs_spack_mirror_add h/help scope= type= s3-access-key-id= s3-access-key-secret= s3-access-token= s3-profile= s3-endpoint-url= oci-username= oci-password=
+set -g __fish_spack_optspecs_spack_mirror_add h/help scope= type= unsigned signed s3-access-key-id= s3-access-key-secret= s3-access-token= s3-profile= s3-endpoint-url= oci-username= oci-password=
 complete -c spack -n '__fish_spack_using_command_pos 0 mirror add' -f
 complete -c spack -n '__fish_spack_using_command mirror add' -s h -l help -f -a help
 complete -c spack -n '__fish_spack_using_command mirror add' -s h -l help -d 'show this help message and exit'
@@ -2179,6 +2183,10 @@ complete -c spack -n '__fish_spack_using_command mirror add' -l scope -r -f -a '
 complete -c spack -n '__fish_spack_using_command mirror add' -l scope -r -d 'configuration scope to modify'
 complete -c spack -n '__fish_spack_using_command mirror add' -l type -r -f -a 'binary source'
 complete -c spack -n '__fish_spack_using_command mirror add' -l type -r -d 'specify the mirror type: for both binary and source use `--type binary --type source` (default)'
+complete -c spack -n '__fish_spack_using_command mirror add' -l unsigned -f -a signed
+complete -c spack -n '__fish_spack_using_command mirror add' -l unsigned -d 'do not require signing and signature verification when pushing and installing from this build cache'
+complete -c spack -n '__fish_spack_using_command mirror add' -l signed -f -a signed
+complete -c spack -n '__fish_spack_using_command mirror add' -l signed -d 'require signing and signature verification when pushing and installing from this build cache'
 complete -c spack -n '__fish_spack_using_command mirror add' -l s3-access-key-id -r -f -a s3_access_key_id
 complete -c spack -n '__fish_spack_using_command mirror add' -l s3-access-key-id -r -d 'ID string to use to connect to this S3 mirror'
 complete -c spack -n '__fish_spack_using_command mirror add' -l s3-access-key-secret -r -f -a s3_access_key_secret
@@ -2237,7 +2245,7 @@ complete -c spack -n '__fish_spack_using_command mirror set-url' -l oci-password
 complete -c spack -n '__fish_spack_using_command mirror set-url' -l oci-password -r -d 'password to use to connect to this OCI mirror'
 
 # spack mirror set
-set -g __fish_spack_optspecs_spack_mirror_set h/help push fetch type= url= scope= s3-access-key-id= s3-access-key-secret= s3-access-token= s3-profile= s3-endpoint-url= oci-username= oci-password=
+set -g __fish_spack_optspecs_spack_mirror_set h/help push fetch type= url= unsigned signed scope= s3-access-key-id= s3-access-key-secret= s3-access-token= s3-profile= s3-endpoint-url= oci-username= oci-password=
 complete -c spack -n '__fish_spack_using_command_pos 0 mirror set' -f -a '(__fish_spack_mirrors)'
 complete -c spack -n '__fish_spack_using_command mirror set' -s h -l help -f -a help
 complete -c spack -n '__fish_spack_using_command mirror set' -s h -l help -d 'show this help message and exit'
@@ -2249,6 +2257,10 @@ complete -c spack -n '__fish_spack_using_command mirror set' -l type -r -f -a 'b
 complete -c spack -n '__fish_spack_using_command mirror set' -l type -r -d 'specify the mirror type: for both binary and source use `--type binary --type source`'
 complete -c spack -n '__fish_spack_using_command mirror set' -l url -r -f -a url
 complete -c spack -n '__fish_spack_using_command mirror set' -l url -r -d 'url of mirror directory from \'spack mirror create\''
+complete -c spack -n '__fish_spack_using_command mirror set' -l unsigned -f -a signed
+complete -c spack -n '__fish_spack_using_command mirror set' -l unsigned -d 'do not require signing and signature verification when pushing and installing from this build cache'
+complete -c spack -n '__fish_spack_using_command mirror set' -l signed -f -a signed
+complete -c spack -n '__fish_spack_using_command mirror set' -l signed -d 'require signing and signature verification when pushing and installing from this build cache'
 complete -c spack -n '__fish_spack_using_command mirror set' -l scope -r -f -a '_builtin defaults system site user command_line'
 complete -c spack -n '__fish_spack_using_command mirror set' -l scope -r -d 'configuration scope to modify'
 complete -c spack -n '__fish_spack_using_command mirror set' -l s3-access-key-id -r -f -a s3_access_key_id