[PATCH v2 2/3] build: distribute signed sha256sums

[Daniel Kahn Gillmor <dkg@fifthhorseman.net>]

Distribute clearsigned sha256sum file in addition to the detached
signature.

Verifies that use the sha256sum ensure that the thing signed includes
the name of the tarball. This defends the verifier by default against
a freeze, rollback, or project substitution attack.

A verifier can use something like the following (as expressed in
bash):

      set -o pipefail
      wget https://notmuchmail.org/releases/notmuch-$VERSION.tar.gz{,.sha256.asc}
      gpgv --keyring ./notmuch-signers.pgp --output - notmuch-$VERSION.tar.gz.sha256.asc | sha256sum -c -

See id:87r2b8w956.fsf@fifthhorseman.net and other messages in that
thread for discussion.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
---
 Makefile.global | 2 +-
 Makefile.local  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/Makefile.global b/Makefile.global
index 6e17494a..27c82433 100644
--- a/Makefile.global
+++ b/Makefile.global
@@ -43,7 +43,7 @@ RELEASE_URL=https://notmuchmail.org/releases
 TAR_FILE=$(PACKAGE)-$(VERSION).tar.gz
 ELPA_FILE:=$(PACKAGE)-emacs-$(ELPA_VERSION).tar
 DEB_TAR_FILE=$(PACKAGE)_$(VERSION).orig.tar.gz
-SHA256_FILE=$(TAR_FILE).sha256
+SHA256_FILE=$(TAR_FILE).sha256.asc
 GPG_FILE=$(TAR_FILE).asc
 
 PV_FILE=bindings/python/notmuch/version.py
diff --git a/Makefile.local b/Makefile.local
index 8535844a..eb599565 100644
--- a/Makefile.local
+++ b/Makefile.local
@@ -40,7 +40,7 @@ $(TAR_FILE):
 	@echo "Source is ready for release in $(TAR_FILE)"
 
 $(SHA256_FILE): $(TAR_FILE)
-	sha256sum $^ > $@
+	sha256sum $^ | gpg --clear-sign --output $@ -
 
 $(GPG_FILE): $(TAR_FILE)
 	gpg --armor --detach-sign $^
-- 
2.20.1