diff --git a/support/download/helpers b/support/download/helpers
new file mode 100755
index 0000000..0e0432c
--- /dev/null
+++ b/support/download/helpers
@@ -0,0 +1,70 @@
+# Generate a reproducible archive from the content of a directory
+#
+# $1    : input directory
+# $2    : leading component in archive
+# $3    : ISO8601 date: YYYY-MM-DDThh:mm:ssZZ
+# $4    : output file
+# $5... : globs of filenames to exclude from the archive, suitable for
+#         find's -path option, and relative to the input directory $1
+#
+# Notes :
+#   - must not be called with CWD as, or below, the input directory
+#   - some temporary files are created in CWD, and removed at the end
+#
+# Example:
+#   $ find /path/to/temp/dir
+#   /path/to/temp/dir/
+#   /path/to/temp/dir/some-file
+#   /path/to/temp/dir/some-dir/
+#   /path/to/temp/dir/some-dir/some-other-file
+#
+#   $ mk_tar_gz /path/to/some/dir \
+#               foo_bar-1.2.3 \
+#               1970-01-01T00:00:00Z \
+#               /path/to/foo.tar.gz \
+#               '.git/*' '.svn/*'
+#
+#   $ tar tzf /path/to/foo.tar.gz
+#   foo_bar-1.2.3/some-file
+#   foo_bar-1.2.3/some-dir/some-other-file
+#
+mk_tar_gz() {
+    local in_dir="${1}"
+    local base_dir="${2}"
+    local date="${3}"
+    local out="${4}"
+    shift 4
+    local glob tmp pax_options
+    local -a find_opts
+
+    for glob; do
+        find_opts+=( -or -path "./${glob#./}" )
+    done
+
+    pax_options="delete=atime,delete=ctime,delete=mtime"
+    pax_options+=",exthdr.name=%d/PaxHeaders/%f,exthdr.mtime={${date}}"
+
+    tmp="$(mktemp --tmpdir="$(pwd)")"
+    pushd "${in_dir}" >/dev/null
+
+    # Establish list
+    find . -not -type d -and -not \( -false "${find_opts[@]}" \) >"${tmp}.list"
+    # Sort list for reproducibility
+    LC_ALL=C sort <"${tmp}.list" >"${tmp}.sorted"
+
+    # Create POSIX tarballs, since that's the format the most reproducible
+    tar cf - --transform="s#^\./#${base_dir}/#" \
+             --numeric-owner --owner=0 --group=0 --mtime="${date}" \
+             --format=posix --pax-option="${pax_options}" \
+             -T "${tmp}.sorted" >"${tmp}.tar"
+
+    # Compress the archive
+    gzip -6 -n <"${tmp}.tar" >"${out}"
+
+    rm -f "${tmp}"{.list,.sorted,.tar}
+
+    popd >/dev/null
+}
+
+# Keep this line and the following as last lines in this file.
+# vim: ft=bash