Update and improve dropbear

By updating to dropbear 2020.81 (mostly based on OpenWRT), we gain :
 - ed25519 host keys (and pubkeys)
 - curve25519 key exchange
 - rsa-sha2-256 pubkey authentication, instead of ssh-rsa (disabled on newer clients)

Both ed25519 host keys and curve25519 key exchange are much faster than RSA and DH, so
connecting is down from 3.5s to 0.6s.
But unlike host keys, pubkey authentication is faster with RSA (ed25519 is 0.3s slower).

The ed25519 host key is also much faster to generate than the RSA and DSS (deprecated) ones,
which allows to generate it on demand.
This saves a LOT of CPU on boot, dropbear starts about 1 min earlier !
It is now actually ready before the link, so actual gain is more about 30-40s.

Clients supporting ed25519 host keys will connect quickly, older ones
which only have RSA would have a slower first connection.

Now /etc/dropbear is persistent, so :
 - host keys no longer change on reboot
 - public keys can be copied to /etc/dropbear/authorized_keys

.gitignore

@@ -1,2 +1,4 @@
 /firmwares/*/
 /firmwares/*_patched.bin
+/OpenWrt-Toolchain-*
+/patches/0000_dropbear/build

patches/0000_dropbear/Makefile

@@ -0,0 +1,73 @@
+DROPBEAR_RELEASE = 2020.81
+DROPBEAR_TAR = dropbear-$(DROPBEAR_RELEASE).tar.bz2
+DROPBEAR_URL = https://matt.ucc.asn.au/dropbear/releases/$(DROPBEAR_TAR)
+DROPBEAR_SRC = build/dropbear-$(DROPBEAR_RELEASE)
+DROPBEAR_PATCHES = $(addprefix https://raw.githubusercontent.com/openwrt/openwrt/master/package/network/services/dropbear/patches/, \
+    100-pubkey_path.patch \
+    110-change_user.patch \
+    130-ssh_ignore_x_args.patch \
+    140-disable_assert.patch \
+    600-allow-blank-root-password.patch \
+    900-configure-hardening.patch \
+    910-signkey-fix-use-of-rsa-sha2-256-pubkeys.patch \
+)
+
+DROPBEAR_CONF = \
+    --host mips-openwrt-linux-uclibc \
+    --disable-lastlog \
+    --disable-utmpx \
+    --disable-utmp \
+    --disable-wtmp \
+    --disable-wtmpx \
+    --disable-loginfunc \
+    --disable-pututline \
+    --disable-pututxline \
+    --disable-zlib \
+    $()
+
+# chacha20-poly1305 is more than 2x slower than aes-ctr for decryption on this HW.
+# But it would be more than 2x faster for encryption.
+DROPBEAR_DISABLED_FEATURES = \
+    INETD_MODE \
+    DROPBEAR_CLI_NETCAT \
+    DROPBEAR_DSS \
+    DO_MOTD \
+    DROPBEAR_ECDSA \
+    DROPBEAR_ECDH \
+    DROPBEAR_CHACHA20POLY1305
+
+all: dropbear
+
+clean:
+	rm -rf build
+
+build:
+	mkdir -p $@
+
+build/$(DROPBEAR_TAR): | build
+	wget -O $@ $(DROPBEAR_URL)
+
+build/.dropbear_patches: | build
+	rm -f build/*.patch
+	cd build && wget $(DROPBEAR_PATCHES)
+	@touch $@
+
+build/.dropbear_unpacked: build/$(DROPBEAR_TAR) build/.dropbear_patches
+	tar -C build -xf $<
+	set -e; for p in build/*.patch; do patch -d $(DROPBEAR_SRC) -p1 < $$p; done
+	@touch $@
+
+build/.dropbear_autoreconf: build/.dropbear_unpacked
+	cd $(DROPBEAR_SRC) && autoreconf
+	@touch $@
+
+build/.dropbear_configured: build/.dropbear_autoreconf | toolchain
+	cd $(DROPBEAR_SRC) && ./configure $(DROPBEAR_CONF)
+	printf "#define %s 0\n" $(DROPBEAR_DISABLED_FEATURES) > $(DROPBEAR_SRC)/localoptions.h
+	@touch $@
+
+dropbear: build/.dropbear_configured
+	$(MAKE) -C $(DROPBEAR_SRC) PROGRAMS="dropbear dbclient dropbearkey scp" MULTI=1 SCPPROGRESS=1
+	$(OBJCOPY) --strip-all $(DROPBEAR_SRC)/dropbearmulti dropbear
+
+include ../../toolchain.mk

patches/0000_dropbear/generate-host-keys-as-needed.patch

@@ -0,0 +1,46 @@
+--- a/etc/init.d/dropbear	2017-07-11 07:03:05.000000000 +0200
++++ b/etc/init.d/dropbear	2020-12-29 02:09:43.719665917 +0100
+@@ -65,7 +65,7 @@
+ 	local pid_file="/var/run/${NAME}.${PIDCOUNT}.pid"
+
+ 	procd_open_instance
+-	procd_set_param command "$PROG" -F -P "$pid_file"
++	procd_set_param command "$PROG" -F -P "$pid_file" -R
+ 	[ "${PasswordAuth}" -eq 0 ] && procd_append_param command -s
+ 	[ "${GatewayPorts}" -eq 1 ] && procd_append_param command -a
+ 	[ "${RootPasswordAuth}" -eq 0 ] && procd_append_param command -g
+@@ -80,34 +80,8 @@
+ 	procd_close_instance
+ }
+
+-keygen()
+-{
+-	for keytype in rsa dss; do
+-		# check for keys
+-		key=dropbear/dropbear_${keytype}_host_key
+-		[ -f /tmp/$key -o -s /etc/$key ] || {
+-			# generate missing keys
+-			mkdir -p /tmp/dropbear
+-			[ -x /usr/bin/dropbearkey ] && {
+-				/usr/bin/dropbearkey -t $keytype -f /tmp/$key 2>&- >&- && exec /etc/rc.common "$initscript" start
+-			} &
+-		exit 0
+-		}
+-	done
+-
+-	lock /tmp/.switch2jffs
+-	mkdir -p /etc/dropbear
+-	mv /tmp/dropbear/dropbear_* /etc/dropbear/
+-	lock -u /tmp/.switch2jffs
+-	#chown root /etc/dropbear
+-	#chmod 0700 /etc/dropbear
+-}
+-
+ start_service()
+ {
+-	[ -s /etc/dropbear/dropbear_rsa_host_key -a \
+-	  -s /etc/dropbear/dropbear_dss_host_key ] || keygen
+-
+ 	. /lib/functions.sh
+ 	. /lib/functions/network.sh
+

patches/0000_dropbear/patch.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+set -e
+
+ROOTFS=$1
+
+echo "Updating dropbear to 2020.81"
+cp -f dropbear "$ROOTFS/usr/sbin/dropbear"
+
+# Generate host keys as needed (so we must not have placeholders)
+patch -p1 -d "$ROOTFS" < generate-host-keys-as-needed.patch
+rm -f "$ROOTFS"/etc/dropbear/dropbear_*_host_key
+
+# Make dropbear config (host keys, authorized_keys) persistent
+echo "/etc/dropbear:/configs/etc/dropbear" >> "$ROOTFS/usr/cfg/etc.cfg"

toolchain.mk

@@ -0,0 +1,27 @@
+TOOLCHAIN_NAME = OpenWrt-Toolchain-lantiq-for-mips_34kc+dsp-gcc-4.8-linaro_uClibc-0.9.33.2
+TOOLCHAIN_TAR = $(TOOLCHAIN_NAME).tar.bz2
+TOOLCHAIN_URL = https://archive.openwrt.org/barrier_breaker/14.07/lantiq/xrx200/$(TOOLCHAIN_TAR)
+
+ROOT_DIR := $(dir $(abspath $(lastword $(MAKEFILE_LIST))))
+CROSS_COMPILE = $(ROOT_DIR)/$(TOOLCHAIN_NAME)/toolchain-mips_34kc+dsp_gcc-4.8-linaro_uClibc-0.9.33.2/bin/mips-openwrt-linux-uclibc-
+
+# We use the *.bin directly : the wrapper is not needed and add rpath...
+export AR = $(CROSS_COMPILE)ar
+export AS = $(CROSS_COMPILE)as.bin
+export CC = $(CROSS_COMPILE)gcc.bin
+export CXX = $(CROSS_COMPILE)g++.bin
+export LD = $(CROSS_COMPILE)ld.bin
+export OBJCOPY = $(CROSS_COMPILE)objcopy
+export RANLIB = $(CROSS_COMPILE)ranlib
+export STRIP = $(CROSS_COMPILE)strip
+
+# Prevent "warning: environment variable 'STAGING_DIR' not defined"
+export STAGING_DIR
+
+toolchain: $(CC)
+
+$(ROOT_DIR)/$(TOOLCHAIN_TAR):
+	wget -O $@ $(TOOLCHAIN_URL)
+
+$(CC): | $(ROOT_DIR)/$(TOOLCHAIN_TAR)
+	tar -C $(ROOT_DIR) -xf $(TOOLCHAIN_TAR)