[PATCH v4 1/3] dropbear: create a submenu for public key algorithms

Rui Salvaterra rsalvaterra at gmail.com
Tue Dec 15 11:59:40 EST 2020 

This allows the user to select only the public key algorithms (s)he requires
(e.g., disabling RSA and keeping only Ed25519). The default selection maintains
the current functionality.

Additionally, make sure at least one public key algorithm is selected, lest the
build would fail.

Dropbear executable sizes (ath79, -O2):

	RSA + Ed25519:	210101 bytes
	RSA only:	197765 bytes
	Ed25519 only:	189637 bytes

Signed-off-by: Rui Salvaterra <rsalvaterra at gmail.com>
---
 package/network/services/dropbear/Config.in   | 27 ++++++++++++++-----
 package/network/services/dropbear/Makefile    | 23 +++++++++++-----
 .../dropbear/files/dropbear.failsafe.ecc      |  8 ++++++
 .../dropbear/files/dropbear.failsafe.ed25519  |  8 ++++++
 ...ropbear.failsafe => dropbear.failsafe.rsa} |  0
 ...nkey-fix-use-of-rsa-sha2-256-pubkeys.patch | 12 ++++++---
 6 files changed, 62 insertions(+), 16 deletions(-)
 create mode 100644 package/network/services/dropbear/files/dropbear.failsafe.ecc
 create mode 100644 package/network/services/dropbear/files/dropbear.failsafe.ed25519
 rename package/network/services/dropbear/files/{dropbear.failsafe => dropbear.failsafe.rsa} (100%)
 mode change 100755 => 100644

diff --git a/package/network/services/dropbear/Config.in b/package/network/services/dropbear/Config.in
index 15000eff53..5b7be04ade 100644
--- a/package/network/services/dropbear/Config.in
+++ b/package/network/services/dropbear/Config.in
@@ -1,14 +1,13 @@
 menu "Configuration"
 	depends on PACKAGE_dropbear
 
-config DROPBEAR_CURVE25519
-	bool "Curve25519 support"
+menu "Public key algorithm selection"
+
+config DROPBEAR_RSA
+	bool "RSA support"
 	default y
 	help
-		This enables the following key exchange algorithm:
-		  curve25519-sha256 at libssh.org
-
-		Increases binary size by about 4 kB (MIPS).
+		Enable support for the RSA public key algorithm.
 
 config DROPBEAR_ECC
 	bool "Elliptic curve cryptography (ECC)"
@@ -58,6 +57,13 @@ config DROPBEAR_ED25519
 
 		Increases binary size by about 12 kB (MIPS).
 
+config DROPBEAR_AUTOSEL_PK
+	def_bool y
+	depends on !(DROPBEAR_ECC || DROPBEAR_ED25519)
+	select DROPBEAR_RSA
+
+endmenu
+
 config DROPBEAR_CHACHA20POLY1305
 	bool "Chacha20-Poly1305 support"
 	default y
@@ -67,6 +73,15 @@ config DROPBEAR_CHACHA20POLY1305
 
 		Increases binary size by about 4 kB (MIPS).
 
+config DROPBEAR_CURVE25519
+	bool "Curve25519 support"
+	default y
+	help
+		This enables the following key exchange algorithm:
+		  curve25519-sha256 at libssh.org
+
+		Increases binary size by about 4 kB (MIPS).
+
 config DROPBEAR_ZLIB
 	bool "Enable compression"
 	default n
diff --git a/package/network/services/dropbear/Makefile b/package/network/services/dropbear/Makefile
index 8bbb26f829..d0b6a4b7ea 100644
--- a/package/network/services/dropbear/Makefile
+++ b/package/network/services/dropbear/Makefile
@@ -32,7 +32,8 @@ PKG_CONFIG_DEPENDS:= \
 	CONFIG_DROPBEAR_CURVE25519 CONFIG_DROPBEAR_ZLIB \
 	CONFIG_DROPBEAR_ED25519 CONFIG_DROPBEAR_CHACHA20POLY1305 \
 	CONFIG_DROPBEAR_UTMP CONFIG_DROPBEAR_PUTUTLINE \
-	CONFIG_DROPBEAR_DBCLIENT CONFIG_DROPBEAR_SCP CONFIG_DROPBEAR_ASKPASS
+	CONFIG_DROPBEAR_DBCLIENT CONFIG_DROPBEAR_SCP CONFIG_DROPBEAR_ASKPASS \
+	CONFIG_DROPBEAR_RSA
 
 include $(INCLUDE_DIR)/package.mk
 
@@ -67,9 +68,9 @@ define Package/dropbear/description
 endef
 
 define Package/dropbear/conffiles
+$(if $(CONFIG_DROPBEAR_RSA),/etc/dropbear/dropbear_rsa_host_key)
 $(if $(CONFIG_DROPBEAR_ED25519),/etc/dropbear/dropbear_ed25519_host_key)
 $(if $(CONFIG_DROPBEAR_ECC),/etc/dropbear/dropbear_ecdsa_host_key)
-/etc/dropbear/dropbear_rsa_host_key
 /etc/config/dropbear
 endef
 
@@ -137,7 +138,7 @@ DB_OPT_CONFIG = \
 	!!DROPBEAR_ECC_384|CONFIG_DROPBEAR_ECC_FULL|1|0 \
 	!!DROPBEAR_ECC_521|CONFIG_DROPBEAR_ECC_FULL|1|0 \
 	DROPBEAR_CLI_ASKPASS_HELPER|CONFIG_DROPBEAR_ASKPASS|1|0 \
-
+	DROPBEAR_RSA|CONFIG_DROPBEAR_RSA|1|0 \
 
 TARGET_CFLAGS += -DARGTYPE=3 -ffunction-sections -fdata-sections -flto
 TARGET_LDFLAGS += -Wl,--gc-sections -flto=jobserver
@@ -199,10 +200,18 @@ define Package/dropbear/install
 	$(INSTALL_DIR) $(1)/usr/lib/opkg/info
 	$(INSTALL_DIR) $(1)/etc/dropbear
 	$(INSTALL_DIR) $(1)/lib/preinit
-	$(INSTALL_DATA) ./files/dropbear.failsafe $(1)/lib/preinit/99_10_failsafe_dropbear
-	$(if $(CONFIG_DROPBEAR_ED25519),touch $(1)/etc/dropbear/dropbear_ed25519_host_key)
-	$(if $(CONFIG_DROPBEAR_ECC),touch $(1)/etc/dropbear/dropbear_ecdsa_host_key)
-	touch $(1)/etc/dropbear/dropbear_rsa_host_key
+
+ifdef CONFIG_DROPBEAR_ED25519
+	$(INSTALL_DATA) ./files/dropbear.failsafe.ed25519 $(1)/lib/preinit/99_10_failsafe_dropbear
+else ifdef CONFIG_DROPBEAR_ECC
+	$(INSTALL_DATA) ./files/dropbear.failsafe.ecc $(1)/lib/preinit/99_10_failsafe_dropbear
+else ifdef CONFIG_DROPBEAR_RSA
+	$(INSTALL_DATA) ./files/dropbear.failsafe.rsa $(1)/lib/preinit/99_10_failsafe_dropbear
+endif
+
+	$(if $(CONFIG_DROPBEAR_ED25519),touch $(1)/etc/dropbear/dropbear_ed25519_host_key,)
+	$(if $(CONFIG_DROPBEAR_ECC),touch $(1)/etc/dropbear/dropbear_ecdsa_host_key,)
+	$(if $(CONFIG_DROPBEAR_RSA),touch $(1)/etc/dropbear/dropbear_rsa_host_key,)
 endef
 
 define Package/dropbearconvert/install
diff --git a/package/network/services/dropbear/files/dropbear.failsafe.ecc b/package/network/services/dropbear/files/dropbear.failsafe.ecc
new file mode 100644
index 0000000000..924938bd55
--- /dev/null
+++ b/package/network/services/dropbear/files/dropbear.failsafe.ecc
@@ -0,0 +1,8 @@
+#!/bin/sh
+
+failsafe_dropbear () {
+	dropbearkey -t ecdsa -s 256 -f /tmp/dropbear_failsafe_host_key
+	dropbear -r /tmp/dropbear_failsafe_host_key <> /dev/null 2>&1
+}
+
+boot_hook_add failsafe failsafe_dropbear
diff --git a/package/network/services/dropbear/files/dropbear.failsafe.ed25519 b/package/network/services/dropbear/files/dropbear.failsafe.ed25519
new file mode 100644
index 0000000000..46b4918014
--- /dev/null
+++ b/package/network/services/dropbear/files/dropbear.failsafe.ed25519
@@ -0,0 +1,8 @@
+#!/bin/sh
+
+failsafe_dropbear () {
+	dropbearkey -t ed25519 -f /tmp/dropbear_failsafe_host_key
+	dropbear -r /tmp/dropbear_failsafe_host_key <> /dev/null 2>&1
+}
+
+boot_hook_add failsafe failsafe_dropbear
diff --git a/package/network/services/dropbear/files/dropbear.failsafe b/package/network/services/dropbear/files/dropbear.failsafe.rsa
old mode 100755
new mode 100644
similarity index 100%
rename from package/network/services/dropbear/files/dropbear.failsafe
rename to package/network/services/dropbear/files/dropbear.failsafe.rsa
diff --git a/package/network/services/dropbear/patches/910-signkey-fix-use-of-rsa-sha2-256-pubkeys.patch b/package/network/services/dropbear/patches/910-signkey-fix-use-of-rsa-sha2-256-pubkeys.patch
index b774a38b1a..b2846ea87b 100644
--- a/package/network/services/dropbear/patches/910-signkey-fix-use-of-rsa-sha2-256-pubkeys.patch
+++ b/package/network/services/dropbear/patches/910-signkey-fix-use-of-rsa-sha2-256-pubkeys.patch
@@ -21,18 +21,24 @@ Signed-off-by: Petr Štetiar <ynezz at true.cz>
 
 --- a/signkey.c
 +++ b/signkey.c
-@@ -657,8 +657,12 @@ int buf_verify(buffer * buf, sign_key *k
+@@ -657,9 +657,19 @@ int buf_verify(buffer * buf, sign_key *k
  	sigtype = signature_type_from_name(type_name, type_name_len);
  	m_free(type_name);
  
--	if (expect_sigtype != sigtype) {
--			dropbear_exit("Non-matching signing type");
++#if DROPBEAR_RSA
 +	if (sigtype == DROPBEAR_SIGNATURE_NONE) {
 +		dropbear_exit("No signature type");
 +	}
 +
 +	if ((expect_sigtype != DROPBEAR_SIGNATURE_RSA_SHA256) && (expect_sigtype != sigtype)) {
++		dropbear_exit("Non-matching signing type");
++	}
++#else
+ 	if (expect_sigtype != sigtype) {
+-			dropbear_exit("Non-matching signing type");
 +		dropbear_exit("Non-matching signing type");
  	}
++#endif
  
  	keytype = signkey_type_from_signature(sigtype);
+ #if DROPBEAR_DSS
-- 
2.29.2

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