dropbear: Disable legacy/insecure options

Dropbear by default enables a number of algorithms that are now considered
insecure and should only be used when legacy support is required:
   3DES encryption
   Blowfish encryption
   SHA1-96 message integrity
   CBC encryption mode
   DSA public keys
   Diffie-Hellman Group1 key exchange

So disable them by default, but add a config option for bringing them back.
Furthermore the Blowfish legacy algorithm is unconditionally disabled

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Reviewed-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/dropbear/Config.in

@@ -56,4 +56,14 @@ config BR2_PACKAGE_DROPBEAR_LASTLOG
 	  Enable logging of dropbear access to lastlog. Notice that
 	  Buildroot does not generate lastlog by default.
 
+config BR2_PACKAGE_DROPBEAR_LEGACY_CRYPTO
+	bool "enable legacy crypto"
+	help
+	  Enable legacy and possibly insecure algorithms:
+	    3DES encryption
+	    SHA1-96 message integrity
+	    CBC encryption mode
+	    DSA public keys
+	    Diffie-Hellman Group1 key exchange
+
 endif

package/dropbear/dropbear.mk

@@ -56,13 +56,23 @@ endef
 DROPBEAR_POST_EXTRACT_HOOKS += DROPBEAR_SVR_PASSWORD_AUTH
 endif
 
+define DROPBEAR_DISABLE_LEGACY_CRYPTO
+	echo '#define DROPBEAR_3DES 0'                  >> $(@D)/localoptions.h
+	echo '#define DROPBEAR_ENABLE_CBC_MODE 0'       >> $(@D)/localoptions.h
+	echo '#define DROPBEAR_SHA1_96_HMAC 0'          >> $(@D)/localoptions.h
+	echo '#define DROPBEAR_DSS 0'                   >> $(@D)/localoptions.h
+	echo '#define DROPBEAR_DH_GROUP1 0'             >> $(@D)/localoptions.h
+endef
+ifneq ($(BR2_PACKAGE_DROPBEAR_LEGACY_CRYPTO),y)
+DROPBEAR_POST_EXTRACT_HOOKS += DROPBEAR_DISABLE_LEGACY_CRYPTO
+endif
+
 define DROPBEAR_ENABLE_REVERSE_DNS
 	echo '#define DO_HOST_LOOKUP 1'                 >> $(@D)/localoptions.h
 endef
 
 define DROPBEAR_BUILD_FEATURED
 	echo '#define DROPBEAR_SMALL_CODE 0'            >> $(@D)/localoptions.h
-	echo '#define DROPBEAR_BLOWFISH 1'              >> $(@D)/localoptions.h
 	echo '#define DROPBEAR_TWOFISH128 1'            >> $(@D)/localoptions.h
 	echo '#define DROPBEAR_TWOFISH256 1'            >> $(@D)/localoptions.h
 endef