clean up some default options

- move hmac-sha2-512, hmac-md5, twofish_ctr to sysoptions.h, off by default
- try and improve text for KEX methods

default_options.h.in

@@ -9,9 +9,9 @@ Local customisation should be added to localoptions.h which is
 used if it exists. Options defined there will override any options in this
 file.
 
-Options can also be defined with -DDROPBEAR_XXX in Makefile CFLAGS
+Options can also be defined with -DDROPBEAR_XXX=[0,1] in Makefile CFLAGS
 
-IMPORTANT: Many options will require "make clean" after changes */
+IMPORTANT: Some options will require "make clean" after changes */
 
 #define DROPBEAR_DEFPORT "22"
 
@@ -41,9 +41,9 @@ IMPORTANT: Many options will require "make clean" after changes */
 #define DEBUG_TRACE 0
 
 /* Set this if you want to use the DROPBEAR_SMALL_CODE option. This can save
-several kB in binary size however will make the symmetrical ciphers and hashes
-slower, perhaps by 50%. Recommended for small systems that aren't doing
-much traffic. */
+ * several kB in binary size however will make the symmetrical ciphers and hashes
+ * slower, perhaps by 50%. Recommended for small systems that aren't doing
+ * much traffic. */
 #define DROPBEAR_SMALL_CODE 1
 
 /* Enable X11 Forwarding - server only */
@@ -52,7 +52,6 @@ much traffic. */
 /* Enable TCP Fowarding */
 /* 'Local' is "-L" style (client listening port forwarded via server)
  * 'Remote' is "-R" style (server listening port forwarded via client) */
-
 #define DROPBEAR_CLI_LOCALTCPFWD 1
 #define DROPBEAR_CLI_REMOTETCPFWD 1
 
@@ -85,8 +84,8 @@ much traffic. */
 #define DROPBEAR_AES128 1
 #define DROPBEAR_3DES 1
 #define DROPBEAR_AES256 1
-#define DROPBEAR_TWOFISH256 1
-#define DROPBEAR_TWOFISH128 1
+#define DROPBEAR_TWOFISH256 0
+#define DROPBEAR_TWOFISH128 0
 /* Compiling in Blowfish will add ~6kB to runtime heap memory usage */
 #define DROPBEAR_BLOWFISH 0
 
@@ -99,21 +98,11 @@ much traffic. */
  * and forwards compatibility */
 #define DROPBEAR_ENABLE_CTR_MODE 1
 
-/* Twofish counter mode is disabled by default because it 
-has not been tested for interoperability with other SSH implementations.
-If you test it please contact the Dropbear author */
-#define DROPBEAR_TWOFISH_CTR 0
-
 /* Message integrity. sha2-256 is recommended as a default, 
    sha1 for compatibility */
 #define DROPBEAR_SHA1_HMAC 1
 #define DROPBEAR_SHA1_96_HMAC 1
 #define DROPBEAR_SHA2_256_HMAC 1
-/* Default is to include it if sha512 is being compiled in for ECDSA */
-#define DROPBEAR_SHA2_512_HMAC (DROPBEAR_ECDSA)
-
-/* XXX needed for fingerprints */
-#define DROPBEAR_MD5_HMAC 0
 
 /* Hostkey/public key algorithms - at least one required, these are used
  * for hostkey as well as for verifying signatures with pubkey auth.
@@ -139,30 +128,33 @@ If you test it please contact the Dropbear author */
    with badly seeded /dev/urandom when systems first boot. */
 #define DROPBEAR_DELAY_HOSTKEY 1
 
-/* Enable Curve25519 for key exchange. This is another elliptic
- * curve method with good security properties. Increases binary size
- * by ~8kB on x86-64 */
-#define DROPBEAR_CURVE25519 1
-
-/* Enable elliptic curve Diffie Hellman key exchange, see note about
- * ECDSA above */
-#define DROPBEAR_ECDH 1
 
 /* Key exchange algorithm.
+
  * group14_sha1 - 2048 bit, sha1
  * group14_sha256 - 2048 bit, sha2-256
  * group16 - 4096 bit, sha2-512
  * group1 - 1024 bit, sha1
+ * curve25519 - elliptic curve DH
+ * ecdh - NIST elliptic curve DH (256, 384, 521)
  *
- * group14 is supported by most implementations.
- * group16 provides a greater strength level but is slower and increases binary size
  * group1 is too small for security though is necessary if you need 
      compatibility with some implementations such as Dropbear versions < 0.53
+ * group14 is supported by most implementations.
+ * group16 provides a greater strength level but is slower and increases binary size
+ * curve25519 and ecdh algorithms are faster than non-elliptic curve methods
+ * curve25519 increases binary size by ~8kB on x86-64
+ * including either ECDH or ECDSA increases binary size by ~30kB on x86-64
+
+ * Small systems should generally include either curve25519 or ecdh for performance.
+ * curve25519 is less widely supported but is faster
  */ 
 #define DROPBEAR_DH_GROUP1 1
 #define DROPBEAR_DH_GROUP14_SHA1 1
 #define DROPBEAR_DH_GROUP14_SHA256 1
 #define DROPBEAR_DH_GROUP16 0
+#define DROPBEAR_CURVE25519 1
+#define DROPBEAR_ECDH 1
 
 /* Control the memory/performance/compression tradeoff for zlib.
  * Set windowBits=8 for least memory usage, see your system's
@@ -178,8 +170,6 @@ If you test it please contact the Dropbear author */
 
 /* Whether to print the message of the day (MOTD). */
 #define DO_MOTD 0
-
-/* The MOTD file path */
 #define MOTD_FILENAME "/etc/motd"
 
 /* Authentication Types - at least one required.

sysoptions.h

@@ -99,6 +99,23 @@
 #define MAX_MAC_LEN 20
 #endif
 
+/* sha2-512 is not necessary unless unforseen problems arise with sha2-256 */
+#ifndef DROPBEAR_SHA2_512_HMAC
+#define DROPBEAR_SHA2_512_HMAC 0
+#endif
+
+/* might be needed for compatibility with very old implementations */
+#ifndef DROPBEAR_MD5_HMAC
+#define DROPBEAR_MD5_HMAC 0
+#endif
+
+/* Twofish counter mode is disabled by default because it 
+has not been tested for interoperability with other SSH implementations.
+If you test it please contact the Dropbear author */
+#ifndef DROPBEAR_TWOFISH_CTR
+#define DROPBEAR_TWOFISH_CTR 0
+#endif
+
 
 #define DROPBEAR_ECC ((DROPBEAR_ECDH) || (DROPBEAR_ECDSA))