Add Ed25519 support (#91)

* Add support for Ed25519 as a public key type

Ed25519 is a elliptic curve signature scheme that offers
better security than ECDSA and DSA and good performance. It may be
used for both user and host keys.

OpenSSH key import and fuzzer are not supported yet.

Initially inspired by Peter Szabo.

* Add curve25519 and ed25519 fuzzers

* Add import and export of Ed25519 keys

signkey.h

@@ -28,6 +28,7 @@
 #include "buffer.h"
 #include "dss.h"
 #include "rsa.h"
+#include "ed25519.h"
 
 enum signkey_type {
 #if DROPBEAR_RSA
@@ -41,6 +42,9 @@ enum signkey_type {
 	DROPBEAR_SIGNKEY_ECDSA_NISTP384,
 	DROPBEAR_SIGNKEY_ECDSA_NISTP521,
 #endif /* DROPBEAR_ECDSA */
+#if DROPBEAR_ED25519
+	DROPBEAR_SIGNKEY_ED25519,
+#endif
 	DROPBEAR_SIGNKEY_NUM_NAMED,
 	DROPBEAR_SIGNKEY_ECDSA_KEYGEN = 70, /* just "ecdsa" for keygen */
 	DROPBEAR_SIGNKEY_ANY = 80,
@@ -78,6 +82,9 @@ struct SIGN_key {
 	ecc_key * ecckey521;
 #endif
 #endif
+#if DROPBEAR_ED25519
+	dropbear_ed25519_key * ed25519key;
+#endif
 };
 
 typedef struct SIGN_key sign_key;