move crypto parts from stdi2p to crypto package

lib/crypto/encrypt.go

@@ -0,0 +1,16 @@
+package crypto
+
+type Encrypter interface {
+  // encrypt a block of data
+  // return encrypted block or nil and error if an error happened
+  Encrypt(data []byte) (enc []byte, err error)
+}
+
+type PublicEncryptionKey interface {
+
+  // create a new encrypter to encrypt data to this public key
+  NewEncrypter() (Encrypter, error)
+  
+  // length of this public key in bytes
+  Len() int
+}