have cipher specify header size

Really it is more about mimimum cipher block size.

 Please enter the commit message for your changes. Lines starting

src/crypto/mod.rs

@@ -11,6 +11,7 @@ pub enum CipherError {
 pub trait PSOCipher {
     fn encrypt(&mut self, data: &Vec<u8>) -> Result<Vec<u8>, CipherError>;
     fn decrypt(&mut self, data: &Vec<u8>) -> Result<Vec<u8>, CipherError>;
+    fn header_size(&self) -> usize;
 }
 
 
@@ -26,4 +27,8 @@ impl PSOCipher for NullCipher {
     fn decrypt(&mut self, data: &Vec<u8>) -> Result<Vec<u8>, CipherError> {
         Ok(data.clone())
     }
+
+    fn header_size(&self) -> usize {
+        4
+    }
 }

src/crypto/pc.rs

@@ -111,6 +111,10 @@ impl PSOCipher for PSOPCCipher {
     fn decrypt(&mut self, data: &Vec<u8>) -> Result<Vec<u8>, CipherError> {
         self.encrypt(data)
     }
+
+    fn header_size(&self) -> usize {
+        4
+    }
 }