added some comments to explain some cryptography-related classes and to-dos

src/com/fsck/k9/activity/MessageView.java

@@ -1031,6 +1031,10 @@ public class MessageView extends K9Activity implements OnClickListener
         mAttachments.removeAllViews();
         findSurroundingMessagesUid();
 
+        // grab a new crypto provider object, as the account may have changed, and currently
+        // the decrypted data and signature are stored in the crypto provider object
+        // TODO: separate that storage from the provider
+        // TODO: then move the provider object directly into the account object
         mCrypto = null;
         initializeCrypto();
 

src/com/fsck/k9/crypto/Apg.java

@@ -23,6 +23,9 @@ import com.fsck.k9.mail.MessagingException;
 import com.fsck.k9.mail.Part;
 import com.fsck.k9.mail.internet.MimeUtility;
 
+/**
+ * APG integration.
+ */
 public class Apg extends CryptoProvider
 {
     static final long serialVersionUID = 0x21071235;

src/com/fsck/k9/crypto/CryptoProvider.java

@@ -9,6 +9,11 @@ import android.content.Intent;
 import com.fsck.k9.Account;
 import com.fsck.k9.mail.Message;
 
+/**
+ * A CryptoProvider provides functionalities such as encryption, decryption, digital signatures.
+ * It currently also stores the results of such encryption or decryption.
+ * TODO: separate the storage from the provider
+ */
 abstract public class CryptoProvider implements Serializable
 {
     static final long serialVersionUID = 0x21071234;

src/com/fsck/k9/crypto/None.java

@@ -6,6 +6,10 @@ import android.content.Context;
 import com.fsck.k9.Account;
 import com.fsck.k9.mail.Message;
 
+/**
+ * Dummy CryptoProvider for when cryptography is disabled. It is never "available" and doesn't
+ * do anything.
+ */
 public class None extends CryptoProvider
 {
     static final long serialVersionUID = 0x21071230;