pv/crypto: Improve comment about the AES 256 GCM key

The size of an AES key depends on the mode of operation, so add this
information at least in the comments, as the API cannot be changed as it
is already published.

Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

rust/pv/src/crypto.rs

@@ -18,7 +18,7 @@ use openssl::{
 };
 use std::{convert::TryInto, ops::Range};
 
-/// An AES256-key that will purge itself out of the memory when going out of scope
+/// An AES256-GCM key that will purge itself out of the memory when going out of scope
 pub type Aes256Key = Confidential<[u8; 32]>;
 pub(crate) const AES_256_GCM_TAG_SIZE: usize = 16;
 
@@ -31,7 +31,7 @@ pub(crate) type Sha256Hash = [u8; SHA_256_HASH_SIZE as usize];
 #[non_exhaustive]
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum SymKeyType {
-    /// AES 256 key (32 bytes)
+    /// AES 256 GCM key (32 bytes)
     Aes256,
 }
 
@@ -39,7 +39,7 @@ pub enum SymKeyType {
 #[non_exhaustive]
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum SymKey {
-    /// AES 256 key (32 bytes)
+    /// AES 256 GCM key (32 bytes)
     Aes256(Aes256Key),
 }