Add documentation to SessionKey

pgpainless-core/src/main/java/org/pgpainless/util/SessionKey.java

@@ -9,24 +9,49 @@ import javax.annotation.Nonnull;
 import org.bouncycastle.openpgp.PGPSessionKey;
 import org.pgpainless.algorithm.SymmetricKeyAlgorithm;
 
+/**
+ * A {@link SessionKey} is the symmetric key that is used to encrypt/decrypt an OpenPGP message.
+ * The OpenPGP message header contains a copy of the session key, encrypted for the public key of each recipient.
+ */
 public class SessionKey {
 
-    private SymmetricKeyAlgorithm algorithm;
-    private byte[] key;
+    private final SymmetricKeyAlgorithm algorithm;
+    private final byte[] key;
 
+    /**
+     * Constructor to create a session key from a BC {@link PGPSessionKey} object.
+     *
+     * @param sessionKey BC session key
+     */
     public SessionKey(@Nonnull PGPSessionKey sessionKey) {
         this(SymmetricKeyAlgorithm.fromId(sessionKey.getAlgorithm()), sessionKey.getKey());
     }
 
+    /**
+     * Create a session key object from an algorithm and a key.
+     *
+     * @param algorithm algorithm
+     * @param key key
+     */
     public SessionKey(@Nonnull SymmetricKeyAlgorithm algorithm, @Nonnull byte[] key) {
         this.algorithm = algorithm;
         this.key = key;
     }
 
+    /**
+     * Return the symmetric key algorithm.
+     *
+     * @return algorithm
+     */
     public SymmetricKeyAlgorithm getAlgorithm() {
         return algorithm;
     }
 
+    /**
+     * Return the bytes of the key.
+     *
+     * @return key
+     */
     public byte[] getKey() {
         byte[] copy = new byte[key.length];
         System.arraycopy(key, 0, copy, 0, copy.length);