OpenSSH key parsing: minor clean-ups in new AEAD code paths

Guard against corrupted key files with excessive or negative key data
lengths (AEAD encryption only; other code paths were already guarding
against this.)

Avoid an extra copy of decrypted key data if no MAC is used by the
cipher.

sshd-common/src/main/java/org/apache/sshd/common/config/keys/loader/openssh/OpenSSHKeyPairResourceParser.java

@@ -154,7 +154,7 @@ public Collection<KeyPair> extractKeyPairs(
 
         CipherFactory cipherSpec = BuiltinCiphers.resolveFactory(cipher);
         if (cipherSpec == null || !cipherSpec.isSupported()) {
-            throw new NoSuchAlgorithmException("Unsupported cipher: " + cipher);
+            throw new NoSuchAlgorithmException("Unsupported cipher: " + cipher + " for encrypted key in " + resourceKey);
         }
 
         byte[] encryptedData;
@@ -164,6 +164,13 @@ public Collection<KeyPair> extractKeyPairs(
             // to follow the payload data.
             int authTokenLength = cipherSpec.getAuthenticationTagSize();
             int encryptedLength = KeyEntryResolver.decodeInt(stream);
+            if (encryptedLength < 0) {
+                throw new StreamCorruptedException(
+                        "Key length " + encryptedLength + " negative for encrypted key in " + resourceKey);
+            } else if (encryptedLength > MAX_PRIVATE_KEY_DATA_SIZE) {
+                throw new StreamCorruptedException("Key length " + encryptedLength + " > allowed maximum "
+                                                   + MAX_PRIVATE_KEY_DATA_SIZE + " for encrypted key in " + resourceKey);
+            }
             encryptedData = new byte[encryptedLength + authTokenLength];
             IoUtils.readFully(stream, encryptedData);
         } else {

sshd-common/src/main/java/org/apache/sshd/common/config/keys/loader/openssh/kdf/BCryptKdfOptions.java

@@ -137,6 +137,12 @@ public byte[] decodePrivateKeyBytes(
             int macLength = cipherSpec.getAuthenticationTagSize();
             cipher.init(Cipher.Mode.Decrypt, kv, iv);
             cipher.update(sensitive, 0, sensitive.length - macLength);
+            if (macLength == 0) {
+                // Avoid an extra copy
+                byte[] result = sensitive;
+                sensitive = null; // Don't clear in finalization
+                return result;
+            }
             return Arrays.copyOf(sensitive, sensitive.length - macLength);
         } catch (RuntimeException e) {
             Throwable t = ExceptionUtils.peelException(e);