Validate 7z AES KDF limits before unsalted key derivation

docs/API.md

@@ -486,6 +486,8 @@ using (var archive = ZipArchive.OpenArchive("file.zip"))
 
 `CheckCrc` validates archive-level payload checksums when the format stores reliable metadata, such as ZIP CRC32 values. Formats without payload checksums skip this validation. Decompressor integrity checks that are required to decode a stream may still fail even when `CheckCrc` is disabled.
 
+7z AES key derivation accepts iteration exponents 0 through 24 and the special `0x3F` direct-key mode. Larger iteration exponents throw `NotSupportedException` before key derivation, including when salt and IV are absent.
+
 When using `SymbolicLinkHandler`, directory extraction rejects link targets outside the extraction root and never follows symbolic links or reparse points while extracting later entries. The handler itself remains trusted application code.
 
 ### Options matrix

src/SharpCompress/Compressors/LZMA/AesDecoderStream.cs

@@ -24,7 +24,7 @@ internal sealed partial class AesDecoderStream : DecoderStream2
     public AesDecoderStream(Stream input, byte[] info, IPasswordProvider pass, long limit)
     {
         var password = pass.CryptoGetTextPassword();
-        if (password == null)
+        if (password is null)
         {
             throw new SharpCompress.Common.CryptographicException(
                 "Encrypted 7Zip archive has no password specified."
@@ -43,7 +43,7 @@ internal sealed partial class AesDecoderStream : DecoderStream2
 
         var passwordBytes = Encoding.Unicode.GetBytes(password);
         var key = InitKey(numCyclesPower, salt, passwordBytes);
-        if (key == null)
+        if (key is null)
         {
             throw new ArchiveOperationException("Initialized with null key");
         }
@@ -147,13 +147,24 @@ internal sealed partial class AesDecoderStream : DecoderStream2
 
     private void Init(byte[] info, out int numCyclesPower, out byte[] salt, out byte[] iv)
     {
+        if (info.Length == 0)
+        {
+            throw new InvalidFormatException("Missing 7z AES properties.");
+        }
         var bt = info[0];
         numCyclesPower = bt & 0x3F;
 
+        // Check before the no-salt/no-IV return, and before starting expensive key derivation.
+        // 0x3F is the format's direct-key mode, not an iteration exponent.
+        if (numCyclesPower > 24 && numCyclesPower != 0x3F)
+        {
+            throw new NotSupportedException("Unsupported 7z AES KDF count.");
+        }
+
         if ((bt & 0xC0) == 0)
         {
             salt = Array.Empty<byte>();
-            iv = Array.Empty<byte>();
+            iv = new byte[16];
             return;
         }
 
@@ -183,11 +194,6 @@ internal sealed partial class AesDecoderStream : DecoderStream2
         {
             iv[i] = info[i + saltSize + 2];
         }
-
-        if (numCyclesPower > 24)
-        {
-            throw new NotSupportedException();
-        }
     }
 
     private byte[]? InitKey(int mNumCyclesPower, byte[] salt, byte[] pass)

tests/SharpCompress.Test/SevenZip/SevenZipAesKdfTests.cs

@@ -0,0 +1,35 @@
+using System;
+using System.IO;
+using SharpCompress.Compressors.LZMA;
+using SharpCompress.Compressors.LZMA.Utilities;
+using Xunit;
+
+namespace SharpCompress.Test.SevenZip;
+
+public class SevenZipAesKdfTests
+{
+    [Theory]
+    [InlineData(25)]
+    [InlineData(31)]
+    [InlineData(62)]
+    public void SevenZipAes_Rejects_Excessive_Kdf_Without_Salt(int exponent)
+    {
+        using var input = new MemoryStream();
+        Assert.Throws<NotSupportedException>(() =>
+            new AesDecoderStream(input, [(byte)exponent], new Password(), 0)
+        );
+    }
+
+    [Fact]
+    public void SevenZipAes_Allows_Direct_Key_Without_Iv()
+    {
+        using var input = new MemoryStream();
+        using var decoder = new AesDecoderStream(input, [0x3F], new Password(), 0);
+        Assert.Equal(0, decoder.Read(new byte[1], 0, 1));
+    }
+
+    private sealed class Password : IPasswordProvider
+    {
+        public string CryptoGetTextPassword() => "test";
+    }
+}