AesGcmStrict 1.0.2

.NET 6.0 .NET Standard 2.1
NuGet\Install-Package AesGcmStrict -Version 1.0.2
This command is intended to be used within the Package Manager Console in Visual Studio, as it uses the NuGet module's version of Install-Package.
dotnet add package AesGcmStrict --version 1.0.2
<PackageReference Include="AesGcmStrict" Version="1.0.2" />
For projects that support PackageReference, copy this XML node into the project file to reference the package.
paket add AesGcmStrict --version 1.0.2
#r "nuget: AesGcmStrict, 1.0.2"
#r directive can be used in F# Interactive, C# scripting and .NET Interactive. Copy this into the interactive tool or source code of the script to reference the package.
// Install AesGcmStrict as a Cake Addin
#addin nuget:?package=AesGcmStrict&version=1.0.2

// Install AesGcmStrict as a Cake Tool
#tool nuget:?package=AesGcmStrict&version=1.0.2

AesGcmStrict (.NET) NuGet

by Stan Drapkin

AesGcmStrict class:

  • Wraps AesGcm and enforces AesGcm Tag to be exactly 16 bytes. Use AesGcmStrict instead of AesGcm.
  • AesGcm allows truncated Tags: any AesGcm-provided Tag truncated to ex. 12 bytes will successfully decrypt.

Example of the problem:

Span<byte> plaintext = Encoding.UTF8.GetBytes("Hello World!");
Span<byte> key = new byte[32];
Span<byte> nonce = new byte[12];
Span<byte> ciphertext = new byte[plaintext.Length];
Span<byte> tag = new byte[16]; // generating a 16-byte Tag!

using var gcm = new AesGcm(key);

Console.WriteLine(Encoding.UTF8.GetString(plaintext));

gcm.Encrypt(nonce, plaintext, ciphertext, tag);
plaintext.Clear();
tag = tag.Slice(0, 12); // truncating the Tag to ex. 12 bytes
gcm.Decrypt(nonce, ciphertext, tag, plaintext); // decrypts successfully (PROBLEM)
Console.WriteLine(Encoding.UTF8.GetString(plaintext));

Most users of AesGcm might expect ~128 bits of Tag-security, but effectively get only ~96 bits at best.

How to fix with AesGcmStrict:

  1. Add the namespace: using SecurityDriven;
  2. Replace AesGcm with AesGcmStrict

Same example with AesGcmStrict:

// using SecurityDriven;
Span<byte> plaintext = Encoding.UTF8.GetBytes("Hello World!");
Span<byte> key = new byte[32];
Span<byte> nonce = new byte[12];
Span<byte> ciphertext = new byte[plaintext.Length];
Span<byte> tag = new byte[16]; // generating a 16-byte Tag!

using var gcm = new AesGcmStrict(key); // switching to AesGcmStrict

Console.WriteLine(Encoding.UTF8.GetString(plaintext));

gcm.Encrypt(nonce, plaintext, ciphertext, tag);
plaintext.Clear();
tag = tag.Slice(0, 12); // truncating the Tag to ex. 12 bytes
gcm.Decrypt(nonce, ciphertext, tag, plaintext); // throws ArgumentException (tag must be 16 bytes)
Console.WriteLine(Encoding.UTF8.GetString(plaintext));
Product Versions
.NET net5.0 net5.0-windows net6.0 net6.0-android net6.0-ios net6.0-maccatalyst net6.0-macos net6.0-tvos net6.0-windows
.NET Core netcoreapp3.0 netcoreapp3.1
.NET Standard netstandard2.1
MonoAndroid monoandroid
MonoMac monomac
MonoTouch monotouch
Tizen tizen60
Xamarin.iOS xamarinios
Xamarin.Mac xamarinmac
Xamarin.TVOS xamarintvos
Xamarin.WatchOS xamarinwatchos
Compatible target framework(s)
Additional computed target framework(s)
Learn more about Target Frameworks and .NET Standard.
  • .NETStandard 2.1

    • No dependencies.
  • net6.0

    • No dependencies.

NuGet packages

This package is not used by any NuGet packages.

GitHub repositories

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Version Downloads Last updated
1.0.2 91 6/30/2022

Replacement for System.Security.Cryptography.AesGcm.