Authenticate multiple inputs easily using keyed BLAKE2b in libsodium.

The standard way of safely computing a MAC for multiple inputs requires worrying about concatenating arrays and converting the length of each array to a fixed number of bytes, such as 4 bytes to represent an integer, consistently in either big- or little-endian, regardless of the endianness of the machine. This is annoying to implement and possibly less efficient than the following approach discussed by Neil Madden, author of API Security in Action.

You can find the NuGet package here.

The easiest way to install this is via the NuGet Package Manager in Visual Studio, as explained here. JetBrains Rider also has a package manager, and instructions can be found here.

1. Install the Sodium.Core NuGet package in Visual Studio.
2. Download the latest release.
3. Move the downloaded .dll file into your Visual Studio project folder.
4. Click on the Project tab and Add Project Reference... in Visual Studio.
5. Go to Browse, click the Browse button, and select the downloaded .dll file.
6. Add using MultiMAC; to the top of each code file that will use the library.

Note that the libsodium library requires the Visual C++ Redistributable for Visual Studio 2015-2019 to work on Windows. If you want your program to be portable, then you must keep the relevant (x86 or x64) vcruntime140.dll file in the same folder as your executable on Windows.

⚠️WARNING: Never use the same key for key1 and key2.

const TagLength tagLength = TagLength.BLAKE2b256;

// Both keys should be derived using a KDF in practice (e.g. Argon2, HKDF, etc)
byte[] key1 = SodiumCore.GetRandomBytes((int)tagLength);

// The keys must be the same size as the tag length
byte[] key2 = SodiumCore.GetRandomBytes((int)tagLength);

// Gather up the byte arrays to authenticate
byte[] input1 = Encoding.UTF8.GetBytes("Po");

byte[] input2 = Encoding.UTF8.GetBytes("ta");

byte[] input3 = Encoding.UTF8.GetBytes("toes");

byte[] input4 = Encoding.UTF8.GetBytes("Boil 'em, mash 'em, stick 'em in a stew");

// Compute a 256-bit tag
byte[] tag = MultiMac.Compute(key1, key2, tagLength, input1, input2, input3, input4);

// Verify a tag
bool validTag = MultiMac.Verify(tag, key1, key2, input1, input2, input3, input4);

The following benchmarks were done using BenchmarkDotNet in a .NET 6 console application. A plaintext file and 16 random bytes were used as inputs, and the key size was equal to the tag length.

• BLAKE2b refers to concatenating multiple inputs via array copying (BLAKE2b(message: additionalData || fileBytes || additionalDataLength || fileBytesLength, key)).
• MultiMAC-BLAKE2b refers to using this library, which employs a cascade construction (BLAKE2b(message: BLAKE2b(message: fileBytes, key: BLAKE2b(message: additionalData, key1)), key2)).

In sum, the MultiMAC cascade approach is noticeably faster for large inputs, slightly faster for medium inputs, and slower for very small inputs. It is also significantly easier and safer to use, which suggests it should be offered in cryptographic libraries.