Guideline about cryptography with RMS

[LeaFrock]

I've read a related issue #131. But about 3 years passed, something changes and it doesn't solve my confusion.

I'm going to use RMS in a list of ASP.NET Core 8 web APIs. Some of them need to encrypt the user-committed content (such as a form-file) with AES (CBC mode for example).

With the help of Issue#131, I write the following codes,

        private async Task<byte[]> EncryptCore(IFormFile file, byte[] key, byte[] iv)
        {
            using var ms = RMS.GetStream(Guid.NewGuid(), "myTag", file.Length);
            using var aes = Aes.Create();
            aes.Key = key;
            aes.IV = iv;
            using var transform = aes.CreateEncryptor(key, iv);
            using var cs = new CryptoStream(ms, transform, CryptoStreamMode.Write);
            await file.CopyToAsync(cs);
            await cs.FlushFinalBlockAsync();
            var data = ms.ToArray(); // Is it risky?
            return data;
        }

But the README says,

Most applications should not call ToArray and should avoid calling GetBuffer if possible.

Also, .NET 6+ adds new simplified cryptography APIs such as AES.EncryptCbc.

        private async Task<byte[]> EncryptCore(IFormFile file, byte[] key, byte[] iv)
        {
            using var ms = RMS.GetStream(Guid.NewGuid(), "myTag", file.Length);
            await file.CopyToAsync(ms);
            using var aes = Aes.Create();
            aes.Key = key;
            aes.IV = iv;
            var data = aes.EncryptCbc(ms.ToArray(), iv, PaddingMode.PKCS7); // ToArray seems to be risky. How should I do so?
            return data;
        }

The README says GetReadOnlySequence() should be used for reading, but EncryptCbc only accepts byte[] and ReadOnlySpan<byte>. I'm not sure whether it's possible to use RMS here.

Overall, I hope you can give the guideline. Thanks in advance!

[Ivan Maximov]

You may use ToArray safely in both cases but the main idea for RMS usage is to not allocate arrays again and again. The all project is about reusing buffers. If called/returning APIs support only arrays then you may find RMS useless. In the second example I suggest to use byte[] EncryptCbc(readonlyspan) overload so you save one array allocation.

[LeaFrock]

I appreciate your reply @sungam3r .

In the second example I suggest to use byte[] EncryptCbc(readonlyspan) overload so you save one array allocation

Would you mind paste some codes as a demo here? I don't know how to convert ReadOnlySequence to Span. What I know is the following way, which seems not correct when using AES.

foreach(var m in ms.GetReadOnlySequence())
{
    byte[] block = aes.EncryptCbc(m.Span, iv, PaddingMode.PKCS7); // Then combine several blocks into a full byte array? Seems not correct.
}

(updated)
Well, I realize the way. Do you mean using GetBuffer instead?

        private async Task<byte[]> EncryptCore(IFormFile file, byte[] key, byte[] iv)
        {
            using var ms = RMS.GetStream(Guid.NewGuid(), "myTag", file.Length);
            await file.CopyToAsync(ms);
            using var aes = Aes.Create();
            aes.Key = key;
            var buffer = ms.GetBuffer();
            var data = aes.EncryptCbc(buffer.AsSpan(0, (int)ms.Length), iv, PaddingMode.PKCS7);
            return data;
        }

[Ivan Maximov]

Yes - just use span projection over buffer returned from RMS.

[Ivan Maximov]

In that way RMS ecosystem does not allocate arrays. But you as the caller of .EncryptCbc still allocate since that API was designed to return array. You may try to find other overload or combination of calls to save another array allocation if clients of EncryptCore method can work with shared buffers as well.

[LeaFrock]

You may try to find other overload...

Got it. It's another issue but still thanks a lot!