CryptoStream.FlushFinalBlockAsync method missing

[rupertsciamenna89]

Background and Motivation

I'm creating a web API for file encryption with an AES Key. I creates an helper functions that generate an instance of EncryptedFileResult (an extension of ActionResult class that I created) where I encrypt the file.

I override the two methods ExecuteResult and ExecuteResultAsync.

The body of ExecuteResultAsync is the next:

var fs = GetCryptoStream(ms, Cypher);
await fs.CopyToAsync(ms);
fs.FlushFinalBlock();
await fs.FlushAsync();

When the program runs, at the line fs.FlushFinalBlock();, it generates an exception:

'System.InvalidOperationException' in Microsoft.AspNetCore.Server.IIS.dll
Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.

For the moment I set the AllowSynchronousIO to true, but it will be good to have an async overload for the FlushFinalBlock, to have the benefits of async methods.
I would like to implements it.

Proposed API

partial class CryptoStream
{
    public ValueTask FlushFinalBlockAsync(CancellationToken cancellationToken = default) => throw null;
}

Usage Examples

Previous code could be this:

var fs = GetCryptoStream(ms, Cypher);
await fs.CopyToAsync(ms);
await fs.FlushFinalBlockAsync();
await fs.FlushAsync();

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[vcsjones]

cc @bartonjs

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[vcsjones]

It looks like the FlushFinalBlock implementation is already async:

runtime/src/libraries/System.Security.Cryptography.Primitives/src/System/Security/Cryptography/CryptoStream.cs

Line 106 in 643f5e1

private async ValueTask FlushFinalBlockAsync(bool useAsync)

Though a public version is not directly exposed. However, DisposeAsync will perform the asynchronous flush final block.

Assuming you are done with the CryptoStream as well, you could use DisposeAsync to cause the final block to be flushed, asynchronously.

runtime/src/libraries/System.Security.Cryptography.Primitives/src/System/Security/Cryptography/CryptoStream.cs

Line 695 in 643f5e1

await FlushFinalBlockAsync(useAsync: true).ConfigureAwait(false);

This might be a good work around for now instead of setting AllowSynchronousIO.

[bartonjs]

@stephentoub It doesn't look like it occurred to me to ask the question back then (dotnet/corefx#33422), but: Did you consider adding public (Value)Task FlushFinalBlockAsync(CancellationToken cancellationToken=default) when you added DisposeAsync()?

I'm guessing it was just a narrow scope of "override DisposeAsync in all the right places"

[stephentoub]

Did you consider adding public (Value)Task FlushFinalBlockAsync(CancellationToken cancellationToken=default) when you added DisposeAsync()?

I don't think I considered it.

I'm guessing it was just a narrow scope of "override DisposeAsync in all the right places"

Yup.

[bartonjs]

Seems like we can do

partial class CryptoStream
{
    public ValueTask FlushFinalBlockAsync(CancellationToken cancellationToken = default) =>
        FlushFinalBlockAsync(useAsync: true, cancellationToken);
}

And plumb the token into all of the relevant places in the existing private FlushFinalBlockAsync(bool)

[terrajobst]

Video

Looks good as proposed.

namespace System.Security.Cryptography
{
    partial class CryptoStream
    {
        public ValueTask FlushFinalBlockAsync(CancellationToken cancellationToken = default);
    }
}