Javascript - Amazon S3 direct file upload from client browser (CORS) - AWS Signature Version 4

I am trying to upload some images directly to the Amazon Simple Storage Service (S3) located in Frankfurt (eu-central-1). With this method: Authenticating Requests in Browser-Based Uploads Using POST (AWS Signature Version 4)

On the server side I am creating the policy & signature and on the client side I am building the request. (For the answers everything can be done in the client side, security aspects will come later.)

But I am always getting a error from S3:

<Error>
    <Code>InvalidRequest</Code>
    <Message>The authorization mechanism you have provided is not supported. Please use AWS4-HMAC-SHA256.</Message>
    <RequestId>blah</RequestId>              
    <HostId>blah</HostId>
</Error>

So my questions:

• How to create a correct policy?


• How to create a correct signingkey?


• How to create the signature (with the signingkey & policy)?


• How looks a correct HTML form?

Thanks in advance for any help