Why Does base64.StdEncoding.DecodeString(str) Fail with 'illegal base64 data at input byte 4' Error?

"Illegal base64 data at input byte 4" When Using base64.StdEncoding.DecodeString(str)

When attempting to decode a Base64Image using base64.StdEncoding.DecodeString(str), users may encounter the error "illegal base64 data at input byte 4." This occurs because the input string is not entirely Base64 encoded.

The input provided is a Data URI scheme, a format used to embed data, such as images, within web pages. It follows this structure:

data:[<MIME-type>][;charset=<encoding>][;base64],<data>

In the given example, the MIME-type is "image/png," and ";base64" indicates that the data component is Base64 encoded.

To extract the Base64 encoded data, remove the prefix up to and including the comma:

input := "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAGQAAABkCAYA"

b64data := input[strings.IndexByte(input, ',')+1:]

Now, you can decode the Base64 string:

data, err := base64.StdEncoding.DecodeString(b64data)
if err != nil {
    fmt.Println("error:", err)
}

This process will successfully decode the image data without encountering the "illegal base64 data at input byte 4" error.

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