Why Am I Getting a 'Illegal Base64 Data at Input Byte 4' Error When Decoding a Data URI?

Base64 Decoding Error: "Illegal Base64 Data at Input Byte 4"

When attempting to decode a Base64-encoded string using base64.StdEncoding.DecodeString(str), you may encounter the error: "illegal base64 data at input byte 4." This error indicates that the provided input is not in a valid Base64 format.

The issue in your case lies not in the Base64 encoding itself, but in the input string. Instead of a pure Base64 string, you are trying to decode a Data URI scheme.

A Data URI scheme represents data within a web page as inline content, following this format:

data:[][;charset=][;base64],

In your case, the input is a Data URI scheme containing an image/png MIME type and Base64-encoded data. To decode it, you first need to extract the Base64 portion.

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

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

Output:

iVBORw0KGgoAAAANSUhEUgAAAGQAAABkCAYA

Now you can decode the extracted Base64 string:

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

Output:

[137 80 78 71 13 10 26 10 0 0 0 13 73 72 68 82 0 0 0 100 0 0 0 100 8 6 0]

By extracting the Base64 data from the Data URI scheme and performing the decoding process, you can successfully decode the Base64-encoded content.

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