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

Troubleshooting "Illegal Base64 Data at Input Byte 4" Error in Base64 Decoding

While using base64.StdEncoding.DecodeString(str), an error indicating "illegal base64 data at input byte 4" might occur. This issue arises when the input string provided for decoding contains non-Base64 encoded data.

Understanding Data URI Scheme

Often, the input string is not directly Base64 encoded but rather a part of a Data URI scheme. This scheme embeds data within web pages as inline resources using the following format:

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

In the case of the provided error, the input string represents a Data URI with the image/png MIME type. To extract the actual Base64 encoded data:

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

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

This eliminates the prefix and leaves only the Base64 encoded data.

Decoding the Extracted Base64 Data

Once the Base64 encoded data (b64data) is obtained, it can be decoded using the base64.StdEncoding.DecodeString() function to extract the raw data. For example:

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

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