This Commented CUE demonstrates how to use the built-in function encoding/base64.Decode to decode data that is encoded as Base64.

file.cue
package example

import "encoding/base64"

// encoded contains some Base64-encoded data
encoded: "c29tZSBCYXNlNjQtZW5jb2RlZCBkYXRh"

// decoded contains the string interpolation of
// the byte array returned by base64.Decode
decoded: "\(base64.Decode(null, encoded))"
TERMINAL
$ cue export
{
    "encoded": "c29tZSBCYXNlNjQtZW5jb2RlZCBkYXRh",
    "decoded": "some Base64-encoded data"
}