Doing base64 decoding on a string in Go - string

Question

I have a particular string that I need to run base64 decode on in Go. This string looks something like this:
qU4aaakFmjaaaaI5aaa\/EN\/aaa\/SaaaJaaa6aa+nGnk=
Please note this is not the exact same string but it does have the same shape and number of characters, padding characters and it has those \/ things on the same positions in the string.
Let's call it key.
In PHP if I run
base64_decode($key);
the decode operation is successful
If In Python I run
base64.b64decode(key)
the decode operation is once more successful. Problem is, I can't do base64 decoding on this thing in Go.
dcd, err := base64.StdEncoding.DecodeString("qU4aaakFmjaaaaI5aaa\\/EN\\/aaa\\/SaaaJaaa6aa+nGnk=")
if err != nil {
log.Fatal(err)
}
return dcd
This will return the error
illegal base64 data at input byte 19
In the Go version, I have to escape those backslashes. It seems that the error appears at byte 19. Bearing in mind that this string that I am using as an example has the same length as the string that is actually causing the problem I would believe that the error happens right at the byte with the \ character. What can I do about this?

Answer 1

The alphabet of the standard Base64 does not contain backslash. So the qU4aaakFmjaaaaI5aaa\/EN\/aaa\/SaaaJaaa6aa+nGnk= input is not valid Base64 encoded string.
The forward slash is valid character in Base64, just not the backslash. It's possible the \/ is a sequence designating a single slash. If so, replace the \/ sequences with a single / and you're good to go.
For example:
s := `qU4aaakFmjaaaaI5aaa\/EN\/aaa\/SaaaJaaa6aa+nGnk=`
s = strings.ReplaceAll(s, `\/`, `/`)
dcd, err := base64.StdEncoding.DecodeString(s)
if err != nil {
log.Fatal(err)
}
fmt.Println(string(dcd))
Which outputs (try it on the Go Playground):
�Ni��6�i�9i����i��i��i��i��y
If \/ is not a special sequence and you want to discard all invalid characters from the input, this is how it could be done:
var valid = map[rune]bool{}
func init() {
for _, r := range "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=" {
valid[r] = true
}
}
func clean(s string) string {
return strings.Map(func(r rune) rune {
if valid[r] {
return r
}
return -1
}, s)
}
func main() {
s := `qU4aaakFmjaaaaI5aaa\/EN\/aaa\/SaaaJaaa6aa+nGnk=`
s = clean(s)
dcd, err := base64.StdEncoding.DecodeString(s)
if err != nil {
log.Fatal(err)
}
fmt.Println(string(dcd))
}
Output is the same. Try this one on the Go Playground.