Normal [base64]-encoded data cannot be used directly in [URL]s without
escaping the +, /, and = characters. There is a popular URL-safe variant of
base64 that avoids this problem by replacing these characters. The + and /
are replaced with - and _ respectively, while the = padding characters are
omitted completely as the correct padding can always be recreated if you
know the length of the data.
It is very easy to encode and decode strings in this format using the normal
base64 encoding provided by Tcl's [binary] command, so long as you don't mind
a second pass over the data. Given that URLs are typically short, this is often
not a concern.
Here is a basic implementation in pure Tcl, which I've also uploaded to GitHub
at https://github.com/NeilMadden/tclbase64url
======
# base64url.tcl --
#
# Implementation of the URL-safe Base64 variant described
# in section 5 of RFC 4648.
#
# Copyright 2020 Neil Madden.
# See the file LICENSE.txt for license terms.
package require Tcl 8.6
package provide base64url 1.0.0
namespace eval ::base64url {
namespace export encode decode
namespace ensemble create
proc encode data {
regexp {[^=]*(=*)} [binary encode base64 $data] -> padding
string map {+ - / _ = ""} [binary encode base64 $data]
}
proc decode str {
binary decode base64 [pad [string map {- + _ /} $str]]
}
proc pad str {
return $str[padding $str]
}
proc padding str {
string repeat = [expr {(4 - [string length $str]) & 3}]
}
}
======