base64 , a Tcllib module, provides commands for encoding and decoding binary data in base64. In Tcl 8.6, it has largely been superseded by binary, which also provides base64 encoding and decoding.
The following information is no longer particularly relevant. It remains for historical purposes.
The base64 which is part of tcllib is slow, and catastrophically memory-hungry for large (on the order of a megabyte or more) sources. The memory disaster results from what Donal Fellows reports as a bug to SourceForge [L1 ]. He provided a patch as well [L2 ]. Although it doesn't help performance significantly in the cases of interest to Cameron Laird, a simple fix for the memory woe is to overide base64::decode with a version which substitutes
set length [string length $string]
for {set ii 0} {$ii < $length} {incr ii} {
set char [string range $string $ii $ii]
... for
foreach char [split $string {}] {
...Note: in the current version in tcllib (version 2.2.1 of base64), this code has been replaced by
binary scan $string c* X
foreach x $X {