Binary to decimal

I'm not sure what you're expecting the decimal value to be, but here is perhaps one way to do this:

set data "nVkVI5xxeXvvzbGfFmarAfaDMTRTPnRlKVedR5oqcRGNaZFV8X3HfJ8RCLhIzdM5"

binary scan [b64decode $data] H* hdata
log local0. $hdata

set first [string range $hdata 0 15]
set last [string range $hdata 16 end]

log local0. $first

set dec [expr 0x$first]
log local0. $dec