This  is  a  loadable  extension  to Tcl  providing  commands  for  data
conversion, message  digests, zlib based  compression, error-correction,
channel-based manipulation of  binary data. Trf extends  the language at
the C-level with so-called  ``transformer''-procedures. With the help of
some  patches (*)  to the  core  the package  is able  to intercept  all
read/write operations on designated channels, thus giving it the ability
to transform  the buffer  contents as desired.  This allows  things like
transparent encryption,  compression, charset recoding, etc.  Build upon
this  framework (and  as proof  of  concept) a  collection of  tcl-level
commands was implemented.

WWW: https://wiki.tcl-lang.org/479