The current 7-bit charset accepts any character code that fits in 7 bits (i.e. 0-127). However, the file specification that led the creation of the 7-bit charset does not allow for most of the non-visible characters. The codes that are not allowed are 0-9, 11, 12, and 14-31. We either need to modify our 7-bit encoder and decoder to not allow these character codes and return a malformed error, or we should create a new 7-bit charset specific to this file format and it's 7-bit restrictions.