#Whitespace, 119 bytes
Whitespace, 119 bytes
[S S S N _Push_0][N S S N _Create_Label_LOOP][S N S _Duplicate_top][S N S _Duplicate_top][T N T S _Read_STDIN_as_character][T T T _Retrieve][S N S _Duplicate_input][S S S T S S S T T N _Push_35][T S S T _Subtract][N T S S N _If_0_Jump_to_Label_PRINT_CHAR][S S S T T T S T T N _Push_59][T S S T _Subtract][N T S T N _If_0_Jump_to_Label_INCREASE][N S N N _Jump_to_Label_LOOP][N S S S N _Create_Label_PRINT_CHAR][S N N _Discard_top][S S S T T T T T T T N _Push_127][T S T T _Modulo][T N S S _Print_as_character][S S S N _Push_0][N S N N _Jump_to_Label_LOOP][N S S T N _Create_Label_INCREASE][S S S T N _Push_1][T S S S _Add][N S N N _Jump_to_Label_LOOP] Letters S (space), T (tab), and N (new-line) added as highlighting only.
[..._some_action] added as explanation only.
Try it online (with raw spaces, tabs and new-lines only).
Pseudo-code:
Integer i = 0 Label LOOP: Character c = STDIN as character If(c == '#'): Jump to Label PRINT_CHAR If(c == ';'): Jump to Label INCREASE Jump to Label LOOP Label PRINT_CHAR: i = i modulo-127 Print i as character to STDOUT i = 0 Jump to Label LOOP Label INCREASE: i = i + 1 Jump to Label LOOP 
