Read a password

HTML, 20 bytes

<input type=password

Alternative: HTML + JavaScript, 51 bytes

Although the OP has confirmed that to be valid, here's a solution using JS for the purists!

<input id=i oninput=i.value=i.value.replace(/./g,8)

Vim, 36 bytes:

:im <C-v><CR> <C-v><esc>ZQ<CR>:au I<tab><tab> * let v:char=0<CR>i

This uses vim-key notation, so <C-v> is control-v, <CR> is enter, <esc> is the escape key, and <tab> is the tab key.

c is '0'.

Here is a hexdump to prove the byte count is accurate:

00000000: 3a69 6d20 160a 2016 1b5a 510a 3a61 7520  :im .. ..ZQ.:au 
00000010: 4909 0920 2a20 6c65 7420 763a 6368 6172  I.. * let v:char
00000020: 3d30 0a69                                =0.i

This works by running the following two ex commands:

:imap <CR> <esc>ZQ
:autocmd InsertCharPre * let v:char=0

The first one means

:imap               " Anytime the following is pressed in insert mode:
      <CR>          "   (the 'enter' key)
           <esc>ZQ  " Then act as if the user instead pressed '<esc>ZQ' (close the buffer)

And the second one means

:autocmd                                " Automatically as vim runs:
         InsertCharPre                  "   Any time the user is about to insert a char
                       *                "   In any type of file
                         let v:char=0   "     Then instead insert a '0' character

Ruby with Shoes, 29 characters

Shoes.app{edit_line secret:1}

Sample output: