been there, done that; it is very useful.

  1> [(file-get-string "/bin/ls") 0..15]
  "\x7F;ELF\x02\x01\x01\xDC00\xDC00\xDC00\xDC00\xDC00\xDC00\xDC00\xDC00"
  2> (file-put-string "foo" (file-get-string "/bin/ls"))
  t
  3> (sh "cmp /bin/ls foo")
  0
  4> (sh "sha256sum /bin/ls foo")
  a90ba058c747458330ba26b5e2a744f4fc57f92f9d0c9112b1cb2f76c66c4ba0  /bin/ls
  a90ba058c747458330ba26b5e2a744f4fc57f92f9d0c9112b1cb2f76c66c4ba0  foo
  0

There is no need to handle any fractional bytes if the original input is a sequence of bytes; so many whole bytes have to be recovered, not so many whole bytes plus three bits or whatever.