The lost key

Here's a challenge for you.
I lost my key, so it's not locked.
28489 46260 53815 79183 67000 86384 87725 71624 15594 14403
09147 18110 41059 73715 23675 11716 31344 61936 58405 71789
67996 04102 81613 41793 49867 77121 88375 80775 60638 90347
24676 39058 85504 92003 48722 10115 46990 44086 24854 24159
20664 01522 57274 15899 85065 70377 35752 53492 58559 88988
35370 43081 68166 86373 65926 75374 33291 85632 67446 59349
63704 65749 72091 66837 50171 43248 57595 16454 91499 39226
96517 88044 56393 01838 17430 52594 84964 23196 23914 22203
38920 55335 19311 43922 28110 85530 38860 62709 80794 00892
79201 97026 95796 57573 01408 45600 05825 23271 53168 89392
79895 92055 56120 97719 88677 37834 52558 70854 7
The spaces are for readability only - the ciphertext consists entirely of digits. If you paste the above into a new file, s/ //g (remove all spaces) and add an ASCII 0x0a newline (which vim will do automatically for you), the resulting file will give you an md5sum of:
   f5dfacff56e1494715ead7028fc288b5
The newline is not part of the ciphertext, of course.
(The plaintext is in plain English (straight ASCII) and has an md5sum of 2f682c420d4f5cd443719f33050eac67.)
There are 541 ciphertext characters. Since they're all digits, you'd expect 54 of each. What you get is:
Code  53 ( 5)     64 ( 11.81%)
Code  55 ( 7)     58 ( 10.70%)
Code  56 ( 8)     56 ( 10.33%)
Code  57 ( 9)     56 ( 10.33%)
Code  49 ( 1)     53 (  9.78%)
Code  51 ( 3)     53 (  9.78%)
Code  52 ( 4)     53 (  9.78%)
Code  48 ( 0)     52 (  9.59%)
Code  54 ( 6)     49 (  9.04%)
Code  50 ( 2)     47 (  8.67%)
For anyone not in my killfile I will try to answer any questions (either here or in email) if I can do so without completely giving the game away.
--
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within