The 16-byte (128-bit) ASCII encoding for the 32-bit checksum provides a large degree of redundancy in the precise string that is used to express any given value. A straightforward estimate suggests that there are about 1029 = 2128/232 different ASCII patterns that represent each unsigned 32-bit integer. If only the 62 alpha-numeric characters are allowed in the encoded strings, this number is reduced by a factor of about 1010 = (62/256)16 to 1019 total ASCII expressions for each possible FITS checksum.
This proposal does not mandate a single ASCII encoding algorithm selected from this very large number of possibilities, although the encoding described in §3.4 and §3.5 and in the code in the appendices is recommended. The following constraints on the ASCII encoded values are required, however: