md5 checksum of a given/previous row

Hi,

I have a requirement to create an tamper proof chain of records for audit
purposes. The pseudo code is as follows

before_insert:
1. compute checksum of previous row (or conditionally selected row)
2. insert the computed checksum in the current row

For now lets assume we are using md5 checksum, I can easily get the md5 of
current row using

select md5(textin(record_out(test))) as md5_checksum FROM test;

but how do I compute md5 of previous row or of a row selected by where
column=val;?

Thanks.
Onkara