md5 checksum of a 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
3. using on-update or on-delete trigger raise error to prevent
update/delete of any row.

Here are the different options that I have tried using lag and md5 functions

http://www.sqlfiddle.com/#!17/69843/2

CREATE TABLE test
("id" uuid DEFAULT uuid_generate_v4() NOT NULL,
"value" decimal(5,2) NOT NULL,
"delta" decimal(5,2),
"created_at" timestamp default current_timestamp,
"words" text,
CONSTRAINT pid PRIMARY KEY (id)
)
;

INSERT INTO test
(value, words)
VALUES
(51.0, 'A'),
(52.0, 'B'),
(54.0, 'C'),
(57.0, 'D')
;

select
created_at, value,
value - lag(value, 1, 0.0) over(order by created_at) as delta,
md5(lag(words,1,words) over(order by created_at)) as the_word,
md5(textin(record_out(test))) as Hash
FROM test
ORDER BY created_at;

But how do I use lag function or something like lag to read the previous
record as whole.

Thanks,
Onkara
PS: This was earlier posted in 'pgsql-in-general' mailing list, but I think
this is a more appropriate list, if I am wrong I am sorry