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CMS: Comment on checksum algorithm.

1998-11-14 16:34:00
```Couple of comments re the checksum algorithm:

```
```12.6.1  Sum of Sums Key Checksum

The Sum of Sums [SUM] key checksum algorithm is:

1.  Initialize two 16 bit integers, sum1 and sum2, to zero.
2.  Loop through the octets of the content-encryption key, most
significant octet to least significant octet.
2a.  Create a 16 bit integer, called temp, by concatenating
eight zero bits and the key octet.
2b.  sum1 = sum1 + temp.
2c.  sum2 = sum2 + sum1.
3.  Use sum2 as the checksum value.

```
```
IMHO "most significant octet to least significant octet" might be
considered ambiguous. Perhaps "first to last octet" (assuming that's
what it means) would be clearer.

I assume 2a this is just setting temp to the value of the key octet
(i.e. treat key octet as an unsigned 8 bit integer).

Finally isn't the whole thing equivalent to:

{n*K_1 + (n-1)*K_2 + ... + K_n} mod 2^16

Where an K_i represents the i'th octet of an n byte key? Or am I
misinterpreting something?

Steve.
--
Dr Stephen N. Henson. UK based freelance Cryptographic Consultant.
For info see homepage at http://www.drh-consultancy.demon.co.uk/
Email: shenson(_at_)drh-consultancy(_dot_)demon(_dot_)co(_dot_)uk
PGP key: via homepage.

```
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