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                                                                   EXHIBIT 10.39



                THE ISSUANCE OF THIS NOTE HAS NOT BEEN REGISTERED
              UNDER THE SECURITIES ACT AND NO OFFER OR SALE HEREOF
              MAY BE MADE BY THE HOLDER HEREOF ABSENT REGISTRATION
               UNDER THE SECURITIES ACT OR AN EXEMPTION THEREFROM.


                                   DATUM INC.


           10.25% SENIOR SECURED SERIES B NOTE DUE SEPTEMBER 27, 2003


No. B-1                                                       September 30, 1996
$12,000,000


         FOR VALUE RECEIVED, the undersigned, Datum Inc. (herein called the
"Company"), a corporation organized and existing under the laws of the State of
Delaware, hereby promises to pay to THE PRUDENTIAL INSURANCE COMPANY OF AMERICA
or registered assigns, the principal sum of TWELVE MILLION DOLLARS on September
27, 2003, with interest (computed on the basis of a 360-day year--30-day month)
(a) on the unpaid balance thereof at the rate of 10.25% per annum from the date
hereof, payable quarterly on the 27th day of March, June, September and December
in each year, commencing with the March 27, June 27, September 27 or December 27
next succeeding the date hereof, until the principal hereof shall have become
due and payable, and (b) on any overdue payment (including any overdue
prepayment) of principal, any overdue payment of interest and any overdue
payment of any Yield-Maintenance Amount (as defined in the Agreement referred to
below), payable quarterly as aforesaid (or, at the option of the registered
holder hereof, on demand), at a rate per annum from time to time equal to the
greater of (i) 12.25% or (ii) 2.0% over the rate of interest publicly announced
by Morgan Guaranty Trust Company of New York from time to time in New York City
as its prime rate.

         Payments of principal of, interest on and any Yield-Maintenance Amount
payable with respect to this Note are to be made at the main office of Bank of
New York in New York City or at such other place as the holder hereof shall
designate to the Company in writing, in lawful money of the United States of
America.

         This Note is one of a series of Senior Secured Notes (herein called the
"Notes") issued pursuant to a Note and Warrant Purchase Agreement, dated as of
September 27, 1996 (herein called the "Agreement"), between the Company and The
Prudential Insurance Company of America and is entitled to the benefits thereof.
The Notes are secured and guaranteed as contemplated by the Agreement.


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         This Note is a registered Note and, as provided in the Agreement, upon
surrender of this Note for registration of transfer, duly endorsed, or
accompanied by a written instrument of transfer duly executed, by the registered
holder hereof or such holder's attorney duly authorized in writing, a new Note
for a like principal amount will be issued to, and registered in the name of,
the transferee. Prior to due presentment for registration of transfer, the
Company may treat the person in whose name this Note is registered as the owner
hereof for the purpose of receiving payment and for all other purposes, and the
Company shall not be affected by any notice to the contrary.

         The Company agrees to make required prepayments of principal on the
dates and in the amounts specified in the Agreement. This Note is also subject
to optional prepayment, in whole or from time to time in part, on the terms
specified in the Agreement.

         In case an Event of Default, as defined in the Agreement, shall occur
and be continuing, the principal of this Note may be declared or otherwise
become due and payable in the manner and with the effect provided in the
Agreement.

         This Note shall be construed and enforced in accordance with the
internal law of the State of California.



                                                    DATUM INC.



                                                    By: /s/ David A. Young
                                                       -------------------------
                                                    Its: Chief Financial Officer
                                                        ------------------------



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