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                                                                   EXHIBIT 10.43

                                   DATUM INC.

                       NONQUALIFIED STOCK OPTION AGREEMENT
                       -----------------------------------

        This Nonqualified Stock Option Agreement (the "Agreement") is entered
into as of April 6, 1998, by and between Datum Inc., a Delaware corporation (the
"Company") and Erik van der Kaay (the "Optionee") pursuant to the Company's 1994
Stock Incentive Plan (the "Plan").

        1. GRANT OF OPTION. The Company hereby grants to Optionee an option (the
"Option") to purchase all or any portion of a total of One Hundred Twenty
Thousand (120,000) shares (the "Shares") of the Common Stock of the Company at a
purchase price of Thirteen Dollars & Seventy-Five Cents ($13.75) per share (the
"Exercise Price"), subject to the terms and conditions set forth herein and the
provisions of the Plan. This Option is not intended to qualify as an "incentive
stock option" as defined in Section 422 of the Internal Revenue Code of l986, as
amended (the "Code").

        2. VESTING OF OPTION. The right to exercise this Option shall vest in
installments, and this Option shall be exercisable from time to time in whole or
in part as to any vested installment, as follows:

<TABLE>
<CAPTION>
                                                           This Option shall be
                      On or After:                         Exercisable as to:
                      ------------                         ------------------
<C>                   <S>                                     <C>
        (i)           April 6, 1999:                          30,000 shares

        (ii)          April 6, 2000:  an additional           30,000 shares

        (iii)         April 6, 2001:  an additional           30,000 shares

        (iv)          April 6, 2002:  an additional           30,000 shares
</TABLE>

        No additional shares shall vest after the date of termination of
Optionee's "Continuous Service" (as defined in Section 3 below), but this Option
shall continue to be exercisable in accordance with Section 3 hereof with
respect to that number of shares that have vested as of the date of termination
of Optionee's Continuous Service.

        3. TERM OF OPTION. Optionee's right to exercise this Option shall
terminate upon the first to occur of the following:

               (a) the expiration of ten (10) years from the date of this
Agreement;

               (b) the expiration of three (3) months from the date of
termination of Optionee's Continuous Service if such termination occurs for any
reason other than permanent disability or death; provided, however, that if
Optionee dies during such three-month period, the provisions of Section 3(d)
below shall apply;


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               (c) the expiration of one (1) year from the date of termination
of Optionee's Continuous Service if such termination is due to permanent
disability of the Optionee (as defined in Section 22(e)(3) of the Code); or

               (d) the expiration of one (1) year from the date of termination
of Optionee's Continuous Service if such termination is due to Optionee's death
or if death occurs during the three-month period following termination of
Optionee's Continuous Service pursuant to Section 3(b) above.

        As used herein, the term "Continuous Service" means employment by either
the Company or any parent or subsidiary corporation of the Company, or by a
corporation or a parent or subsidiary of a corporation issuing or assuming a
stock option in a transaction to which Section 424(a) of the Code applies, which
is uninterrupted except for vacations, illness (except for permanent disability,
as defined in Section 22(e)(3) of the Code), or leaves of absence which are
approved in writing by the Company or any of such other employer corporations,
if applicable.

        4. EXERCISE OF OPTION. On or after the vesting of any portion of this
Option in accordance with Section 2 above, and until termination of this Option
in accordance with Section 3 above, the portion of this Option which has vested
may be exercised in whole or in part by the Optionee (or, after his or her
death, by the person designated in Section 5 below) upon delivery of the
following to the Company at its principal executive offices:

               (a) a written notice of exercise which identifies this Agreement
and states the number of Shares then being purchased (but no fractional Shares
may be purchased);

               (b) a check or cash in the amount of the Exercise Price (or
payment of the Exercise Price in such other form of lawful consideration as the
Administrator may approve from time to time under the provisions of Section 6(a)
of the Plan); and

               (c) a check or cash in the amount reasonably requested by the
Company to satisfy the Company's withholding obligations under federal, state or
other applicable tax laws with respect to the taxable income, if any, recognized
by the Optionee in connection with the exercise of this Option (unless the
Company and Optionee shall have made other arrangements (i) for deductions or
withholding from Optionee's wages, bonus or other compensation payable to
Optionee, or (ii) the withholding of Shares issuable upon exercise of this
Option, or (iii) the delivery of Shares owned by the Optionee in accordance with
Section 14 of the Plan, provided such arrangements satisfy the requirements of
applicable tax, securities and other laws).

        5. DEATH OF OPTIONEE; NO ASSIGNMENT. The rights of the Optionee under
this Agreement may not be assigned or transferred except by will or by the laws
of descent and distribution, and may be exercised during the lifetime of the
Optionee only by such Optionee. Any attempt to sell, pledge, assign,
hypothecate, transfer or dispose of this Option in contravention of this
Agreement or the Plan shall be void and shall have no effect. If the Optionee's
Continuous Service terminates as a result of his or her death, and provided
Optionee's rights hereunder shall have vested pursuant to Section 2 hereof,
Optionee's legal representative, his or her legatee, or the person who acquired
the right to exercise this Option by reason of the death of the Optionee
(individually, a 


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"Successor") shall succeed to the Optionee's rights and obligations under this
Agreement. After the death of the Optionee, only a Successor may exercise this
Option.

        6. RECEIPT OF PLAN. Optionee acknowledges receipt of a copy of the Plan
and understands that all rights and obligations connected with this Option are
set forth in this Agreement and in the Plan.

        7. RESTRICTIVE LEGENDS. Optionee hereby acknowledges that federal
securities laws and the securities laws of the state in which he or she resides
may require the placement of certain restrictive legends upon the Shares issued
upon exercise of this Option, and Optionee hereby consents to the placing of any
such legends upon certificates evidencing the Shares as the Company, or its
counsel, may deem necessary or advisable.

        8. LIMITATION OF COMPANY'S LIABILITY FOR NONISSUANCE. The Company agrees
to use its reasonable best efforts to obtain from any applicable regulatory
agency such authority or approval as may be required in order to issue and sell
the Shares to the Optionee pursuant to this Option. Inability of the Company to
obtain, from any such regulatory agency, authority or approval deemed by the
Company's counsel to be necessary for the lawful issuance and sale of the Shares
hereunder and under the Plan shall relieve the Company of any liability in
respect of the nonissuance or sale of such shares as to which such requisite
authority or approval shall not have been obtained.

        9. ADJUSTMENTS UPON CHANGES IN CAPITAL STRUCTURE. In the event that the
outstanding shares of Common Stock of the Company are hereafter increased or
decreased or changed into or exchanged for a different number or kind of shares
or other securities of the Company by reason of a recapitalization, stock split,
combination of shares, reclassification, stock dividend or other change in the
capital structure of the Company, then appropriate adjustment shall be made by
the Administrator to the number of Shares subject to the unexercised portion of
this Option and to the Exercise Price per share, in order to preserve, as nearly
as practical, but not to increase, the benefits of the Optionee under this
Option, in accordance with the provisions of Section 9 of the Plan.

        10. MERGER, REORGANIZATIONS, ETC. In the event that the Company at any
time proposes to sell substantially all of its assets, merge into, consolidate
with or to enter into any other reorganization in which the Company is not the
surviving corporation, or if the Company is the surviving corporation and the
ownership of the outstanding capital stock of the Company following the
transaction changes by 50% or more as a result of such transaction, this Option
shall terminate upon the effective date of such transaction unless provision is
made in writing in connection with such transaction for (a) the assumption of
this Option or the substitution for this Option of a new option of comparable
value covering shares of a successor corporation, with appropriate adjustments
as to the number and kind of shares and the Exercise Price, in which event this
Option or the new option substituted therefor shall continue in the manner and
under the terms so provided, or (b) the substitution for this Option of a
program or plan to provide rights to Optionee to receive, on exercise of such
rights, the type and amount of consideration Optionee would have received had he
or she exercised this Option prior to such transaction and less the aggregate
Exercise Price therefor. If such provision is not made in such transaction, then
the Administrator shall cause written notice of the proposed transaction to be
given to Optionee not less than thirty (30) days prior to the anticipated
effective date of the proposed transaction, and all of the then unvested portion
of this Option shall 


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vest and concurrent with the effective date of the proposed transaction Optionee
shall have the right to exercise any or all of the then unexercised portion of
this Option.

        11. NO EMPLOYMENT CONTRACT CREATED. Neither the granting of this Option
nor the exercise hereof shall be construed as granting to the Optionee any right
with respect to continuance of employment by the Company or any of its
subsidiaries. The right of the Company or any of its subsidiaries to terminate
at will the Optionee's employment at any time (whether by dismissal, discharge
or otherwise), with or without cause, is specifically reserved.

        12. RIGHTS AS SHAREHOLDER. The Optionee (or transferee of this option by
will or by the laws of descent and distribution) shall have no rights as a
shareholder with respect to any Shares covered by this Option until the date of
the issuance of a stock certificate or certificates to him or her for such
Shares, notwithstanding the exercise of this Option.

        13. INTERPRETATION. This Option is granted pursuant to the terms of the
Plan, and shall in all respects be interpreted in accordance therewith. The
Administrator shall interpret and construe this Option and the Plan, and any
action, decision, interpretation or determination made in good faith by the
Administrator shall be final and binding on the Company and the Optionee. As
used in this Agreement, the term "Administrator" shall refer to the committee of
the Board of Directors of the Company appointed to administer the Plan, and if
no such committee has been appointed, the term Administrator shall mean the
Board of Directors.

        14. NOTICES. Any notice, demand or request required or permitted to be
given under this Agreement shall be in writing and shall be deemed given when
delivered personally or three (3) days after being deposited in the United
States mail, as certified or registered mail, with postage prepaid, and
addressed, if to the Company, at its principal place of business, Attention: the
Chief Financial Officer, and if to the Optionee, at his or her most recent
address as shown in the employment, stock or other records of the Company.

        15. GOVERNING LAW. The validity, construction, interpretation, and
effect of this Option shall be governed by and determined in accordance with the
laws of the State of California.

        16. SEVERABILITY. Should any provision or portion of this Agreement be
held to be unenforceable or invalid for any reason, the remaining provisions and
portions of this Agreement shall be unaffected by such holding.

        17. COUNTERPARTS. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original and all of which
together shall be deemed one instrument.

        18. MODIFICATION. Optionee's rights under this Agreement are subject to
modification in certain events as provided in Section 10 of the Plan.


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        IN WITNESS WHEREOF, the parties have executed this Agreement as of the
date first above written.


DATUM INC.                                       "OPTIONEE"


By: /s/ Louis B. Horwitz                         /s/ Erik van der Kaay
    ---------------------------------------      -------------------------------
    Louis B. Horwitz, Chairman of the Board      Erik van der Kaay


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