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                                                                   EXHIBIT 10.54


            THIRD AMENDED AND RESTATED REVOLVING LINE OF CREDIT NOTE


                                                              Irvine, California
$10,000,000.00                                                      July 7, 2000

         FOR VALUE RECEIVED, the undersigned DATUM INC., a Delaware corporation
("Borrower") promises to pay to the order of WELLS FARGO BANK, NATIONAL
ASSOCIATION ("Bank") at its office at Orange Coast Regional Commercial Banking
Office, 2030 Main Street, Suite 900, Irvine, California 92614, or at such other
place as the holder hereof may designate, in lawful money of the United States
of America and in immediately available funds, the principal sum of Ten Million
Dollars ($10,000,000.00), or so much thereof as may be advanced and be
outstanding, with interest thereon, to be computed on each advance from the date
of its disbursement as set forth herein. This Note amends and restates in its
entirety that certain Amended and Restated Revolving Line of Credit Note dated
June 15, 1999 executed and delivered by Borrower to the order of Bank in the
original principal amount of up to $10,000,000.00 (the "Prior Note"). Amounts
outstanding and committed under the Prior Note shall, upon the effectiveness of
this Note be deemed to be outstanding and committed hereunder and evidenced
hereby, subject, however, to all terms and conditions hereunder and under the
Credit Agreement referred to below. This Note is the "Line of Credit Note"
referred to in the Credit Agreement.

DEFINITIONS:

         As used herein, the following terms shall have the meanings set forth
after each, and any other term defined in this Note shall have the meaning set
forth at the place defined:

         1.

         2.

         (a) "Business Day" means any day except a Saturday, Sunday or any other
day on which commercial banks in California are authorized or required by law to
close.

         (b) "Credit Agreement" means that certain Second Amended and Restated
Credit Agreement between Borrower and Bank dated as of July 7, 2000, either as
originally executed or as the same may from time to time be supplemented,
modified, amended, restated, extended or supplemented.

         (c) "Fixed Rate Term" means a period commencing on a Business Day and
continuing for (1) one month, (2) months, (3) months or (6) months, as
designated by Borrower, during which all or a portion of the outstanding
principal balance of this Note bears interest determined in relation to LIBOR;
provided however, that no Fixed Rate Term may be selected for a principal amount
less than Five Hundred Thousand Dollars ($500,000.00); and provided further,
that no Fixed Rate Term shall extend beyond the scheduled maturity date hereof.
If any Fixed Rate Term would end on a day which is not a Business Day, then such
Fixed Rate Term shall be extended to the next succeeding Business Day.


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         (d) "LIBOR" means the rate per annum (rounded upward, if necessary, to
the nearest whole 1/8 of 1%) and determined pursuant to the following formula:

                           LIBOR =          Base LIBOR
                                   -------------------------------
                                   100% - LIBOR Reserve Percentage

                           (i) "Base LIBOR" means the rate per annum for United
                  States dollar deposits quoted by Bank as the Inter-Bank Market
                  Offered Rate, with the understanding that such rate is quoted
                  by Bank for the purpose of calculating effective rates of
                  interest for loans making reference thereto, on the first day
                  of a Fixed Rate Term for delivery of funds on said date for a
                  period of time approximately equal to the number of days in
                  such Fixed Rate Term and in an amount approximately equal to
                  the principal amount to which such Fixed Rate Term applies.
                  Borrower understands and agrees that Bank may base its
                  quotation of the Inter-Bank Market Offered Rate upon such
                  offers or other market indicators of the Inter-Bank Market as
                  Bank in its discretion deems appropriate including, but not
                  limited to, the rate offered for U.S. dollar deposits on the
                  London Inter-Bank Market.

                           (ii) "LIBOR Reserve Percentage" means the reserve
                  percentage prescribed by the Board of Governors of the Federal
                  Reserve System (or any successor) for "Eurocurrency
                  Liabilities" (as defined in Regulation D of the Federal
                  Reserve Board, as amended), adjusted by Bank for expected
                  changes in such reserve percentage during the applicable Fixed
                  Rate Term.

         (e) "Prime Rate" means at any time the rate of interest most recently
announced within Bank at its principal office as its Prime Rate, with the
understanding that the Prime Rate is one of Bank's base rates and serves as the
basis upon which effective rates of interest are calculated for those loans
making reference thereto, and is evidenced by the recording thereof after its
announcement in such internal publication or publications as Bank may designate.

INTEREST:

         (f) Interest. The outstanding principal balance of this Note shall bear
interest (computed on the basis of a 360-day year, actual days elapsed) either
(i) at a fluctuating rate per annum equal to the Prime Rate in effect from time
to time, or (ii) at a fixed rate per annum determined by Bank to be two and one
quarter percent (2.25%) above LIBOR in effect on the first day of the applicable
Fixed Rate Term. When interest is determined in relation to the Prime Rate, each
change in the rate of interest hereunder shall become effective on the date each
Prime Rate change is announced within Bank. With respect to each LIBOR selection
hereunder, Bank is hereby authorized to note the date, principal amount,
interest rate and Fixed Rate Term applicable thereto and any payments made
thereon on Bank's books and records (either manually or by electronic entry)
and/or on any schedule attached to this Note, which notations shall be prima
facie evidence of the accuracy of the information noted.

         (g) Selection of Interest Rate Options. At any time any portion of this
Note bears interest determined in relation to LIBOR, it may be continued by
Borrower at the end of the Fixed Rate Term applicable thereto so that all or a
portion thereof bears interest determined in relation to the Prime Rate or to
LIBOR for a new Fixed Rate Term designated by Borrower. At any time any portion
of this Note bears interest determined in relation to the Prime Rate, Borrower
may convert all or a portion thereof so that it bears interest determined in
relation to LIBOR for a Fixed Rate Term designated by Borrower. At such time as
Borrower requests an

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advance hereunder or wishes to select a LIBOR option for all or a portion of the
outstanding principal balance hereof, and at the end of each Fixed Rate Term,
Borrower shall give Bank notice specifying: (i) the interest rate option
selected by Borrower; (ii) the principal amount subject thereto; and (iii) for
each LIBOR selection, the length of the applicable Fixed Rate Term. Any such
notice may be given by telephone (or such other electronic method as Bank may
permit) so long as, with respect to each LIBOR selection, (A) if requested by
Bank, Borrower provides to Bank written confirmation thereof not later than
three (3) Business Days after such notice is given, and (B) such notice is given
to Bank prior to 10:00 a.m. on the first day of the Fixed Rate Term, or at a
later time during any Business Day if Bank, at it's sole option but without
obligation to do so, accepts Borrower's notice and quotes a fixed rate to
Borrower. If Borrower does not immediately accept a fixed rate when quoted by
Bank, the quoted rate shall expire and any subsequent LIBOR request from
Borrower shall be subject to a redetermination by Bank of the applicable fixed
rate. If no specific designation of interest is made at the time any advance is
requested hereunder or at the end of any Fixed Rate Term, Borrower shall be
deemed to have made a Prime Rate interest selection for such advance or the
principal amount to which such Fixed Rate Term applied.

         (h) Limitation on LIBOR Portions. Unless Bank otherwise consents, no
more than one (1) portion of the outstanding principal balance of this Note
shall bear interest in relation to Bank's LIBOR at any time.

         (i) Taxes and Regulatory Costs. Borrower shall pay to Bank immediately
upon demand, in addition to any other amounts due or to become due hereunder,
any and all (i) withholdings, interest equalization taxes, stamp taxes or other
taxes (except income and franchise taxes) imposed by any domestic or foreign
governmental authority and related in any manner to LIBOR, and (ii) future,
supplemental, emergency or other changes in the LIBOR Reserve Percentage,
assessment rates imposed by the Federal Deposit Insurance Corporation, or
similar requirements or costs imposed by any domestic or foreign governmental
authority or resulting from compliance by Bank with any request or directive
(whether or not having the force of law) from any central bank or other
governmental authority and related in any manner to LIBOR to the extent they are
not included in the calculation of LIBOR. In determining which of the foregoing
are attributable to any LIBOR option available to Borrower hereunder, any
reasonable allocation made by Bank among its operations shall be conclusive and
binding upon Borrower.

         (j) Payment of Interest. Interest accrued on this Note shall be payable
on the first day of each month, commencing July 1, 2000.

         (k) Default Interest. From and after the maturity date of this Note, or
such earlier date as all principal owing hereunder becomes due and payable by
acceleration or otherwise, the outstanding principal balance of this Note shall
bear interest until paid in full at an increased rate per annum (computed on the
basis of a 360-day year, actual days elapsed) equal to four percent (4%) above
the rate of interest from time to time applicable to this Note.


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BORROWING AND REPAYMENT:

         (l) Borrowing and Repayment. Borrower may from time to time during the
term of this Note borrow, partially or wholly repay its outstanding borrowings,
and reborrow, subject to all of the limitations, terms and conditions of this
Note and of any document executed in connection with or governing this Note;
provided however, that the total outstanding borrowings under this Note shall
not at any time exceed the principal amount stated above. The unpaid principal
balance of this obligation at any time shall be the total amounts advanced
hereunder by the holder hereof less the amount of principal payments made hereon
by or for Borrower, which balance may be endorsed hereon from time to time by
the holder. The outstanding principal balance of this Note shall be due and
payable in full on June 15, 2001.

         (m) Advances. Advances hereunder, to the total amount of the principal
sum stated above, may be made by the holder at the oral or written request of
(i) Erik H. van der Kaay or Chris Felfe or David A. Young, any one acting alone,
who are authorized to request advances and direct the disposition of any
advances until written notice of the revocation of such authority is received by
the holder at the office designated above, or (ii) any person, with respect to
advances deposited to the credit of any deposit account of Borrower, which
advances, when so deposited, shall be conclusively presumed to have been made to
or for the benefit of Borrower regardless of the fact that persons other than
those authorized to request advances may have authority to draw against such
account. The holder shall have no obligation to determine whether any person
requesting an advance is or has been authorized by Borrower.

         (n) Application of Payments. Each payment made on this Note shall be
credited first, to any interest then due and second, to the outstanding
principal balance hereof. All payments credited to principal shall be applied
first, to the outstanding principal balance of this Note which bears interest
determined in relation to the Prime Rate, if any, and second, to the outstanding
principal balance of this Note which bears interest determined in relation to
LIBOR, with such payments applied to the oldest Fixed Rate Term first.

PREPAYMENT:

         (o) Prime Rate. Borrower may prepay principal on any portion of this
Note which bears interest determined in relation to the Prime Rate at any time,
in any amount and without penalty.

         (p) LIBOR. Borrower may prepay principal on any portion of this Note
which bears interest determined in relation to LIBOR at any time and in the
minimum amount of Two Hundred Fifty Thousand Dollars ($250,000.00); provided
however, that if the outstanding principal balance of such portion of this Note
is less than said amount, the minimum prepayment amount shall be the entire
outstanding principal balance thereof. In consideration of Bank providing this
prepayment option to Borrower, or if any such portion of this Note shall become
due and payable at any time prior to the last day of the Fixed Rate Term
applicable thereto by acceleration or otherwise, Borrower shall pay to Bank
immediately upon demand a fee which is the sum of the discounted monthly
differences for each month from the month of prepayment through the month in
which such Fixed Rate Term matures, calculated as follows for each such month:


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                           (i) Determine the amount of interest which would have
                  accrued each month on the amount prepaid at the interest rate
                  applicable to such amount had it remained outstanding until
                  the last day of the Fixed Rate Term applicable thereto.

                           (ii) Subtract from the amount determined in (i) above
                  the amount of interest which would have accrued for the same
                  month on the amount prepaid for the remaining term of such
                  Fixed Rate Term at LIBOR in effect on the date of prepayment
                  for new loans made for such term and in a principal amount
                  equal to the amount prepaid.

                           (iii) If the result obtained in (ii) for any month is
                  greater than zero, discount that difference by LIBOR used in
                  (ii) above.

Each Borrower acknowledges that prepayment of such amount may result in Bank
incurring additional costs, expenses and/or liabilities, and that it is
difficult to ascertain the full extent of such costs, expenses and/or
liabilities. Each Borrower, therefore, agrees to pay the above-described
prepayment fee and agrees that said amount represents a reasonable estimate of
the prepayment costs, expenses and/or liabilities of Bank. If Borrower fails to
pay any prepayment fee when due, the amount of such prepayment fee shall
thereafter bear interest until paid at a rate per annum four percent (4.00%)
above the Prime Rate in effect from time to time (computed on the basis of a
360-day year, actual days elapsed). Each change in the rate of interest on any
such past due prepayment fee shall become effective on the date each Prime Rate
change is announced within Bank.

EVENTS OF DEFAULT:

         This Note is made pursuant to and is subject to the terms and
conditions of the Credit Agreement. Any default in the payment or performance of
any obligation under this Note, or any defined event of default under the Credit
Agreement, shall constitute an "Event of Default" under this Note.


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MISCELLANEOUS:

         (q) Remedies. Upon the sale, transfer, hypothecation, assignment or
other encumbrance, whether voluntary, involuntary or by operation of law, of all
or any interest in any real property securing this Note, or upon the occurrence
of any Event of Default, the holder of this Note, at the holder's option, may
declare all sums of principal and interest outstanding hereunder to be
immediately due and payable without presentment, demand, notice of
nonperformance, notice of protest, protest or notice of dishonor, all of which
are expressly waived by Borrower, and the obligation, if any, of the holder to
extend any further credit hereunder shall immediately cease and terminate.
Borrower shall pay to the holder immediately upon demand the full amount of all
payments, advances, charges, costs and expenses, including reasonable attorneys'
fees (to include outside counsel fees and all allocated costs of the holder's
in-house counsel), expended or incurred by the holder in connection with the
enforcement of the holder's rights and/or the collection of any amounts which
become due to the holder under this Note, and the prosecution or defense of any
action in any way related to this Note, including without limitation, any action
for declaratory relief, whether incurred at the trial or appellate level, in an
arbitration proceeding or otherwise, and including any of the foregoing incurred
in connection with any bankruptcy proceeding (including without limitation, any
adversary proceeding, contested matter or motion brought by Bank or any other
person) relating to Borrower or any other person or entity.

         (r) Obligations Joint and Several. Should more than one person or
entity sign this Note as a Borrower, the obligations of each such Borrower shall
be joint and several.

         (s) Governing Law. This Note shall be governed by and construed in
accordance with the laws of the State of California.


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         IN WITNESS WHEREOF, the undersigned has executed this Note as of the
date first written above.


                                            DATUM INC.,
                                            a Delaware corporation


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



                                            By: /s/ Erik H. van der Kaay
                                                --------------------------------
                                                Name: Erik H. van der Kaay
                                                Title: Chairman and CEO


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