<PAGE>

                                                                    EXHIBIT 10.4

================================================================================





                                 CERTICOM CORP.

                                       and

                      COMPUTERSHARE TRUST COMPANY OF CANADA

                                     Trustee


                      ____________________________________

                          First Supplemental Indenture

                           Dated as of August 30, 2001

                                       to

                                 Trust Indenture

                           Dated as of August 30, 2001

                           providing for the issue of
           7.25% Senior Convertible Unsecured Subordinated Debentures
                               Due August 30, 2004


                      ____________________________________


================================================================================

<PAGE>

     THIS FIRST SUPPLEMENTAL INDENTURE is made as of August 30, 2001

B E T W E E N:

                 CERTICOM CORP., a corporation continued
                 under the laws of the Yukon Territory and
                 having an office in the City of Toronto
                 in the Province of Ontario, Canada (the
                 "Corporation"),



                               -and-

                 COMPUTERSHARE TRUST COMPANY OF CANADA, a
                 trust company incorporated under the laws
                 of Canada and duly authorized to carry on
                 the trust business in each province of
                 Canada (the "Trustee").

     WHEREAS by a trust indenture (hereinafter referred to as the "Trust
Indenture") made as of August 30, 2001 between the Corporation and the Trustee
provision was made for the issue of Debentures of the Corporation;

     AND WHEREAS the Corporation and the Trustee have agreed to enter into this
Supplemental Indenture in order to clarify certain terms of the Trust Indenture;

     AND WHEREAS the Corporation is not in default under the Trust Indenture;

     AND WHEREAS all necessary acts and proceedings have been done and taken and
all necessary resolutions passed to authorize the execution and delivery of this
Supplemental Indenture and to make the same legal and valid and binding upon the
Corporation;

     AND WHEREAS the foregoing recitals are made as representations and
statements of fact by the Corporation and not by the Trustee;

     NOW THEREFORE THIS FIRST SUPPLEMENTAL INDENTURE WITNESSES that, in
consideration of the premises and the covenants herein contained, the parties
hereto agree as follows:

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                                      -2-

                                   ARTICLE 1

                        DEFINITIONS AND OTHER PROVISIONS
                             OF GENERAL APPLICATION

Section 1.01   Definitions
               -----------

         For all purposes of this Supplemental Indenture, except as otherwise
expressly provided or unless the subject matter or context otherwise requires:

         "Trust Indenture" means the trust indenture made as of August 30, 2001
         between the Corporation and the Trustee relating to the creation and
         issue of $13,500,000 aggregate principal amount of 7.25% Senior
         Convertible Unsecured Subordinated Debentures due August 30, 2004;

         "Supplemental Indenture", "hereto", "herein", "hereof", "hereby",
         "hereunder" and similar expressions refer to this First Supplemental
         Indenture made as of August 30, 2001 and not to any particular Article,
         Section or other portion hereof, and include any and every instrument
         supplemental or ancillary hereto or in implementation hereof, and the
         expressions "Article" and "Section" followed by a number mean and refer
         to the specified Article or Section of this Supplemental Indenture,

and all other terms and expressions used herein shall have the same meanings as
corresponding terms and expressions defined in the Trust Indenture.

Section 1.02   To be Read with Trust Indenture; Governing Law
               ----------------------------------------------

         This Supplemental Indenture is a supplemental indenture within the
meaning of the Trust Indenture and this Supplemental Indenture shall be read
together and shall have effect, so far as practicable, as though all the
provisions of the Trust Indenture and this Supplemental Indenture were contained
in one instrument, which instrument shall be governed by and construed in
accordance with the laws of the Province of Ontario and the laws of Canada
applicable therein.

                                   ARTICLE 2

                                   AMENDMENTS

Section 2.01   Article 2 - The Debentures
               --------------------------

     (1) Subsection 2.2.(d) of the Trust Indenture is hereby amended by deleting
the reference in the first line thereof to "sections 3.9 and 3.18" and
substituting therefor a reference to "section 3.18".

     (2) Subsection 2.6(a) of the Trust Indenture is hereby deleted in its
entirety and the following shall be substituted therefor:

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                                      -3-

               "(a) Every Debenture, whether issued originally or in exchange
               for other Debentures, shall bear interest from and including the
               date of issue or from and including the last interest payment
               date to which interest shall have been paid or made available for
               payment on the Debentures, whichever shall be later."

Section 2.02   Article 3 - Redemption and Purchase For Cancellation of
               -------------------------------------------------------

Debentures
----------

     (1)       Subsection 3.6(a) of the Trust Indenture is hereby amended by
deleting the words "Subject to section 3.9, upon" in the first line thereof and
substituting therefor the word "Upon".

     (2)       Section 3.7 of the Trust Indenture is hereby deleted in its
entirety and the following shall be substituted therefor:

               "3.7     Failure to Surrender Debentures Called for Redemption
                        -----------------------------------------------------

                        If the Holder of any Debentures called for redemption
               shall, within 30 days from the date fixed for redemption, fail to
               surrender any of such Debentures or shall not within such time
               accept payment of the Redemption Price payable in respect
               thereof, including certificates representing Common Shares
               issuable to such Holder, as the case may be, or give such receipt
               therefor, if any, as the Trustee may require, such Redemption
               Price shall be set aside in trust for such Holder, in accordance
               with section 12.10; and such setting aside shall for all purposes
               be deemed a payment to the Debentureholder of the sum so set
               aside, and to that extent such Debentures shall on and after the
               redemption date thereafter not be considered as outstanding
               hereunder and the Debentureholder shall have no right except to
               receive payment out of the moneys so paid and deposited, upon
               surrender of his Debentures, of the Redemption Price of such
               Debentures without interest thereon."

     (3)       Section 3.9 of the Trust Indenture is hereby deleted in its
entirety.

     (4)       Section 3.10 of the Trust Indenture is hereby deleted in its
entirety.

     (5)       Section 3.11 of the Trust Indenture is hereby deleted in its
entirety and the following shall be substituted therefor:


               "3.11    Restricted Debentures
                        ---------------------

                        The Debentures and Common Shares issuable upon
               conversion or repurchase of the Debentures have not been and will
               not be registered under the U.S. Securities Act and Debentures
               and Common Shares issuable upon conversion or repurchase of
               Debentures may not be offered, sold, pledged or transferred
               within the United States or to, or for the account or benefit of,
               a U.S. Person and may be offered, sold, pledged or

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                                      -4-

               otherwise transferred only outside the United States in
               accordance with the U.S. Securities Act."

        (6)    Section 3.12 of the Trust Indenture is hereby deleted in its
entirety.

        (7)    Section 3.13 of the Trust Indenture is hereby deleted in its
entirety.

        (8)    Subsection 3.16(c)(ii) of the Trust Indenture is hereby amended
by inserting the words ", which date shall not be prior to the tenth Business
Day following the date of mailing of such notice" in the fourth line immediately
following the words "payment date".

        (9)    Subsection 3.16(i) of the Trust Indenture is hereby deleted in
its entirety and the following shall be substituted therefor:

               "(i)    The provisions of section 3.7 apply mutatis mutandis to
               the Corporation's obligation to satisfy the purchase price under
               an Offer to Purchase except that reference to the Redemption
               Price in such section shall be deemed to be a reference to the
               Offer to Purchase Price for the purpose of this section 3.16;"

        (10)   Article 3 of the Trust Indenture is hereby supplemented by
inserting the following immediately after Section 3.17 of the Trust Indenture:

               "3.18   Payment of Offer to Purchase Price
                       ----------------------------------

                       Subject to section 3.19 and applicable law and
               regulatory approval, and notwithstanding any other provision of
               this Indenture, the Corporation may, at its option, in the case
               of an Offer to Purchase on notice given not less than five
               Trading Days preceding the Change of Control Date to the Trustee
               and the Holders of the Debentures and in accordance with
               subsection 3.20 and Article 11, elect to satisfy and discharge
               its obligation hereunder and under the Debentures to pay all or
               any part of the Offer to Purchase Price by paying (A) cash, (B)
               issuing and delivering, or causing to be issued and delivered,
               freely tradeable, fully paid and non-assessable Common Shares or
               freely tradeable, fully paid and non-assessable securities of the
               successor Person resulting from the Change of Control or of the
               Person that made the offer to purchase Common Shares resulting in
               the Change of Control or such Person's Affiliate, provided in
               each case that such securities were issued to holders of Common
               Shares in connection with the Change of Control or (C) any
               combination of cash, Common Shares or such securities. The
               satisfaction of the Offer to Purchase Price payable to a Holder
               shall occur by the delivery to the Holder of cash and/or
               securities (or securities that are convertible thereinto or
               exchangeable therefor) as provided under this section 3.18, such
               securities being valued, in the case of Common Shares, at 95% of
               the Current Market Price as at the Change of Control Date and, in
               the case of other securities, at 95% of the weighted average
               trading price of such

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                                      -5-

               securities on the principal stock exchange, quotation system or
               market on which such securities are listed, quoted or traded for
               the period of 20 consecutive trading days on such exchange,
               quotation system or market ending five trading days before the
               Change of Control Date. The Corporation may not exercise the
               right conferred by this section 3.18 if an Event of Default
               hereunder has occurred and is continuing at the date of the
               notice referred to in this section 3.18.

               3.19     Issue of Common Shares on Repurchase of Debentures Upon
                        -------------------------------------------------------
                        a Change of Control
                        -------------------

               (a)      If the Corporation elects under section 3.18 to satisfy
               its obligation to pay all or any part of the Offer to Purchase
               Price on the Payment Date by the issue of Common Shares or other
               securities and if otherwise permitted to do so by law, the
               Corporation will issue or cause to be issued that number of
               Common Shares or other securities determined under section 3.18,
               and will deliver to the Trustee the following:

                        (i)    a Certificate of the Corporation certifying
                               that no Event of Default hereunder has occurred
                               and is continuing as at the date of the notice
                               referred to in section 3.18; and

                        (ii)   an opinion of Counsel that (i) all requirements
                               imposed by this Indenture or by the laws of
                               Canada and any applicable province thereof (based
                               on the addresses of Holders set forth in the
                               general register kept by the Corporation pursuant
                               to section 2.8 as at the Payment Date) in
                               connection with the proposed issue of Common
                               Shares or other securities have been complied
                               with including that no prospectus or similar
                               document is required to be filed or
                               authorizations of regulatory authorities required
                               to be obtained under applicable legislation of
                               any province of Canada (other than as may have
                               been filed or obtained) before such Common Shares
                               or other securities may properly and legally be
                               issued and, once issued, traded without being
                               subject to any general restriction as to the
                               resale thereof provided such Common Shares or
                               other securities are traded through persons
                               registered, if required, under applicable laws,
                               (ii) on the Payment Date the Common Shares or
                               other securities will have been validly issued
                               and upon such issue in satisfaction of the
                               Corporation's obligation to pay all or any part
                               of the Offer to Purchase Price will be
                               outstanding as fully paid and non-assessable
                               securities, and (iii) if the Common Shares or
                               other securities are then quoted on NASDAQ and/or
                               listed on the TSE, application to quote the
                               Common Shares or

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                                      -6-

                               other securities so issued has been made to
                               NASDAQ and the Common Shares or other securities
                               so issued have been conditionally approved for
                               listing on the TSE (or on such other principal
                               stock exchange, quotation system or market as the
                               Common Shares or other securities may then be
                               listed, quoted or traded), subject to compliance
                               with the requirements of such stock exchange,
                               quotation system or market.

               (b)      If any order, ruling, registration, notice or filing
               pursuant to any securities laws of Canada or any province thereof
               is required to ensure that any Common Shares or other securities
               issuable on the Payment Date are issued in compliance with all
               such laws or to ensure that any such Common Shares or other
               securities, once issued, are not subject to any general
               restriction as to the resale thereof provided that such Common
               Shares or other securities are traded through persons registered,
               if required under applicable laws, the Corporation covenants that
               it will take or cause to be taken all action as may be necessary
               to make or obtain such order, ruling, registration or filing, or
               give such notice, as the case may be.

               (c)      If the provisions of subsections 3.19 (a)(i) and (ii)
               are not complied with, the Offer to Purchase Price payable on the
               Payment Date will be payable in lawful money of Canada as
               otherwise provided hereunder. If such provisions are complied
               with, the issue by the Corporation or such other Person referred
               to in clause 3.18(B) or such Person's Affiliates of that number
               of Common Shares or other securities determined under section
               3.18 shall fully satisfy and discharge the obligation of the
               Corporation to pay all or any part of the Offer to Purchase Price
               determined to be satisfied in accordance with subsection 3.18 on
               the Payment Date.

               3.20     General Requirements
                        --------------------

               (a)      The notice to the Trustee and Holders of Debentures to
               be given by the Corporation pursuant to section 3.18 must:

                        (i)    state that the Corporation has exercised its
                               option to pay all or any part of the Offer to
                               Purchase Price payable to the Holders of
                               Debentures on the Payment Date by the issue of
                               Common Shares or other securities to the Holders
                               of Debentures;

                        (ii)   advise each Holder of Debentures that the
                               Common Shares or other securities to be issued in
                               respect of such Holder's Debentures will be
                               registered in the name of the Holder unless the
                               Trustee receives from such Holder, on or before
                               5:00 p.m. (Toronto time) on the fifth Business
                               Day prior to

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                                      -7-

                               the Payment Date at its principal stock transfer
                               office in the City of Toronto written notice in
                               form and substance satisfactory to the Trustee
                               directing the Corporation to register or to
                               direct the registration of such Common Shares or
                               other securities in some other name or names and
                               stating the name or names (with addresses),
                               accompanied by payment to the Trustee of any
                               transfer tax which may be payable by reason
                               thereof; and

                        (iii)  advise each Holder that such Holder may, on or
                               after the Payment Date and on proof of identity
                               satisfactory to the Trustee, take personal
                               delivery of the certificates representing that
                               Holder's Common Shares or other securities so
                               issued, at the principal stock transfer office of
                               the Trustee in the City of Toronto if the Trustee
                               receives from such Holder at such principal stock
                               transfer office, in addition to any other notice
                               or delivery required by this subsection and on or
                               before 5:00 p.m. (Toronto time) on the fifth
                               Business Day prior to the Payment Date written
                               notice in form and substance satisfactory to the
                               Trustee, stating that such Holder wishes to take
                               personal delivery of the Common Shares or other
                               securities issued pursuant hereto at the
                               principal stock transfer office of the Trustee in
                               the City of Toronto; and

               (b) On the Payment Date the Corporation will, subject to
               subsections 3.20(a)(ii) and (iii), cause to be sent by prepaid
               ordinary mail (or, in the event of mail service interruption, by
               such other means as the Trustee and the Corporation determine to
               be appropriate), certificates for Common Shares or other
               securities issued pursuant to section 3.18 to each Holder of
               Common Shares or other securities in respect of which Debentures
               have been surrendered in accordance with the requirements of the
               notice, given pursuant to subsection 3.20(a), at their addresses
               as shown on the records of the Corporation.

               (c) On or after the Payment Date the Corporation will deliver
               certificates representing the Common Shares or other securities
               issued pursuant to section 3.19 to any other registered Holder
               thereof, upon presentation and surrender of the Debentures in
               respect of which such Common Shares or other securities were
               issued.

               (d) Each certificate delivered pursuant to this section 3.20 will
               be for that number of Common Shares or other securities that is
               the Holder's proportionate share of the number of Common Shares
               or other securities determined in accordance with section 3.18.

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                                      -8-

               (e)      Interest accrued and unpaid on the Debentures on the
               Payment Date will be paid to the Holders of Debentures in the
               manner contemplated in Article 2 except that the Corporation may,
               at its option, forward payment of interest contemporaneously with
               the sending of Common Shares or other securities under
               subsections 3.20(a)(iii) or 3.20(b).

               3.21     No Requirement to Issue Fractional Shares or Other
                        --------------------------------------------------
                        Security
                        --------

                        The Corporation shall not be required to issue or cause
               to be issued fractional Common Shares or other securities upon
               the issue of Common Shares or other securities pursuant to
               section 3.18. If any fractional interest in a Common Share or
               other security would, except for the provisions of this section,
               be deliverable upon the issue of any Common Shares or other
               securities pursuant to section 3.18, the Corporation shall, in
               lieu of delivering any certificate representing such fractional
               interest, satisfy such fractional interest by paying to the
               registered holder of such shares an amount in lawful money of
               Canada equal (computed to the nearest cent) to the appropriate
               fraction, in the case of any fraction of any Common Share, of the
               Current Market Price of the Common Shares as at the Payment Date
               and, in the case of a fraction of any other security, of the
               weighted average trading price of such securities on the
               principal stock exchange, quotation system or market on which
               such securities are listed, quoted or traded for the period of 20
               consecutive trading days on such exchange, quotation system or
               market as at the date ending five trading days before the Payment
               Date."

Section 2.03   Article 4 - Conversion
               ----------------------

        (1)    Subsection 4.2(e)(i) of the Trust Indenture is hereby amended by
deleting the reference to "June 30, 2003" in the first line thereof and
substituting therefor a reference to "August 30, 2003".

        (2)    Subsection 4.12(b) of the Trust Indenture is hereby amended by
deleting the reference to "section 3.19" and substituting therefor a reference
to "section 3.17".

Section 2.04   Article 8 - Satisfaction and Discharge
               --------------------------------------

               Section 8.3 of the Trust Indenture is hereby amended by
deleting the reference to "sections 3.6, 3.9, 3.16, 3.17, 3.18" in the first
line thereof and substituting therefor a reference to "sections 3.6, 3.7, 3.16
and 3.19".

Section 2.05   Schedule "A" (Form of Debenture)
               --------------------------------

               Schedule "A" of the Trust Indenture is hereby amended by
deleting the words "the date hereof" in the eighth line thereof and substituting
therefor the words "the date of issue hereof".

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                                       -9-

                                    ARTICLE 3

                            MISCELLANEOUS PROVISIONS

Section 3.01   Confirmation of Trust Indenture
               -------------------------------

     The Trust Indenture, as amended and supplemented by this Supplemental
Indenture, is in all respects confirmed.

Section 3.02   Acceptance of Trusts
               --------------------

     The Trustee hereby accepts the trusts in this Supplemental Indenture
declared and provided for and agrees to perform the same upon the terms and
conditions and subject to the provisions set forth in the Trust Indenture as
supplemented by this Supplemental Indenture.

Section 3.03   Counterparts and Formal Date
               ----------------------------

     This Supplemental Indenture may be executed in any number of counterparts,
each of which so executed shall be deemed to be an original; but and all such
counterparts together shall constitute but one and the same instrument and
notwithstanding the date of execution shall be deemed to bear the date August
30, 2001.

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                                      -10-

     IN WITNESS WHEREOF the parties hereto have caused this Supplemental
Indenture to be duly executed, and their respective corporate seals to be
hereunto affixed, all as of the day and year first above written.

                                       CERTICOM CORP.

                                       By:   ______________________________



                                       By:   ______________________________




                                       COMPUTERSHARE TRUST
                                       COMPANY OF
                                       CANADA

                                       By:   ______________________________



                                       By:   ______________________________