ENTRUST, INC.

Exhibit 10.15

ENTRUST, INC.

1999 NON-OFFICER EMPLOYEE STOCK INCENTIVE PLAN

The purpose of this 1999 Non-Officer Employee Stock Incentive Plan (the “Plan”) of Entrust, Inc., a Delaware corporation (the “Company”), is to advance the interests of the Company’s stockholders by enhancing the Company’s ability to attract, retain and motivate persons who make (or are expected to make) important contributions to the Company by providing such persons with equity ownership opportunities and performance-based incentives and thereby better aligning the interests of such persons with those of the Company’s stockholders. Except where the context otherwise requires, the term “Company” shall include any of the Company’s present or future subsidiary corporations as defined in Section 424(f) of the Internal Revenue Code of 1986, as amended, and the regulations promulgated thereunder (the “Code”).

All of the Company’s employees (and any individuals who have accepted an offer for employment), consultants and advisors, other than those who are also officers (within the meaning of Section 16 of the Securities Exchange Act of 1934, as amended, and the rules and regulations promulgated thereunder (the “Exchange Act”)) or directors of the Company, are eligible to be granted options, restricted stock awards or other stock-based awards (each, an “Award”) under the Plan. Each person who has been granted an Award under the Plan shall be deemed a “Participant.”

(a) Administration by Board of Directors. The Plan will be administered by the Board of Directors of the Company (the “Board”). The Board shall have authority to grant Awards and to adopt, amend and repeal such administrative rules, guidelines and practices relating to the Plan as it shall deem advisable. The Board may correct any defect, supply any omission or reconcile any inconsistency in the Plan or any Award in the manner and to the extent it shall deem expedient to carry the Plan into effect and it shall be the sole and final judge of such expediency. All decisions by the Board shall be made in the Board’s sole discretion and shall be final and binding on all persons having or claiming any interest in the Plan or in any Award. No director or person acting pursuant to the authority delegated by the Board shall be liable for any action or determination relating to or under the Plan made in good faith.

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(b) Delegation to Executive Officers. To the extent permitted by applicable law, the Board may delegate to one or more executive officers of the Company the power to make Awards and exercise such other powers under the Plan as the Board may determine, provided that the Board shall fix the maximum number of shares subject to Awards and the maximum number of shares for any one Participant to be made by such executive officers.

(c) Appointment of Committees. To the extent permitted by applicable law, the Board may delegate any or all of its powers under the Plan to one or more committees or subcommittees of the Board (a “Committee”). All references in the Plan to the “Board” shall mean the Board or a Committee of the Board or the executive officer referred to in Section 3(b) to the extent that the Board’s powers or authority under the Plan have been delegated to such Committee or executive officer.

Subject to adjustment under Section 8, Awards may be made under the Plan for up to 2,500,000 shares of common stock, $.01 par value per share, of the Company (the “Common Stock”). If any Award expires or is terminated, surrendered or canceled without having been fully exercised or is forfeited in whole or in part or results in any Common Stock not being issued, the unused Common Stock covered by such Award shall again be available for the grant of Awards under the Plan. Shares issued under the Plan may consist in whole or in part of authorized but unissued shares or treasury shares.

(a) General. The Board may grant nonstatutory stock options to purchase Common Stock (each, an “Option”) and determine the number of shares of Common Stock to be covered by each Option, the exercise price of each Option and the conditions and limitations applicable to the exercise of each Option, including conditions relating to applicable federal or state securities laws, as it considers necessary or advisable. No Option granted under the Plan shall be intended to be an “incentive stock option” as defined in Section 422 of the Code.

(b) Exercise Price. The Board shall establish the exercise price at the time each Option is granted and specify it in the applicable option agreement.

(c) Duration of Options. Each Option shall be exercisable at such times and subject to such terms and conditions as the Board may specify in the applicable option agreement.

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(d) Exercise of Option. Options may be exercised by delivery to the Company of a written notice of exercise signed by the proper person or by any other form of notice (including electronic notice) approved by the Board together with payment in full as specified in Section 5(e) for the number of shares for which the Option is exercised.

(e) Payment Upon Exercise. Common Stock purchased upon the exercise of an Option granted under the Plan shall be paid for as follows:

(1) in cash or by check, payable to the order of the Company;

(2) except as the Board may, in its sole discretion, otherwise provide in an Option Agreement, by (i) delivery of an irrevocable and unconditional undertaking by a creditworthy broker to deliver promptly to the Company sufficient funds to pay the exercise price or (ii) delivery by the Participant to the Company of a copy of irrevocable and unconditional instructions to a creditworthy broker to deliver promptly to the Company cash or a check sufficient to pay the exercise price;

(3) to the extent permitted by the Board and explicitly provided in an Option Agreement (i) by delivery of shares of Common Stock owned by the Participant valued at their fair market value as determined by the Board in good faith (“Fair Market Value”), which Common Stock was owned by the Participant at least six months prior to such delivery, (ii) by delivery of a promissory note of the Participant to the Company on terms determined by the Board or (iii) by payment of such other lawful consideration as the Board may determine; or

(4) by any combination of the above permitted forms of payment.

(f) Deferral. Any Participant who is a participant in a deferred compensation plan established by the Company may elect with the permission of the Board and in accordance with rules established by the Board to defer the receipt of any shares of Common Stock issuable upon the exercise of an Option provided that such election is irrevocable and made at least that number of days prior to the exercise of the Option which shall be determined by the Board. The Participant’s account under such deferred compensation plan shall be credited with a number of stock units equal to the number of shares so deferred.

(a) Grants. The Board may grant Awards entitling recipients to acquire shares of Common Stock, subject to the right of the Company to repurchase all or part

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of such shares at their issue price or other stated or formula price (or to require forfeiture of such shares if issued at no cost) from the recipient in the event that conditions specified by the Board in the applicable Award are not satisfied prior to the end of the applicable restriction period or periods established by the Board for such Award (each, a “Restricted Stock Award”).

(b) Terms and Conditions. The Board shall determine the terms and conditions of any such Restricted Stock Award, including the conditions for repurchase (or forfeiture) and the issue price, if any. Any stock certificates issued in respect of a Restricted Stock Award shall be registered in the name of the Participant and, unless otherwise determined by the Board, deposited by the Participant, together with a stock power endorsed in blank, with the Company (or its designee). At the expiration of the applicable restriction periods, the Company (or such designee) shall deliver the certificates no longer subject to such restrictions to the Participant or if the Participant has died, to the beneficiary designated, in a manner determined by the Board, by a Participant to receive amounts due or exercise rights of the Participant in the event of the Participant’s death (the “Designated Beneficiary”). In the absence of an effective designation by a Participant, Designated Beneficiary shall mean the Participant’s estate.

The Board shall have the right to grant other Awards based upon the Common Stock having such terms and conditions as the Board may determine, including the grant of shares based upon certain conditions, the grant of securities convertible into Common Stock and the grant of stock appreciation rights.

(a) Changes in Capitalization. In the event of any stock split, reverse stock split, stock dividend, recapitalization, combination of shares, reclassification of shares, spin-off or other similar change in capitalization or event, or any distribution to holders of Common Stock other than a normal cash dividend, (i) the number and class of securities available under this Plan, (ii) the number and class of securities and exercise price per share subject to each outstanding Option, (iii) the repurchase price per share subject to each outstanding Restricted Stock Award, and (iv) the terms of each other outstanding Award shall be appropriately adjusted by the Company (or substituted Awards may be made, if applicable) to the extent the Board shall determine, in good faith, that such an adjustment (or substitution) is necessary and appropriate. If this Section 8(a) applies and Section 8(b) also applies to any event, Section 8(b) shall be applicable to such event, and this Section 8(a) shall not be applicable.

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(b) Acquisition Events

(1) Consequences of Acquisition Events. Upon the occurrence of an Acquisition Event (as defined below), or the execution by the Company of any agreement with respect to an Acquisition Event, the Board shall take any one or more of the following actions with respect to then outstanding Awards: (i) provide that outstanding Options shall be assumed, or equivalent Options shall be substituted, by the acquiring or succeeding entity (or an affiliate thereof); (ii) upon written notice to the Participants, provide that all then unexercised Options will become exercisable in full and will terminate immediately prior to the consummation of such Acquisition Event, except to the extent exercised by the Participants within a specified period following the date of such notice; (iii) in the event of an Acquisition Event under the terms of which holders of Common Stock will receive upon consummation thereof a cash payment for each share of Common Stock surrendered pursuant to such Acquisition Event (the “Acquisition Price”), provide that all outstanding Options shall terminate upon consummation of the Acquisition Event and that Participants shall receive, in exchange therefor, a cash payment equal to the amount (if any) by which (A) the Acquisition Price multiplied by the number of shares of Common Stock subject to such outstanding Options (whether or not then exercisable), exceeds (B) the aggregate exercise price of such Options; (iv) provide that all Restricted Stock Awards then outstanding shall become free of all restrictions prior to the consummation of the Acquisition Event; and (v) provide that any other stock-based Awards outstanding (A) shall become exercisable, realizable or vested in full, or shall be free of all conditions or restrictions, as applicable to each such Award, prior to the consummation of the Acquisition Event, or (B) shall be assumed, or equivalent Awards shall be substituted, by the acquiring or succeeding entity (or an affiliate thereof).

An “Acquisition Event” shall mean: (a) any merger or consolidation which would result in the voting securities of the Company outstanding immediately prior thereto continuing to represent (either by remaining outstanding or by being converted into voting securities of the surviving or acquiring entity) less than 60% of the combined voting power of the voting securities of the Company or such surviving or acquiring entity outstanding immediately after such merger or consolidation; (b) any sale of all or substantially all of the assets of the Company; (c) the complete liquidation of the Company; or (d) the acquisition of “beneficial ownership” (as defined in Rule 13d-3 under the Exchange Act) of securities of the Company representing 60% or more of the combined voting power of the Company’s then outstanding securities (other than through an acquisition of securities directly from the Company) by any “person” as such term is used in Sections 13(d) and 14 (d) of the Exchange Act other than the Company, any trustee or other fiduciary holding securities under an employee benefit

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plan of the Company, or any entity owned directly or indirectly by the stockholders of the Company in substantially the same proportion as their ownership of stock of the Company.

(2) Assumption of Awards Upon Certain Events. The Board may grant Awards under the Plan in substitution for stock and stock-based awards held by employees of another entity who become employees of the Company as a result of a merger or consolidation of the employing entity with the Company or the acquisition by the Company of property or stock of the employing entity. The substitute Awards shall be granted on such terms and conditions as the Board considers appropriate in the circumstances.

(a) Transferability of Awards. Except as the Board may otherwise determine or provide in an Award, Awards shall not be sold, assigned, transferred, pledged or otherwise encumbered by the person to whom they are granted, either voluntarily or by operation of law, except by will or the laws of descent and distribution, and, during the life of the Participant, shall be exercisable only by the Participant. References to a Participant, to the extent relevant in the context, shall include references to authorized transferees.

(b) Documentation. Each Award shall be evidenced by a written instrument in such form as the Board shall determine, it being understood that an electronic form of Award shall be deemed to be a written instrument for purposes of the Plan. Each Award may contain terms and conditions in addition to those set forth in the Plan.

(c) Board Discretion. Except as otherwise provided by the Plan, each Award may be made alone or in addition or in relation to any other Award. The terms of each Award need not be identical, and the Board need not treat Participants uniformly.

(d) Termination of Status. The Board shall determine the effect on an Award of the disability, death, retirement, authorized leave of absence or other change in the employment or other status of a Participant and the extent to which, and the period during which, the Participant, the Participant’s legal representative, conservator, guardian or Designated Beneficiary may exercise rights under the Award.

(e) Withholding. Each Participant shall pay to the Company, or make provision satisfactory to the Board for payment of, any taxes required by law to be withheld in connection with Awards to such Participant no later than the date of the event creating the tax liability. Except as the Board may otherwise provide in an

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Award, when the Common Stock is registered under the Exchange Act, Participants may, to the extent then permitted under applicable law, satisfy such tax obligations in whole or in part by delivery of shares of Common Stock, including shares retained from the Award creating the tax obligation, valued at their Fair Market Value. The Company may, to the extent permitted by law, deduct any such tax obligations from any payment of any kind otherwise due to a Participant.

(f) Amendment of Award. The Board may amend, modify or terminate any outstanding Award, including but not limited to, substituting therefor another Award of the same or a different type and changing the date of exercise or realization, provided that the Participant’s consent to such action shall be required unless the Board determines that the action, taking into account any related action, would not materially and adversely affect the Participant. Without intending to limit the generality of the preceding sentence, the Board may, without amending the Plan, modify Awards granted to Participants who are foreign nationals or employed outside the United States to recognize differences in laws, rules, regulations or customers of such foreign jurisdiction with respect to tax, securities, currency, employee benefits or other matters.

(g) Conditions on Delivery of Stock. The Company will not be obligated to deliver any shares of Common Stock pursuant to the Plan or to remove restrictions from shares previously delivered under the Plan until (i) all conditions of the Award have been met or removed to the satisfaction of the Company, (ii) in the opinion of the Company’s counsel, all other legal matters in connection with the issuance and delivery of such shares have been satisfied, including any applicable securities laws and any applicable stock exchange or stock market rules and regulations, and (iii) the Participant has executed and delivered to the Company such representations or agreements as the Company may consider appropriate to satisfy the requirements of any applicable laws, rules or regulations.

(h) Acceleration. The Board may at any time provide that any Options shall become immediately exercisable in full or in part, that any Restricted Stock Awards shall be free of restrictions in full or in part or that any other Awards may become exercisable in full or in part or free of some or all restrictions or conditions, or otherwise realizable in full or in part, as the case may be.

(a) No Right To Employment or Other Status. No person shall have any claim or right to be granted an Award, and the grant of an Award shall not be construed as giving a Participant the right to continued employment or any other relationship with the Company. The Company expressly reserves the right at any time

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to dismiss or otherwise terminate its relationship with a Participant free from any liability or claim under the Plan, except as expressly provided in the applicable Award.

(b) No Rights As Stockholder. Subject to the provisions of the applicable Award, no Participant or Designated Beneficiary shall have any rights as a stockholder with respect to any shares of Common Stock to be distributed with respect to an Award until becoming the record holder of such shares. Notwithstanding the foregoing, in the event the Company effects a split of the Common Stock by means of a stock dividend and the exercise price of and the number of shares subject to such Option are adjusted as of the date of the distribution of the dividend (rather than as of the record date for such dividend), then a Participant who exercises an Option between the record date and the distribution date for such stock dividend shall be entitled to receive, on the distribution date, the stock dividend with respect to the shares of Common Stock acquired upon such Option exercise, notwithstanding the fact that such shares were not outstanding as of the close of business on the record date for such stock dividend.

(c) Effective Date and Term of Plan. The Plan is effective as of December 10, 1999 the date on which it was adopted by the Board (the “Effective Date”). No Awards shall be granted under the Plan after the completion of ten years from the Effective Date, but Awards previously granted may extend beyond that date.

(d) Amendment of Plan. The Board may amend, suspend or terminate the Plan or any portion thereof at any time.

(e) Governing Law. The provisions of the Plan and all Awards made hereunder shall be governed by and interpreted in accordance with the laws of the State of Maryland, without regard to any applicable conflicts of law.

Adopted by the Board of Directors on

December 10, 1999

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AMENDMENT NO. 1

TO THE

1999 NON-OFFICER EMPLOYEE STOCK INCENTIVE PLAN

OF

ENTRUST, INC.

The 1999 Non-Officer Employee Stock Incentive Plan (the “Plan”) of Entrust, Inc. is hereby amended as follows (all capitalized terms used herein and not defined herein shall have the respective meanings ascribed to such terms in the Plan):

“Subject to adjustment under Section 8, Awards may be made under the Plan for up to 7,600,000 shares of common stock, $.01 par value per share, of the Company (the “Common Stock”).

Adopted by the Board of Directors on

December 1, 2000

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AMENDMENT NO. 2

TO THE

1999 NON-OFFICER EMPLOYEE STOCK INCENTIVE PLAN

OF

ENTRUST, INC.

The 1999 Non-Officer Employee Stock Incentive Plan, as amended (the “Plan”), of Entrust, Inc. is hereby further amended as follows (all capitalized terms used herein and not defined herein shall have the respective meanings ascribed to such terms in the Plan):

“Subject to adjustment under Section 8, Awards may be made under the Plan for up to 9,600,000 shares of common stock, $.01 par value per share, of the Company (the “Common Stock”).

Adopted by the Board of Directors on

April 26, 2001

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AMENDMENT NO. 3

TO THE

1999 NON-OFFICER EMPLOYEE STOCK INCENTIVE PLAN

OF

ENTRUST, INC.

The 1999 Non-Officer Employee Stock Incentive Plan, as amended (the “Plan”), of Entrust, Inc. is hereby further amended as follows (all capitalized terms used herein and not defined herein shall have the respective meanings ascribed to such terms in the Plan):

“Subject to adjustment under Section 8, Awards may be made under the Plan for up to 11,600,000 shares of common stock, $.01 par value per share, of the Company (the “Common Stock”).”

Adopted by the Board of Directors on

October 25, 2002

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