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UNITED STATES SECURITIES AND EXCHANGE COMMISSION
 Washington, D.C. 20549
 FORM 10-K/A
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    ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF
1934
 For the fiscal year ended December 31, 2005
 OR
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    TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT
OF 1934
 For the transition period from                          to
 Commission file number: 000-24931
 S1 CORPORATION
 (Exact name of registrant as specified in its charter)
 Delaware
    
    58-2395199(State or other jurisdiction of
    
    (I.R.S. Employerincorporation or organization)
    
    Identification No.)3500 Lenox Road, NE, Suite 200Atlanta, Georgia
    
    30326(Address of principal executive offices)
    
    (Zip Code)
 Registrants telephone number, including area code: (404) 923-3500
 Securities registered pursuant to Section 12(b) of the Act:
 Securities registered pursuant to Section 12(g) of the Act:
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   Documents Incorporated by Reference




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EXPLANATORY NOTE




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TABLE OF CONTENTS  PART IVItem 15. Exhibits and Financial Statement SchedulesSIGNATURESEX-3.6 AMENDED AND RESTATED BYLAWS OF S1EX-31.3 SECTION 302, CERTIFICATION OF THE CEOEX-31.4 SECTION 302, CERTIFICATION OF THE CFO    Table of Contents  
 PART IV
 
 Item 15. Exhibits and Financial Statement Schedules.
  ExhibitNo.
    
    Exhibit Description3.1
    
    Amended and Restated Certificate of Incorporation of S1 Corporation (S1) (filed as
Exhibit 1 to S1s Registration Statement on Form 8-A filed with the Securities and Exchange
Commission (the SEC) on September 30, 1998 and incorporated herein by reference).3.2
    
    Certificate of Amendment of Amended and Restated Certificate of Incorporation of S1 dated
June 3, 1999 (filed as Exhibit 4.2 to S1s Registration Statement on Form S-8 (File No.
333-82369) filed with the SEC on July 7, 1999 and incorporated herein by reference).3.3
    
    Certificate of Amendment of Amended and Restated Certificate of Incorporation of S1 dated
November 10, 1999 (filed as Exhibit 3.3 to S1s Annual Report on Form 10-K for the fiscal year
ended December 31, 1999 and incorporated herein by reference).3.4
    
    Certificate of Designation for S1s Series B Redeemable Convertible Preferred Stock (filed as
Exhibit 2 to S1s Registration Statement on Form 8-A filed with the SEC on September 30, 1998
and incorporated herein by reference).3.5
    
    Certificate of Designations for S1s Series E Convertible Preferred Stock (filed as Exhibit 3
to S1s Registration Statement on Form S-8 (File No. 333-72250) filed with the SEC on October
26, 2001 and incorporated herein by reference).3.6
    
    Amended and Restated Bylaws of S1.4.1
    
    Specimen certificate for S1s common stock (filed as Exhibit 4 to S1s Quarterly Report on
Form 10-Q for the quarterly period ended March 31, 2000 and incorporated herein by reference).4.2
    
    Specimen certificate for S1s Series B Redeemable Convertible Preferred Stock (filed as
Exhibit 4.3 to S1s Annual Report on Form 10-K for the fiscal year ended December 31, 1998 and
incorporated herein by reference).4.3
    
    Specimen certificate for S1s Series E Convertible Preferred Stock (filed as Exhibit 4.2 to
S1s Registration Statement on Form S-3 (File No. 333-75178) filed with the SEC on December
14, 2001 and incorporated herein by reference).
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ExhibitNo.
    
    Exhibit Description10.1
    
    Stock Purchase Agreement, dated as of June 29, 1998, by and among SFNB, S1 and State Farm
Mutual Automobile Insurance Company (filed as Exhibit 10.4 to Pre-Effective Amendment No. 2 to
the S1s Registration Statement on Form S-4 (File No. 333-56181) filed with the SEC on August
21, 1998 and incorporated herein by reference).10.2
    
    Stock Subscription Warrant, dated December 23, 1999, issued by S1 to America Online, Inc.
(filed as Exhibit 10.16 to S1s Annual Report on Form 10-K for the fiscal year ended December
31, 1999 and incorporated herein by reference).10.3
    
    Alliance Center Office Lease Agreement, entered into as of February 25, 2000, by and between
Solano Associates, as Landlord, and Security First Technologies, Inc., as Tenant (filed as
Exhibit 10.3 to S1s Quarterly Report on Form 10-Q for the quarterly period ended September
30, 2000 and incorporated herein by reference).10.4
    
    Security First Technologies Corporation Amended and Restated 1995 Stock Option Plan (filed as
Appendix Bto S1s definitive proxy statement for S1s 1999 annual meeting of shareholders and
incorporated herein by reference).*10.5
    
    Amendment to Security First Technologies Corporation Amended and Restated 1995 Stock Option
Plan (filed as Exhibit 10.3 to S1s Quarterly Report on Form 10-Q for the quarterly period
ended March 31, 2000 and incorporated herein by reference).*10.6
    
    Security First Network Bank Amended and Restated Directors Stock Option Plan (filed as
Exhibit 10.2 to Pre-Effective Amendment No. 2 to S1s Registration Statement on Form S-4 (File
No. 333-56181) filed with the SEC on August 21, 1998 and incorporated herein by reference).*10.7
    
    Amendment to Security First Network Bank Amended and Restated Directors Stock Option Plan
(filed as Exhibit 10.1 to S1s Quarterly Report on Form 10-Q for the quarterly period ended
September 30, 2000 and incorporated herein by reference).*10.8
    
    Security First Technologies Corporation 1998 Directors Stock Option Plan (filed as Exhibit
10.3 to Pre-Effective Amendment No. 1 to S1s Registration Statement on Form S-4 (File No.
333-56181) filed with the SEC on July 30, 1998 and incorporated herein by reference).*10.9
    
    Amendment to Security First Technologies Corporation 1998 Directors Stock Option Plan (filed
as Exhibit 10.2 to S1s Quarterly Report on Form 10-Q for the quarterly period ended September
30, 2000 and incorporated herein by reference).*10.10
    
    Employment Agreement, entered into as of October 5, 2001, by and between S1 and Matthew Hale
(filed as Exhibit 10.36 to S1s Annual Report on Form 10-K for the fiscal year ended December
31, 2001 and incorporated herein by reference).*10.11
    
    Employment Agreement, entered into as of April 27, 2001, by and between S1 and Jaime
Ellertson (filed as Exhibit 10.1 to S1s Quarterly Report on Form 10-Q for the quarterly
period ended March 31, 2002 and incorporated herein by reference).*10.12
    
    Employment Agreement, entered into as of April 30, 2001, by and between S1 and James Mahan
(filed as Exhibit 10.2 to S1s Quarterly Report on Form 10-Q for the quarterly period ended
March 31, 2002 and incorporated herein by reference).*10.13
    
    S1 Corporation 2003 Stock Option Plan (filed as Attachment A to S1s Definitive Proxy
Statement on Schedule 14A filed with the SEC on April 18,2003 and incorporated herein by
reference).*10.14
    
    Stock Purchase Agreement By and Among S1 Corporation, Davidge Data Systems Corp. and GL
Consultants, Inc. Dated as of October 20, 2004 (filed as Exhibit 99 to S1s Current Report on
Form 8-K filed with the SEC on October 25, 2004 and incorporated herein by reference).
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ExhibitNo.
    
    Exhibit Description10.15
    
    Share Purchase Agreement, dated November 6, 2004, among several persons listed in Schedule I thereto and S1 Corporations (filed as Exhibit 2.1 to S1s Current Report on Form 8-K filed with the SEC on November 16, 2004 and incorporated herein by reference).10.16
    
    Agreement and Plan of Merger By and Among S1 Corporation, Edify Corporation, Edify Holding
Company, Inc., Intervoice, Inc., and Arrowhead I, Inc., dated as of November 18, 2005 (filed
as Exhibit 2.1 to S1s Current Report on Form 8-K filed with the SEC on January 4, 2006 and
incorporated herein by reference).10.17
    
    Description of Arrangement for
Directors Fees (incorporated by reference to Exhibit 10.17 of
the Companys Form 10-K filed with the Securities and Exchange Commission on March 16, 2006).21
    
    Subsidiaries of S1 (incorporated by reference to Exhibit 21 of the Companys Form 10-K filed
with the Securities and Exchange Commission on March 16, 2006).23.1
    
    Consent of Independent Registered Public Accounting Firm (incorporated by reference to
Exhibit 23.1 of the Companys Form 10-K filed with the Securities and Exchange Commission on
March 16, 2006).31.1
    
    Certificate of Chief Executive Officer dated March 15, 2006 (incorporated by reference to
Exhibit 31.1 of the Companys Form 10-K filed with the Securities and Exchange Commission on
March 16, 2006).31.2
    
    Certificate of Chief Financial Officer dated March 15, 2006 (incorporated by reference to
Exhibit 31.2 of the Companys Form 10-K filed with the Securities and Exchange Commission on
March 16, 2006).31.3
    
    Certificate of Chief Executive Officer dated March 30, 2006.31.4
    
    Certificate of Chief Financial Officer dated March 30, 2006.32.1
    
    Certificate of Chief Executive
Officer pursuant to §906 of the Sarbanes -Oxley Act of
2002 (incorporated by reference to Exhibit 32.1 of the Company's Form
10-K filed with the Securities and Exchange Commission on March 16,
2006).32.2
    
    Certificate of Chief Financial
Officer pursuant to §906 of the Sarbanes -Oxley Act of 2002
(incorporated by reference to Exhibit 32.2 of the Company's Form
10-K filed with the Securities and Exchange Commission on March 16,
2006).
 
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    Management contract or compensatory plan.  4




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SIGNATURES
  S1 CORPORATIONBy:
    
    /s/ James S. Mahan, IIIJames S. Mahan, IIIChief Executive Officer
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