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TUCoPS :: Crypto :: des.txt

Info on the DES standard in Encryption



The National Institute of Standards and Technology (NIST) of the
Department of Commerce has recently received many inquiries
regarding various aspects of the Data Encryption Standard (DES). 
This document addresses those frequently asked questions and
provides interested individuals with sources of additional
information.  The document is not designed to issue new policy;
rather it summarizes and clarifies existing policies.  Additional
guidance concerning the use of National Security Agency (NSA)
developed Type II and Low-cost Encryption Authentication Devices
(LEAD) is planned to be issued in 1990.


Issued as Federal Information Processing Standard Publication
(FIPS PUB) 46 in 1977, the DES was promulgated by NIST (then the
National Bureau of Standards) to provide a system for the
protection of the confidentiality and integrity of the federal
government's sensitive unclassified computer information.  FIPS
PUB 46 is based upon work by the International Business Machines
Corporation and has been approved as American National Standard
X3.92-1981/R1987.  The DES has been reaffirmed twice, most
recently in 1988.  The current standard, which was issued as FIPS
PUB 46-1, reaffirms the standard until 1993.

Technical Overview 
The Data Encryption Standard specifies a cryptographic algorithm
that converts plaintext to ciphertext using a key, a process
called encryption.  The same algorithm is used with the same key
to convert ciphertext back to plaintext, a process called
decryption.  The DES consists of 16 "rounds" of operations that
mix the data and key together is a prescribed manner using the
fundamental operations of permutation and substitution.  The goal
is to completely scramble the data and key so that every bit of
the ciphertext depends on every bit of the data and every bit of
the key (a 56-bit quantity for the DES).  After sufficient
"rounds" with a good algorithm, there should be no correlation
between the ciphertext and either the original data or key. 
The DES uses 16 rounds for several reasons.  First, a minimum of
12 rounds were needed to sufficiently scramble the key and data
together; the others provided a margin of safety.  Second, the
operation of 16 rounds would return the key back to its original
position in an electronic device for the next use when used in
accordance with the published algorithm.  Third, numerous
"rounds" were needed to keep an analyst or adversary from working
simultaneously forward and backward and "meeting in the middle"
with a solution. 
Security Provided by DES 
The security provided by the DES depends on several factors: 
mathematical soundness, length of key, key management, input data
formatting, mode of operation, implementation, application and
The DES was developed to protect unclassified computer data in
federal computer systems against a number of passive and active
attacks in communications and storage systems.  It was assumed
that a knowledgeable person might seek to comprise the security
system with resources commensurate to the value of the
information to be obtained.  Applications included Electronic
Funds Transfer, privacy protection of personal information,
personal authentication, password protection, access control,
The DES has been evaluated by several organizations and has been
determined to be mathematically sound.  The effective length of
the data key (56-bits) was challenged by several people as being
too short for high security applications.  Several people have
analyzed the algorithm and have concluded that the algorithm is
sound but would not be "if only this simple change was made." 
The most recent charge was that "if the DES has only 6 or 8
rounds instead of 16, then it could be broken on a personal
computer in 0.3 seconds and 3 minutes respectively. 
The two algorithms that were "broken on a personal computer" in
0.3 seconds and 3 minutes respectively WERE NOT THE DES.  There
is only one DES and any change to it results in an algorithm that
IS NOT THE DES.  Cryptographically, any algorithm that is
obtained by any change to the DES may be significantly different
in the security it provides.  Thus, while the DES is sound, many
algorithms that are similar to, but different from, the DES are
not sound. 
NIST has determined that at least until 1993, the DES will
continue to provide more than adequate security for its intended
applications.  It is currently the only cryptographic method to
be used in the federal government to protect unclassified
computer data (except that information described in 10 U.S.C.
Section 2315).  However, NIST does plan to augment the DES with
other cryptographic algorithms in a family of standards that will
provide other types of protection in special applications (e.g.,
digital signatures, key exchange, exportable security).  NIST
will continue to support the use of DES in government security
applications for the foreseeable future. 


Subject to agency waivers as discussed below, use of DES is
mandatory for all federal agencies, including defense agencies,
for the protection of sensitive unclassified data communications
(except information covered by 10 U.S.C. Section 2315, as
described below) when the agency or department determines that
cryptographic protection is required.  Note that the term
unclassified information as used in this document excludes
information covered by 10 U.S.C. 2315.  Use of DES is currently
applicable only to the protection of data communications.  

The National Security Agency (NSA) of the U.S. Department of
Defense develops and promulgates requirements for those
telecommunications and automated information systems operated by
the U.S. Government, its contractors, or agents, that contain
classified information or, as delineated in 10 U.S.C. Section
2315, the function, operation, or use of which:

  -    involves intelligence activities;
  -    involves cryptologic activities related to national
  -    involves the direct command and control of military forces;
  -    involves equipment which is an integral part of a weapon or
       weapon systems; or
  -    is critical to the direct fulfillment of a military or
       intelligence mission.

DES may be used by private-sector individuals or organizations at
their discretion.  

Waivers for the Mandatory Use of DES

The head of a federal department or agency may waive the use of
DES for the protection of unclassified information as discussed

Waivers to the mandatory use of DES are required if:

  -    cryptographic devices perform an algorithm other than DES
       and are used by federal departments or agencies for
       cryptographic protection of information;

  -    DES is implemented in a software-based system (See specific
       exclusions below.); or

  -    the agency or department wishes to use Type II (i.e, for
       unclassified applications) cryptographic devices certified
       by NSA (except for current voice only applications). 
       [Note:  Type I products have been approved by NSA for the
       protection of classified information while Type II products
       have been approved for the protection of unclassified

Waivers to the mandatory use of DES are not required if:

  -    the agency or department wishes to use Type I (i.e., for
       classified applications) cryptographic equipment;

  -    DES is implemented in software for testing or evaluation
       purposes; or

  -    DES is implemented in software for a limited special
       purpose (e.g., encrypting password files).

Additionally, no waivers are currently required for use of Type
II products for voice only applications.  

Waiver Procedures

As mentioned above, the heads of federal departments or agencies
may waive the mandatory use of DES.  This authority may be
redelegated only to a senior official designated pursuant to 44
U.S.C. section 3506(b).  Waivers shall be granted only when:
  -    compliance with the standard would adversely affect the
       accomplishment of the mission of an operator of a federal
       computer system; or

  -    compliance would cause a major adverse financial impact on
       the operator which is not offset by Governmentwide savings.

In addition, when a waiver is being considered to allow for the
use of Type II products, the agency must document that such
devices offer equivalent cost/performance features when compared
to devices conforming to the DES standard.  

Agency heads may act upon a written waiver request containing the
information detailed above.  Agency heads may also act without a
written waiver request when they determine that conditions for
meeting the standard cannot be met.  Agency heads may approve
waivers only by a written decision which explains the basis on
which the agency head made the required finding(s).  A copy of
each such decision, with procurement-sensitive or classified
portions clearly identified, shall be sent to:

  National Institute of Standards and Technology
  Attention: FIPS Waiver Decisions
  Technology Building, Room B-154
  Gaithersburg, MD  20899

In addition, notice of each waiver granted and each delegation of
authority shall be sent promptly to the Committee on Government
Operations of the House of Representatives and the Committee on
Governmental Affairs of the Senate and shall be published
promptly in the Federal Register.

When the determination on a waiver applies to the procurement of
equipment and/or services, a notice of the waiver determination
must be published in the Commerce Business Daily as a part of the
notice of solicitation for offers of an acquisition or, if the
waiver determination is made after that notice is published, by
amendment to such notice.  

A copy of the waiver, any supporting documents, the document
approving the waiver and any supporting or accompanying
documents, with such deletions as the agency is authorized and
decides to make under 5 U.S.C. Section 552(b), shall be part of
the procurement documentation and retained by the agency.  

Endorsement of DES Products

DES products for use in telecommunications equipment and systems
are no longer being endorsed for conformance to FIPS PUB 140
(formerly Federal Standard 1027) by NSA.  NIST has notified the
heads of federal departments that they may wish to consider
waiving the requirements of FIPS PUB 140 in order to buy
equipment which may not meet all of the criteria in the standard. 
This action will enable agencies to procure cost-effective
equipment that meets their needs, but has not been endorsed by

FIPS PUB 140 is currently under revision to be reissued as FIPS
PUB 140-1.  All issues contained within the scope of the document
are being readdressed.  Additionally, NIST is examining various
methods for conducting conformance testing against the
requirements of FIPS PUB 140-1.  Until the NIST FIPS 140-1
program is established, federal agencies may accept written
affirmation of conformance to FIPS PUB 140 from vendors as
sufficient indication of conformance.  

DES Cryptographic Keys

U.S. government users of NSA-endorsed products may obtain DES
cryptographic keys for these products from NSA upon request at no
cost.  Contact your responsible Communications Security (COMSEC)
officer for further information.   
Alternatively, users of the DES, including federal organizations,
may generate their own cryptographic keys.  DES keys must be
properly generated and managed in order to assure a high level of
protection to computer data.  Electronic Key Management includes
generation, distribution, storage, and destruction of
cryptographic keys using automated processes.  Information on
this subject may be obtained from FIPS 74, FIPS 140-1 (future),
ANSI X9.17, and the Secure Data Network System (SDNS) documents
available from NIST.  The specifics of electronic key generation
are outside the scope of this document.

The keys used to protect electronic funds transfers must be able
to be changed and should be changed aperiodically, but at least
annually.  Very large electronic funds transfers should be
protected individually with separate keys and the input data must
be properly formatted to assure high security. 

Exportability of DES Devices and Software Products

Hardware- and software- based implementations of DES are subject
to federal export controls as specified in Title 22, Code of
Federal Regulations (CFR), Parts 120 - 130, the International
Traffic in Arms Regulations (ITAR).  Specific information
regarding export applications, application procedures, types of
licenses, and necessary forms may be found in the CFR. 
Responsibility for granting export licenses (except for those DES
implementations noted below) rests with:

       Office of Munitions Control
       Bureau of Politico-Military Affairs
       U.S. Department of State
       Washington, DC, 20250
       Telephone: (202) 875-6650

The Office of Munitions Control, U.S. Department of State issues
either individual or distribution licenses.  Under a distribution
license, annual reports must be submitted by the distributor
describing to whom the licensed products have been sold.  License
requests for products to be shipped to certain prohibited
countries (see Section 126.1 of the ITAR) are denied for foreign
policy reasons by the Department of State.  

Licenses are normally granted if the end users are either
financial institutions or American subsidiaries abroad.  In
general, either individual or distribution licenses may be used
for financial institutions while only individual licenses may be
used for subsidiaries of U.S. corporations.  

Specific Cryptographic Implementations under Jurisdiction of the
Department of Commerce

The Bureau of Export Administration, U.S. Department of Commerce
is responsible for the granting of export licenses for the
following categories of cryptographic products (including DES):

  -    Authentication.  Software or hardware which calculates a
       Message Authentication Code (MAC) or similar result to
       assure no alteration of text has taken place, or to
       authenticate users, but does not allow for encryption of
       data, text or other media other than that needed for the

  -    Access Control.  Software or hardware which protect
       passwords or Personal Identification Numbers (PIN) or
       similar data to prevent unauthorized access to computing
       facilities, but does not allow for encryption of files or
       text, except as directly related to password or PIN

  -    Proprietary Software Protection.  Decryption-only routines
       for encrypted proprietary software, fonts, or other
       computer-related proprietary information for the purpose of
       maintaining vendor control over said information when such
       decryption routines are not accessible to users of said
       software, font or other information, and cannot be used for
       any other purpose.

  -    Automatic Teller Devices.  Devices limited to the issuance
       of cash or travellers checks, acceptance of deposits, or
       account balance reporting. 

Vendors of products in the above four categories should contact
the following for a product classification determination:

       Bureau of Export Administration
       U.S. Department of Commerce
       P.O. Box 273
       Washington, DC 20044
       Telephone: (202) 377-0708

Following this determination, the vendor will be informed whether
an export license from the U.S. Department of Commerce is
necessary.  The Bureau of Export Administration will provide
vendors with license procedures and further information as

Please note that vendors whose products do not fall clearly into
the above categories should follow procedures set forth in the
ITAR, 22 CFR 120-130.

Validation of Devices for Compliance with FIPS PUBS 46 and 113

NIST performs validations of products for compliance with FIPS
PUBS 46 and 113.  For further information about submitting
products for validation or to obtain a list of devices validated
under either standard, please contact:
       Manager, Security Technology Group
       Computer Security Division
       National Computer Systems Laboratory
       Building 225, Room A266
       National Institute of Standards and Technology
       Gaithersburg, MD  20899
       Telephone (301) 975-2920

Reference Documents

NIST Documents

NIST has issued FIPS PUBS and special publications regarding DES,
its implementation, and modes of operation.

  FIPS PUB 46-1, Data Encryption Standard

  This standard provides the technical specifications for the DES

  FIPS PUB 74, Guidelines for Implementation and Using the NBS
  Data Encryption Standard

  This guideline on DES discusses how and when data encryption
  should be used, various encryption methods, the reduction of
  security threats, implementation of the DES algorithm, and key

  FIPS PUB 81, DES Modes of Operation

  FIPS PUB 81 defines four modes of operation for DES which may be
  used in a wide variety of applications.  The modes specify how
  data will be encrypted and decrypted.  The four modes are: (1)
  Electronic Codebook (ECB), (2) the Cipher Block Chaining (CBC),
  (3) Cipher Feedback (CFB), and (4) Output Feedback (OFB).  

  FIPS PUP 113, Computer Data Authentication

  This standard specifies a Data Authentication Algorithm, based
  upon DES, which may be used to detect unauthorized
  modifications, both intentional and accidental, to data.  The
  Message Authentication Code as specified in ANSI X9.9 is
  computed in the same manner as the Data Authentication Code as
  specified in this standard.  

  FIPS PUB 139, Interoperability and Security Requirements for Use
  of the Data Encryption Standard in the Physical Layer of Data

  This standard specifies interoperability and security-related
  requirements for using encryption at the Physical Layer of the
  ISO Open Systems Interconnection (OSI) Reference Model in
  telecommunications systems conveying digital information.  FIPS
  PUB 139 was previously issued by the General Services
  Administration as Federal Standard 1026.

  FIPS PUB 140, General Security Requirements for Equipment Using
  the Data Encryption Standard

  This document establishes the physical and logical security
  requirements for the design and manufacture of DES equipment. 
  FIPS PUB 140 was previously issued by the General Services
  Administration as Federal Standard 1027.

  FIPS PUB 141, Interoperability and Security Requirements for Use
  of the Data Encryption Standard With CCITT Group 3 Facsimile

  This document specifies interoperability and security related
  requirements for use of encryption with the International
  Telegraph and Telephone Consultative Committee (CCITT), Group 3-
  type facsimile equipment.  

  NBS Special Publication 500-61, Maintenance Testing for the Data
  Encryption Standard

  This special publication describes the design of four
  maintenance tests for the Data Encryption Standard.  The tests
  consist of an iterative procedure that tests the operation of
  DES devices using a small program and minimal data.  The tests
  are defined as four specific stopping points in a general
  testing process and satisfy four testing requirements of
  increasing degree of completeness depending on the thoroughness
  of testing desired.  

  NBS Special Publication 500-156, Message Authentication Code
  (MAC) Validation System: Requirements and Procedures

  This special publication describes a Message Authentication Code
  (MAC) Validation System (MVS) to test message authentication
  devices for conformance to two data authentication standards: 
  FIPS PUB 113 and ANSI X9.9-1986, Financial Institution Message
  Authentication (Wholesale).  The MVS is designed to perform
  automated testing on message authentication devices which are
  remote to NIST.  This publication provides brief overviews of
  the two data authentication standards and introduces the basic
  design and configuration of the MVS.  The requirements and
  administrative procedures to be followed by those seeking formal
  NIST validation of a message authentication device are

Copies of these publications are for sale by the National
Technical Information Service, at:

       National Technical Information Service
       U.S. Department of Commerce
       5285 Port Royal Road
       Springfield, VA 22161
       Telephone (703) 487-4650, FTS: 737-4650

Other Documents

DES has been incorporated into a number of other standards,

  American National Standard for Financial Institution Message
  Authentication, ANSI X9.9-1982, 1430 Broadway, New York, NY.

  American National Standard for Personal Identification Number
  (PIN) Management and Security, ANSI X9.8-1982, 1430 Broadway,
  New York, NY.

  Data Encryption Algorithm (DEA), ANSI X3.92-1981, 1430 Broadway,
  New York, NY.

  Key Management Standard, Document 4.3, American Bankers
  Association, Washington, DC, 1980.

  Management and Use of Personal Identification Numbers, Cat. No.
  207213, American Bankers Association, Washington, DC, 1979.

  Protection of Personal Identification Numbers in Interchange,
  Document 4.5.6, American Bankers Association, Washington, DC,

NIST's Computer Security Program

For further information regarding other aspects of NIST's
computer security program, including NIST's federal agency
assistance program, please contact:

       Computer Security Division
       National Computer Systems Laboratory
       Building 225, Room A216
       National Institute of Standards and Technology
       Gaithersburg, MD  20899
       Telephone (301) 975-2934


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