TUCoPS :: Crypto :: cslbull.txt

Digital Signature Standard

                            CSL BULLETIN
                            January 1993

The National Institute of Standards and Technology (NIST) has
proposed a public key-based Digital Signature Standard (DSS). 
This CSL Bulletin provides agencies with information regarding
the proposed standard and describes several applications which
may benefit from the DSS.

To reduce costs and increase productivity, many federal
government agencies are transforming paper-based systems into
automated electronic systems.  This trend has brought about a
need for a reliable, cost-effective way to replace a handwritten
signature with a digital signature.  Like a handwritten
signature, a digital signature can be used to identify and
authenticate the originator of the information.  A digital
signature can also be used to verify that information has not
been altered after it is signed; this provides message integrity. 
In August 1991, NIST proposed the DSS as a Federal Information
Processing Standard (FIPS).  The proposed DSS specifies a Digital
Signature Algorithm (DSA) for use in computing and verifying
digital signatures.  

Overview of Cryptographic Integrity and the DSS  
Cryptography can be categorized as either secret key cryptography
or public key cryptography.  Secret key cryptography uses a
single cryptographic key shared by two communicating parties. 
For secret key cryptography to be effective, the cryptographic
key must be kept secret and controlled only by the parties that
have access to the key.  FIPS 46-1, Data Encryption Standard
(DES), defines the secret key algorithm to be used by the
government for encrypting unclassified federal information.  

Using the DES, a cryptographic checksum known as a Message
Authentication Code (MAC) can be used to provide message
integrity as specified in FIPS 113, Computer Data Authentication. 
When a key is shared only between the sender and receiver, the
MAC can be used to identify the sender of the information to the
receiver.  However, implementations of this technology cannot
inherently be used to prove to a third party that information
actually originated from the sender.  Since both the sender of
the information and the receiver of the information share the
same key, it is possible that the information could have
originated from either party.    

Public key cryptography is a form of cryptography which makes use
of two keys:  a public key and a private key.  The two keys are
mathematically related, but the private key cannot be determined
from the public key.  In a system implementing public key
technology, each party has its own public/private key pair.  The
public key can be known by anyone; however, no one should be able
to modify it.  The private key is kept secret.  Its use should be
controlled by its owner and it should be protected against
modification as well as disclosure.  

The proposed DSS defines a public key cryptographic system for
generating and verifying digital signatures.  The private key is
randomly generated.  Using this key and a mathematical process
defined in the standard, the public key is generated.  The DSS is
used with the proposed FIPS for a Secure Hash Standard (SHS) to
generate and verify digital signatures.  

To generate a signature on a message, the owner of the private
key first applies the Secure Hash Algorithm (SHA), as defined in
the proposed SHS, to the message.  This action results in a
condensed representation of the message known as a message
digest.  The owner of the private key then applies the private
key to the message digest using the mathematical techniques
specified in the DSA to produce a digital signature.  Any party
with access to the public key, message, and signature can verify
the signature using the DSA.  Public keys are assumed to be known
to the public in general.  If the signature verifies correctly,
the receiver (or any other party) has confidence that the message
was signed by the owner of the public key and the message has not
been altered after it was signed.  

In addition, the verifier can provide the message, digital
signature, and signer's public key as evidence to a third party
that the message was, in fact, signed by the claimed signer. 
Given the evidence, the third party can also verify the
signature.  This capability, an inherent benefit of public key
cryptography, is called non-repudiation.  The DSS does not
provide confidentiality for information.  If confidentiality is
required, the signer could first apply the DES to the message and
then sign it using the DSA.  The figure below illustrates a
typical use of the DSS.  

Applications of Digital Signatures
Because the DSA authenticates both the identity of the signer and
the integrity of the signed information, it can be used in a
variety of applications.  For example, the DSA could be utilized
in an electronic mail system.  After a party generated a message,
that party could sign it using the party's private key.  The
signed message could then be sent to a second party.  After
verifying the received message, the second party would have
confidence that the message was signed by the first party.  The
second party would also know that the message was not altered
after the first party signed it. 

In legal systems, it is often necessary to affix a time stamp to
a document in order to indicate the date and time at which the
document was executed or became effective.  An electronic time
stamp could be affixed to documents in electronic form and then
signed using the DSA.  Applying the DSA to the document would
protect and verify the integrity of the document and its time

The DSA could also be employed in electronic funds transfer
systems.  Suppose an electronic funds transfer message is
generated to request that $100.00 be transferred from one account
to another.  If the message was passed over an unprotected
network, it may be possible for an adversary to alter the message
and request a transfer of $1000.00.  Without additional
information, it would be difficult, if not impossible, for the
receiver to know the message had been altered.  However, if the
DSA was used to sign the message before it was sent, the receiver
would know the message had been altered because it would not
verify correctly.  The transfer request could then be denied.

The DSA could be employed in a variety of business applications
requiring a replacement of handwritten signatures.  One example
is Electronic Data Interchange (EDI).  EDI is the computer-to-
computer interchange of messages representing business documents. 
In the federal government, this technology is being used to
procure goods and services.  Digital signatures could be used to
replace handwritten signatures in these EDI transactions.  For
instance, contracts between the government and its vendors could
be negotiated electronically.  A government procurement official
could post an electronically signed message requesting bids for
office supplies.  Vendors wishing to respond to the request may
first verify the message before they respond.  This action
assures that the contents of the message have not been altered
and that the request was signed by a legitimate procurement

After verifying the bid request, the vendor could generate and
sign an electronic bid.  Upon receiving the bid, the procurement
official could verify that the vendor's bid was not altered after
it was signed.  If the bid is accepted, the electronic message
could be passed to a contracting office to negotiate the final
terms of the contract.  The final contract could be digitally
signed by both the contracting office and the vendor.  If a
dispute arose at some later time, the contents of the contract
and the associated signatures could be verified by a third party.

The DSA could also be useful in the distribution of software.  A
digital signature could be applied to software after it has been
validated and approved for distribution.  Before installing the
software on a computer, the signature could be verified to be
sure no unauthorized changes (such as the addition of a virus)
have been made.  The digital signature could be verified
periodically to ensure the integrity of the software.  

In database applications, the integrity of information stored in
the database is often essential.  The DSA could be employed in a
variety of database applications to provide integrity.  For
example, information could be signed when it was entered into the
database.  To maintain integrity, the system could also require
that all updates or modifications to the information be signed. 
Before signed information was viewed by a user, the signature
could be verified.  If the signature verified correctly, the user
would know the information had not been altered by an
unauthorized party.  The system could also include signatures in
the audit information to provide a record of users who modified
the information.    

These examples show how the DSA can be used in a variety of
applications to improve the integrity of both data and the
application.  CSL anticipates that federal agencies will
incorporate the DSS into a variety of automated electronic
systems that require message integrity and non-repudiation.

Security Provided by the DSS
The security provided by any public key cryptographic system
depends on several factors.  Some important considerations are
the mathematical soundness of the algorithm, the management of
keys, and the implementation of the system in an application. 
The safety of the DSA is dependent on the work needed to find or
compute the discrete logarithm of a very large number. 
Mathematicians and computer scientists have been working to find
a simple solution to the problem of finding logarithms for a long
time.  To date, only incremental improvements in computation have
been attained through the use of more powerful computers.  It is
important to understand that an adversary, who does not know the
private parameters of a party, cannot generate the party's
signature.  Therefore, a digital signature cannot be forged. 

Digital signatures offer protection not available by alternative
signature techniques.  One such alternative is a digitized
signature.  A digitized signature is generated by converting a
visual form of a handwritten signature to an electronic image.
Although a digitized signature resembles its handwritten
counterpart, it does not provide the same protection as a digital
signature.  Digitized signatures can be forged.  They can also be
duplicated and appended to other electronic data.  Digitized
signatures cannot be used to determine if information has been
altered after it is signed.

Supporting Functions 
Functions needed to support the use of the DSS include: 
o    The SHS is required to generate a message digest.  A message
     digest is a condensed representation of the information to
     be signed.  Using the SHS, it is computationally infeasible
     to find a message which corresponds to a given message
     digest, or to find two different messages which will produce
     the same message digest.

o    To use the DSS, a party must be able to generate random
     numbers to produce the public/private key pair and to
     compute the signature.  Random numbers can be generated
     either by a true noise hardware randomizer or by using a
     pseudorandom number generator.  One approved pseudorandom
     number generator is the key generation methodology found in
     Appendix C of the ANSI X9.17, "Financial Institution Key
     Management (Wholesale)."   

o    A means of associating public and private key pairs to the
     corresponding users is required.  That is, there must be a
     binding of a user's identity and the user's public key. 
     This binding may be certified by a mutually trusted party. 
     For example, a certifying authority could sign credentials
     containing a user's public key and identity to form a
     certificate.  Systems for certifying credentials and
     distributing certificates are beyond the scope of the DSS. 
     NIST plans to develop future guidance on certifying
     credentials and distributing certificates.  

User, legal, and technical issues related to the establishment
and operation of digital signature infrastructure are being
explored.  For example, users may require the ability to register
their public key in a directory or obtain a time/date stamp for
legal documents.  Legal issues such as the liabilities of the
certificate management authority, the admissibility of digitally
signed evidence, and the responsibilities of various federal
agencies in supporting the use of the DSS must be examined.  Some
technical requirements which must be addressed include the
interrelationships among users and user communities necessary for
providing services such as certifying credentials and
distributing certificates; the need to interoperate with private
sector and international certificate authorities; and the need to
provide users with the ability to withdraw or immediately revoke
their public key and provide notification to the appropriate
certificate and directory authorities.

NIST expects that this work will harmonize with applicable
international standards such as CCITT X.400 Recommendations,
standards for electronic mail, and CCITT X.500, standards for
directory services.  As this work progresses, NIST will provide
updates to federal departments and agencies.  

Applicability of the DSS
When approved by the Secretary of Commerce, the DSS will be the
governmentwide standard for use by all federal agencies including
defense agencies which require a public key cryptographic
signature system for unclassified information.  

In addition, NIST has been informed by Department of Defense
authorities that the DSS may be used to sign unclassified
information processed by "Warner Amendment" systems (10 U.S.C.
2315 and 44 U.S.C. 3502[2]) and classified data in selected

The General Accounting Office (GAO) has also issued a decision
regarding the use of electronic signatures to create valid
contractual obligations which can be recorded as consistent with
31 U.S.C. 1501.  Under Controller General Decision B-245714, the
GAO has concluded that "Electronic Data Interchange (EDI) systems
using message authentication codes which follow NIST's Computer
Data Authentication Standard (Federal Information Processing
Standard [FIPS] 113) or digital signatures following NIST's
Digital Signature Standard, as currently proposed, can produce a
form of evidence that is acceptable under section 1501." 

Reference Documents

Proposed FIPS for Digital Signature Standard
This proposed standard specifies a Digital Signature Algorithm
appropriate for applications requiring a digital rather than a
written signature.

Proposed FIPS for Secure Hash Standard
This proposed standard specifies a Secure Hash Algorithm (SHA)
for use with the proposed Digital Signature Standard. 
Additionally, for applications not requiring a digital signature,
the SHA is to be used whenever a secure hash algorithm is
required for federal applications.

FIPS 46-1, Data Encryption Standard (DES)
This standard provides the technical specifications for the DES.

FIPS 113, Computer Data Authentication
This standard specifies a Data Authentication Algorithm, based
upon the DES, which may be used to detect unauthorized
modifications to data, both intentional and accidental.  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.

Proposed FIPS 140-1, Security Requirements for Cryptographic
This proposed standard establishes the physical and logical
security requirements for the design and manufacture of modules
implementing NIST-approved cryptographic algorithms.

NIST Special Publication 800-2, Public Key Cryptography by James
Nechvatal.  This publication presents a survey of the state-of-
the-art of public key cryptography circa 1988-1990.

FIPS 171, Key Management Using ANSI X9.17
This standard adopts ANSI X9.17 and specifies a particular
selection of options for the automated distribution of keying
material by the federal government using the protocols of ANSI

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