INLS 187 Presentation on Digital Signatures
March 6, 2002 - Kate Johnson
What is a Digital Signature / What is Not a Digital Signature / How are Signatures Created and Used / Legislation and Legal Status /
Limitations and Criticisms / Additional Definitions / Resources
What is a Digital Signature?
Traditionally, signatures have provided three elements: message integrity, signer
authentication, and nonrepudiation. A digital signature is a code attached to
an electronic document that uniquely identifies the sender (i.e., authentication),
allows for nonrepudiation, and provides for message integrity.
A more formal definition: "(I) A value computed with a cryptographic algorithm
and appended to a data object in such a way that any recipient of the data can
use the signature to verify the data's origin and integrity.
(II) Data appended to, or a cryptographic transformation of, a data unit that
allows a recipient of the data unit to prove the
source and integrity of the data unit and protect against forgery, e.g. by the
recipient."
Source: IETF (http://www.ietf.org/rfc/rfc2828.txt).
What is Not a Digital Signature?
1. Electronic signature and digital signature are not synonyms. An electronic signature can be a symbol, sound, or process used to sign a document. A digital signature, on the other hand, is a secure electronic signature which uses encryption to authenticate the entity who signed the document.
A digital signature is an electronic signature, but an electronic signature is not necessarily a digital signature.
2. Digital signatures are not digital certificates. A digital signature authenticates
both a document and the entity which created that document. A digital certificate
(aka electronic id card) identifies a public key and the key's owner. Certificates
commonly contain the following elements:
a. A public key and the key owner's name
b. The Certifying Authority (CA) that issued the key. Verisign is a well-known
CA.
c. A serial number
d. The digital signature of the CA
Both digital signatures and certificates use asymmetric encryption.
How are Digital Signatures Created and Used (An Example
Using PKI)
On the Sending End:
1. The message to be sent is delimited. (You will see later why this precise
delimitation is needed.)
2. A hash function, within the sender's software, computes a hash result for
the original message.
3. Using the sender's private key, the hash is encrypted.
4. The encrypted hash becomes the digital signature of the message. (Note the
signature will change if the message is changed. The resultant message hashes
will vary.)
5. The digital signature is unique both to the message and the private key used to create it.
On the Receiving End:
1. The receiver computes a hash result for the message.
2. Using the sender's public key, the receiver decrypts the message hash.
3. There are two ways the receiver verifies the digital signature -
a. The hash result the receiver obtains must equal the hash result extracted
from the signature. Only then does the receiver know the message was unaltered.
b. The public key, used by the receiver, verifies the sender's private key was used to create the signature.
Note this is an abbreviated list. Many other statutes and regulations have been published regarding digital signatures. Look under Resources section for additional information.
1. US Federal: In June 2000, President Clinton signed the Federal Electronic Signatures in Global and Network Commerce Act ("E-Sign").
Summary and Comments: The E-Sign act extends nationally what had been implememted by many states through UETA. E-Sign does not aggressively promote or extend the use of electronic signatures. Rather, E-Sign is protective, in that it "assures electronic signatures, contracts and other records shall not be denied legal power and enforceability on the sole ground they are in electronic form." Electronic signatures carry the same legal weight as those on paper, but the Act does not mandate the use of electronic signatures.. (Source: National Governors Association press release)
E-Sign is very broad about what specific technology can be used to create the signature, and does not contain technical requirements. Public-key encryption is not mandated as a requirement, and other technologies, such as biometrics, could be used.
Some document types are exempted from E-Sign. Electronic documents concerning wills, adoption, and divorce, for example, are exempted.
2. US State: Uniform Electronic Transactions Act (UETA) is legislation that has been enacted by several states. Though states can introduce variations into the statute, UETA is an attempt by them to enact common interstate legislation. As of July 2001, 37 states had enacted UETA.
Comments: UETA was the model for E-Sign so there are many similarities. As with E-Sign, UETA grants electronic signatures legal enforcement but does not mandate their use. No technical requirements are made and documents concerning wills, adoption, divorce, and other topics are exempted from this legislation.
3. International: In 1999, the European Union adopted a Directive on a Common Framework for Electronic Signatures.
Comments: Unlike the US approach, the EU framework is more specific. It provides
wording that details how digital signatures need to be stored for authenticity
and nonrepudiation. It strongly advocates that digital signatures be stored
in tamper resistant devices (i.e., smart cards.) Each EU country will develop
its own laws within this framework.
1. A key element a signature (written or digital) must provide is nonrepudiation. The signer cannot say at a later time "I did not sign that" or "I have never seen that document." He or she cannot repudiate, or reject, the signature.
Digital signatures do not provide for a rigorous level of nonrepudiation. A digital signature indicates an individual's private key was used to create the signature. The signature does NOT indicate whether that person signed the document, or even read it. Two plausible scenarios for this are 1) Someone other than the signer (an assistant, for example) has access to the private key, or 2) The software used to create the signature has been cracked.
Schneier's articles (see Resources below) develop this argument more fully.
2. A second key element a signature must provide is authentication. Creation
of a key (private and public) must correctly and rigorously confirm an individual's
identity. Here are two ways this confirmation can be weakened and/or wrecked:
a. The digital certificate linking a public key with an individual can be cracked,
and authentication compromised,
b. A CA is cracked and its data becomes suspect. It may no longer be able to verify the certificates it has issued.
3. The lack of technical specifications within the US statutes may present two problems. First, the lack of standardization may lead to entities using subpar authentication methods. Second, there may be a lack of interoperability as entities settle for different authentication methods.
1. Hashing - Hashing is the transformation of a string of characters into a usually shorter fixed-length value that represents the original string. A message hash can also be referred to as a message digest.
2. Digital Signature Standard - Digital Signature Standard (DSS) is the digital signature algorithm (DSA) developed by the US National Security Agency (GSA) to generate a digital signature for the authentication of electronic documents. DSS was put forth by the National Institute of Standards and Technology (NIST) in 1994, and has become the United States government standard for authentication of electronic documents.
3. Asymmetric Encryption - Encryption method which employs two sets of mathematically related keys: one privately and confidentially held and the other publicly available. It is also termed public-key or Diffie-Hellman encryption. PGP is an example of an asymmetric encryption model.
1. American Bar Association Tutorial on Digital Signatures (JavaScript required)
2. Bruce Schneier has written several articles on digital signatures. Search for digital signatures within his CRYPTO-GRAM newsletters.
3. The Digital Signature Law Survey provides a thorough review of US and international statutes. It was updated October 2001.
4. The law firm, McBride Baker and Coles, also provides a review of US and international statues, with accompanying analyses.
5. The Electronic Privacy Information Center contains several articles on digital signatures, primarily from the privacy perspective.
6. The Computer Security Resource Center, at the National Institute for Standards and Technology, is a good resource for information on digital signatures, public key distribution, and other encryption terminology.