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PKI Basics Digital Signatures
and Public Key Infrastructure (PKI) 101
Introduction
The "paperless office" concept has been around for well over
a decade. It has failed to move from theory to reality, however,
because of cultural reticence, unequal access to technology, and
the lack of an adequate legal and service infrastructure to support
such a paradigm shift. Conspicuously missing from this list of impediments
is the technology itself, because the technology has been available
for the last 20 years or so. Ours is a paper-based society. We are
comfortable with paper, it is tangible, and we feel secure about
the integrity and source of the information it conveys. In short,
we trust it. In traditional commerce, this trust comes from the
use of secure paper, watermarks, letterhead, handwritten signatures,
sealed envelopes and personal contact. In our initial efforts to
"go digital," we have tried to mimic the paper-based world by attempting
to protect the medium through which communications are conveyed.
We have moved to Local Area Networks and Wide Area Networks within
our organizations, and Value Added Networks, T1s and leased lines
to communicate with the external world. The problem with these private
networks is their inability to scale to the proportions necessary
to satisfy the demands of the "global economy." They are based on
existing relationships and require cost-prohibitive connectivity.
The Internet and Electronic Commerce
The Internet is the obvious solution to this problem. Relatively
unnoticed until the past 5 years, the explosion of Internet use
has been tempered only by the lack of sufficient information security
and a legal framework to enable electronic commerce via the Internet
to flourish. Despite these shortcomings, governments, businesses
and individuals are using the Internet more and more as an inexpensive
and ubiquitous means to disseminate and obtain information, goods
and services. With the advent of public key cryptography technology
and the legal recognition of digital signatures at both the state
and federal level, the full potential of the Internet is just beginning
to be discovered. With adequate security and an appropriate legal
and service infrastructure, the Internet may now be used as a Global
Area Network (GAN). The ultimate objective for this GAN is to enable
real-time electronic transactions with strangers that are reliable,
provable and enforceable. While the Internet and public key cryptography
have been around for decades, the legal and service infrastructure
to support their widespread implementation has not. With the establishment
of such a legal framework and infrastructure now underway, the GAN
- and the elimination of paper - is finally becoming a reality.
Overview
All legally binding communications or transactions, whether electronic
or paper-based, must meet these fundamental requirements: The first
requirement is that the message provide for sender authenticity
to enable the recipient (or relying party) to determine who really
sent the message and if that individual is, in fact, authorized
to commit his organization to the transaction. The second major
requirement is that there be some means to ascertain that the message
has integrity. The recipient must be able to determine whether or
not the message received has been altered en route or is incomplete.
The third, and most critical, requirement addresses the ability
to "prove up" the message in court. Referred to as non-repudiation,
this requires some way to ensure that the sender cannot falsely
deny sending the message, nor falsely deny the contents of the message.
Finally, certain signature formalities must be satisfied. For example,
the statute of fraud specifies "in writing" and signature requirements
for transactions over a certain dollar value or time period.
Satisfying the Requirements in Electronic Commerce
In electronic commerce, the focus to date has been on securing the
medium through the use of private leased lines and networks. This
is prohibitively expensive and, in some cases, unfeasible for potential
parties to a transaction. For the Internet to offer an inexpensive
and ubiquitous solution, the focus must be on information security.
The goal here is to protect the message, not the medium. The Internet
is insecure - potentially millions of people have access and "hackers"
can intercept anything traveling over the wire. There is no way
to make it a secure environment; it is, after all, a public network,
hence its availability and affordability. In order for it to serve
our purposes as a vehicle for legally binding transactions, efforts
must be directed at securing the message itself, as opposed to the
transport mechanism. Public key cryptography, a data encryption
technique, provides just that kind of message protection. Originally
recognized within the context of electronic funds transfer and UCC
Article 4A, digital signatures - which are based on public key cryptography
- have been thrust into the legal limelight as the solution to the
problem of guaranteeing secure electronic commerce. The Utah Digital
Signature Act was the first legislative initiative to address secure
electronic commerce, with efforts by other states and the federal
government trailing close behind.
Digital Signatures and Information Security
In defining digital signatures and how they work, it is helpful
to begin by clarifying what they are not. A digital signature is
not a digitized image of a handwritten signature. We are all familiar
with the electronic pad a person signs upon receiving a package
from a delivery service such as Federal Express. In these cases,
the handwritten signature is digitized and the image transferred
to the electronic document. Once captured, these digitized signatures
can be cut and pasted on to any electronic document, making forgery
a simple matter. Digital signatures on the other hand are an actual
transformation of an electronic message using public key cryptography.
Through this process, the digital signature is tied to the document
being signed, as well as to the signer, and therefore cannot be
reproduced. Furthermore, with the passage of the federal digital
signature bill, digitally signed electronic transactions have the
same legal weight as transactions signed in ink. Now, a legally
binding contract may be formed over the Internet by two parties
who have never met, without requiring notarization. This will radically
alter the way business is conducted and accelerate the already rapid
adoption of so-called electronic commerce.
The Basic Principles
The principles underlying the use of cryptography in electronic
communications are as follows:
1. All data entered into a computer is read as a binary number.
For example, when "Jack and Jill went up the hill" is typed in,
the computer reads it as "1000111010100111000101," etc.
2. Because electronic messages are represented numerically in the
computer, it is possible to perform mathematical functions on them.
3. Electronic messages can thus be transformed into alternate representations
that are unique to the original
Public Key Cryptography
There are two distinct encryption techniques. Symmetric cryptography
is the most familiar. It is based on a shared secret, or key, and
works well within isolated environments. An example of symmetric
cryptography is the automated teller machine (ATM) at a bank. When
you use an ATM, you gain access to your account by entering a personal
identification number (PIN). You are, in effect, authenticating
yourself to the bank. You and the bank share a secret, in this case
your PIN, and, as such, can communicate securely upon revealing
knowledge of this secret. The inherent problem with symmetric cryptography
is one of scalability. In order for the communications to be confidential,
the exchange of the key, or shared secret, must be done securely.
Obviously, this type of secure distribution is not feasible when
the number of different people with whom you want to communicate
securely escalates beyond a manageable number. The other encryption
technique is asymmetric cryptography - also known as public key
cryptography - because it involves an asymmetric key pair. This
key pair is comprised of what is referred to as a public key and
a private key. The public key, as its name suggests, may be freely
disseminated. This key does not need to be kept confidential. The
private key, on the other hand, must be kept secret. The owner of
the key pair must guard his private key closely, as sender authenticity
and non-repudiation are based on the signer having sole access to
his private key. There are several important characteristics of
these key pairs. First, while they are mathematically related to
each other, it is impossible to calculate one key from the other.
Therefore, the private key cannot be compromised through knowledge
of the associated public key. Second, each key in the key pair performs
the inverse function of the other. What one key does, only the other
can undo.
Digital Signature Components
Digital signatures are based on asymmetric, or public key, cryptography.
In addition to a key pair and some type of electronic communications,
the digital signing and verification processes involve something
known as a hash algorithm and a signature algorithm. The hash and
signature algorithms are extremely complex mathematical equations.
The hash algorithm is performed on the original electronic message's
binary code, resulting in what is referred to as a message digest,
which is a 160-bit string of digits that is unique to the original
message. The signature algorithm is then performed on this message
digest. The resultant string of digits is the digital signature.
The signer's private key is incorporated into the signature algorithm
during the signing process, and the public key is incorporated into
the signature algorithm during the verification process. An extremely
rudimentary mathematical example of this would be as follows:
For the sake of simplicity, assume that the binary number 100 represents
the original message. Again for simplicity, assume the hash algorithm
is simply to multiply the binary by two. The result of passing the
binary of the original message through the hash algorithm is the
message digest, or the unique fingerprint of the message, which
is 200 in this example. This message digest is then passed through
the signature algorithm, of which the signer's private key is a
component. In this example, the signature algorithm has been drastically
simplified to multiplying by two to the *, where * equals the signer's
private key, in this case 2. The resulting number of 800 is the
digital signature. In contrast to a digitized signature, a digital
signature has nothing to do with the signer's name or handwritten
signature. It is an actual transformation of the message itself
that incorporates a "secret" known only to the signer, and is therefore
tied to both the signer and the message being signed. A signer's
digital signature will be different for each different document
he signs.
Digital Signature Processes
The following are graphical representations of the digital signing
and verification processes, respectively:
Public Key Infrastructure
It is now possible for an individual to purchase digital signature
software, or download it from a browser, and install it on his computer.
He can then generate a key pair and release his public key to the
on-line world, using any identity he chooses, with no guarantee
that the identity is authentic. This scenario underscores the need
for some type of entity to serve as a trusted third party (TTP)
to vouch for individuals' identities, and their relationship to
their public keys. This entity, in public key infrastructure (PKI)
terminology, is referred to as a certification authority (CA). The
CA is a trusted third party that issues digital certificates to
its subscribers, binding their identities to the key pairs they
use to digitally sign electronic communications. Digital certificates
contain the name of the subscriber, the subscriber's public key,
the digital signature of the issuing CA, the issuing CA's public
key, and other pertinent information about the subscriber and his
organization, such as his authority to conduct certain transactions,
etc. These certificates have a default life cycle of 1 year, and
can be revoked upon private key compromise, separation from an organization,
etc. These certificates are stored in an on-line, publicly accessible
repository. The repository also maintains an up-to-date listing
of all the certificates, that have not yet expired, which have been
revoked, referred to as a certificate revocation list (CRL). The
repository also maintains an electronic copy of the certification
practice statement (CPS) of each CA that publishes certificates
to it. The CPS outlines the policies and procedures of each CA's
operations from registration of a subscriber to the physical security
surrounding their CA system.
The following is a graphical representation of the PKI process
flow.
PKI Process Flow
Step 1.
Subscriber applies to Certification Authority for Digital Certificate.
Step 2.
CA verifies identity of Subscriber and issues Digital Certificate.
Step 3.
CA publishes Certificate to Repository.
Step 4.
Subscriber digitally signs electronic message with Private Key to
ensure Sender Authenticity, Message Integrity and Non-Repudiation
and sends to Relying Party.
Step 5.
Relying Party receives message, verifies Digital Signature with
Subscriber's Public Key, and goes to Repository to check status
and validity of Subscriber's Certificate.
Step 6.
Repository returns results of status check on Subscriber's Certificate
to Relying Party.
Digital Signature Applications
Digital signatures are critical to the electronic conversion of
any presently paper-based process that requires strong authentication
of both the sender and the contents of the message, and/or non-repudiation.
The number of such applications is virtually endless, ranging from
purchase order systems, time cards and automated forms processing
to contracts and remote financial transactions or inquiries.
Obligations and Legalities
The effective use of digital signatures imposes certain obligations
on the parties involved. The signers of electronic messages must
protect their private key from compromise. This is the fundamental
building block of the PKI. If a signer's private key is compromised,
he must report it immediately so the CA can revoke his certificate
and place it on a CRL. Certification authorities are obligated to
use due diligence to verify the identity of their subscribers and
their relationship to their public keys. The CA must also promptly
suspend or revoke a certificate at a subscriber's request. Finally,
the reliant parties must actually verify the digital signature and
check its validity against the current CRL maintained by an on-line
repository.
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