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Public-key cryptography and related standards and
techniques underlie security features of many Netscape products, including
signed and encrypted email, form signing, object signing, single sign-on, and
the Secure Sockets Layer (SSL) protocol. This document introduces the basic
concepts of public-key cryptography.
Internet
Security Issues
Encryption
and Decryption
Digital
Signatures
Certificates
and Authentication
Managing
Certificates
For more information on these topics and other aspects of
cryptography, see Security
Resources.
For an overview of SSL, see Introduction
to SSL.
All communication over the Internet uses the Transmission
Control Protocol/Internet Protocol (TCP/IP). TCP/IP allows information to be
sent from one computer to another through a variety of intermediate computers
and separate networks before it reaches its destination.
The great flexibility of TCP/IP has led to its worldwide
acceptance as the basic Internet and intranet communications protocol. At the
same time, the fact that TCP/IP allows information to pass through intermediate
computers makes it possible for a third party to interfere with communications
in the following ways:
Normally, users of the many cooperating computers that make up
the Internet or other networks don't monitor or interfere with the network
traffic that continuously passes through their machines. However, many sensitive
personal and business communications over the Internet require precautions that
address the threats listed above. Fortunately, a set of well-established
techniques and standards known as public-key cryptography make it
relatively easy to take such precautions.
Public-key cryptography facilitates the following tasks:
The sections that follow introduce the concepts of public-key
cryptography that underlie these capabilities.
[Top]
Encryption is the process of transforming information so it is
unintelligible to anyone but the intended recipient. Decryption is the process
of transforming encrypted information so that it is intelligible again. A cryptographic
algorithm, also called a cipher, is a mathematical function used for
encryption or decryption. In most cases, two related functions are employed, one
for encryption and the other for decryption.
With most modern cryptography, the ability to keep encrypted
information secret is based not on the cryptographic algorithm, which is widely
known, but on a number called a key that must be used with the algorithm
to produce an encrypted result or to decrypt previously encrypted information.
Decryption with the correct key is simple. Decryption without the correct key is
very difficult, and in some cases impossible for all practical purposes.
The sections that follow introduce the use of keys for
encryption and decryption.
Symmetric-Key
Encryption
Public-Key
Encryption
Key
Length and Encryption Strength
With symmetric-key encryption, the encryption key can
be calculated from the decryption key and vice versa. With most symmetric
algorithms, the same key is used for both encryption and decryption, as shown in
Figure
1.
Figure 1 Symmetric-key encryption
Implementations of symmetric-key encryption can be highly
efficient, so that users do not experience any significant time delay as a
result of the encryption and decryption. Symmetric-key encryption also provides
a degree of authentication, since information encrypted with one symmetric key
cannot be decrypted with any other symmetric key. Thus, as long as the symmetric
key is kept secret by the two parties using it to encrypt communications, each
party can be sure that it is communicating with the other as long as the
decrypted messages continue to make sense.
Symmetric-key encryption is effective only if the symmetric
key is kept secret by the two parties involved. If anyone else discovers the
key, it affects both confidentiality and authentication. A person with an
unauthorized symmetric key not only can decrypt messages sent with that key, but
can encrypt new messages and send them as if they came from one of the two
parties who were originally using the key.
Symmetric-key encryption plays an important role in the SSL
protocol, which is widely used for authentication, tamper detection, and
encryption over TCP/IP networks. SSL also uses techniques of public-key
encryption, which is described in the next section.
The most commonly used implementations of public-key
encryption are based on algorithms patented by RSA
Data Security. Therefore, this section describes the RSA approach to
public-key encryption.
Public-key encryption (also called asymmetric
encryption) involves a pair of keys--a public key and a private
key--associated with an entity that needs to authenticate its identity
electronically or to sign or encrypt data. Each public key is published, and the
corresponding private key is kept secret. (For more information about the way
public keys are published, see Certificates
and Authentication.) Data encrypted with your public key can be decrypted
only with your private key. Figure
2 shows a simplified view of the way public-key encryption works.
Figure 2 Public-key encryption
The scheme shown in Figure
2 lets you freely distribute a public key, and only you will be able to read
data encrypted using this key. In general, to send encrypted data to someone,
you encrypt the data with that person's public key, and the person receiving the
encrypted data decrypts it with the corresponding private key.
Compared with symmetric-key encryption, public-key encryption
requires more computation and is therefore not always appropriate for large
amounts of data. However, it's possible to use public-key encryption to send a
symmetric key, which can then be used to encrypt additional data. This is the
approach used by the SSL protocol.
As it happens, the reverse of the scheme shown in Figure
2 also works: data encrypted with your private key can be decrypted only
with your public key. This would not be a desirable way to encrypt sensitive
data, however, because it means that anyone with your public key, which is by
definition published, could decrypt the data. Nevertheless, private-key
encryption is useful, because it means you can use your private key to sign data
with your digital signature--an important requirement for electronic commerce
and other commercial applications of cryptography. Client software such as
Communicator can then use your public key to confirm that the message was signed
with your private key and that it hasn't been tampered with since being signed. Digital
Signatures and subsequent sections describe how this confirmation process
works.
In general, the strength of encryption is related to the
difficulty of discovering the key, which in turn depends on both the cipher used
and the length of the key. For example, the difficulty of discovering the key
for the RSA cipher most commonly used for public-key encryption depends on the
difficulty of factoring large numbers, a well-known mathematical problem.
Encryption strength is often described in terms of the size of
the keys used to perform the encryption: in general, longer keys provide
stronger encryption. Key length is measured in bits. For example, 128-bit keys
for use with the RC4 symmetric-key cipher supported by SSL provide significantly
better cryptographic protection than 40-bit keys for use with the same cipher.
Roughly speaking, 128-bit RC4 encryption is 3 x 1026 times stronger
than 40-bit RC4 encryption. (For more information about RC4 and other ciphers
used with SSL, see Introduction
to SSL.)
Different ciphers may require different key lengths to achieve
the same level of encryption strength. The RSA cipher used for public-key
encryption, for example, can use only a subset of all possible values for a key
of a given length, due to the nature of the mathematical problem on which it is
based. Other ciphers, such as those used for symmetric key encryption, can use
all possible values for a key of a given length, rather than a subset of those
values. Thus a 128-bit key for use with a symmetric-key encryption cipher would
provide stronger encryption than a 128-bit key for use with the RSA public-key
encryption cipher.
This difference explains why the RSA public-key encryption
cipher must use a 512-bit key (or longer) to be considered cryptographically
strong, whereas symmetric key ciphers can achieve approximately the same level
of strength with a 64-bit key. Even this level of strength may be vulnerable to
attacks in the near future.
Because the ability to surreptitiously intercept and decrypt
encrypted information has historically been a significant military asset, the
U.S. Government restricts export of cryptographic software, including most
software that permits use of symmetric encryption keys longer than 40 bits. For
detailed information about these restrictions as they apply to Netscape
products, see Export
Restrictions on International Sales.
[Top]
Encryption and decryption address the problem of
eavesdropping, one of the three Internet security issues mentioned at the
beginning of this document. But encryption and decryption, by themselves, do not
address the other two problems mentioned in Internet
Security Issues: tampering and impersonation.
This section describes how public-key cryptography addresses
the problem of tampering. The sections that follow describe how it addresses the
problem of impersonation.
Tamper detection and related authentication techniques rely on
a mathematical function called a one-way hash (also called a message
digest). A one-way hash is a number of fixed length with the following
characteristics:
As mentioned in Public-Key
Encryption, it's possible to use your private key for encryption and your
public key for decryption. Although this is not desirable when you are
encrypting sensitive information, it is a crucial part of digitally signing any
data. Instead of encrypting the data itself, the signing software creates a
one-way hash of the data, then uses your private key to encrypt the hash. The
encrypted hash, along with other information, such as the hashing algorithm, is
known as a digital signature.
Figure
3 shows a simplified view of the way a digital signature can be used to
validate the integrity of signed data.
Figure 3 Using a digital signature to
validate data integrity
Figure
3 shows two items transferred to the recipient of some signed data: the
original data and the digital signature, which is basically a one-way hash (of
the original data) that has been encrypted with the signer's private key. To
validate the integrity of the data, the receiving software first uses the
signer's public key to decrypt the hash. It then uses the same hashing algorithm
that generated the original hash to generate a new one-way hash of the same
data. (Information about the hashing algorithm used is sent with the digital
signature, although this isn't shown in the figure.) Finally, the receiving
software compares the new hash against the original hash. If the two hashes
match, the data has not changed since it was signed. If they don't match, the
data may have been tampered with since it was signed, or the signature may have
been created with a private key that doesn't correspond to the public key
presented by the signer.
If the two hashes match, the recipient can be certain that the
public key used to decrypt the digital signature corresponds to the private key
used to create the digital signature. Confirming the identity of the signer,
however, also requires some way of confirming that the public key really belongs
to a particular person or other entity. For a discussion of the way this works,
see Certificates
and Authentication.
The significance of a digital signature is comparable to the
significance of a handwritten signature. Once you have signed some data, it is
difficult to deny doing so later--assuming that the private key has not been
compromised or out of the owner's control. This quality of digital signatures
provides a high degree of nonrepudiation--that is, digital signatures make it
difficult for the signer to deny having signed the data. In some situations, a
digital signature may be as legally binding as a handwritten signature.
[Top]
A
Certificate Identifies Someone or Something
Authentication
Confirms an Identity
How
Certificates Are Used
Contents
of a Certificate
How
CA Certificates Are Used to Establish Trust
A certificate is an electronic document used to
identify an individual, a server, a company, or some other entity and to
associate that identity with a public key. Like a driver's license, a passport,
or other commonly used personal IDs, a certificate provides generally recognized
proof of a person's identity. Public-key cryptography uses certificates to
address the problem of impersonation (see Internet
Security Issues).
To get a driver's license, you typically apply to a government
agency, such as the Department of Motor Vehicles, which verifies your identity,
your ability to drive, your address, and other information before issuing the
license. To get a student ID, you apply to a school or college, which performs
different checks (such as whether you have paid your tuition) before issuing the
ID. To get a library card, you may need to provide only your name and a utility
bill with your address on it.
Certificates work much the same way as any of these familiar
forms of identification. Certificate authorities (CAs) are entities that
validate identities and issue certificates. They can be either independent third
parties or organizations running their own certificate-issuing server software
(such as Netscape Certificate Server). The methods used to validate an identity
vary depending on the policies of a given CA--just as the methods to validate
other forms of identification vary depending on who is issuing the ID and the
purpose for which it will be used. In general, before issuing a certificate, the
CA must use its published verification procedures for that type of certificate
to ensure that an entity requesting a certificate is in fact who it claims to be.
The certificate issued by the CA binds a particular public key
to the name of the entity the certificate identifies (such as the name of an
employee or a server). Certificates help prevent the use of fake public keys for
impersonation. Only the public key certified by the certificate will work with
the corresponding private key possessed by the entity identified by the
certificate.
In addition to a public key, a certificate always includes the
name of the entity it identifies, an expiration date, the name of the CA that
issued the certificate, a serial number, and other information. Most
importantly, a certificate always includes the digital signature of the issuing
CA. The CA's digital signature allows the certificate to function as a
"letter of introduction" for users who know and trust the CA but don't
know the entity identified by the certificate.
For more information about the role of CAs, see How
CA Certificates Are Used to Establish Trust.
Authentication is the process of confirming an
identity. In the context of network interactions, authentication involves the
confident identification of one party by another party. Authentication over
networks can take many forms. Certificates are one way of supporting
authentication.
Network interactions typically take place between a client,
such as browser software running on a personal computer, and a server,
such as the software and hardware used to host a Web site. Client
authentication refers to the confident identification of a client by a
server (that is, identification of the person assumed to be using the client
software). Server authentication refers to the confident identification
of a server by a client (that is, identification of the organization assumed to
be responsible for the server at a particular network address).
Client and server authentication are not the only forms of
authentication that certificates support. For example, the digital signature on
an email message, combined with the certificate that identifies the sender,
provide strong evidence that the person identified by that certificate did
indeed send that message. Similarly, a digital signature on an HTML form,
combined with a certificate that identifies the signer, can provide evidence,
after the fact, that the person identified by that certificate did agree to the
contents of the form. In addition to authentication, the digital signature in
both cases ensures a degree of nonrepudiation--that is, a digital signature
makes it difficult for the signer to claim later not to have sent the email or
the form.
Client authentication is an essential element of network
security within most intranets or extranets. The sections that follow contrast
two forms of client authentication:
- Password-Based
Authentication. Almost all server software permits client authentication
by means of a name and password. For example, a server might require a user
to type a name and password before granting access to the server. The server
maintains a list of names and passwords; if a particular name is on the
list, and if the user types the correct password, the server grants access.
- Certificate-Based
Authentication. Client authentication based on certificates is part of
the SSL protocol. The client digitally signs a randomly generated piece of
data and sends both the certificate and the signed data across the network.
The server uses techniques of public-key cryptography to validate the
signature and confirm the validity of the certificate.
Figure
4 shows the basic steps involved in authenticating a client by means of a
name and password. Figure 4 assumes the following:
Figure 4 Using a password to
authenticate a client to a server
These are the steps shown in Figure 4:
- In response to an authentication request from the
server, the client displays a dialog box requesting the user's name and
password for that server. The user must supply a name and password
separately for each new server the user wishes to use during a work session.
- The client sends the name and password across the
network, either in the clear or over an encrypted SSL connection.
- The server looks up the name and password in its local
password database and, if they match, accepts them as evidence
authenticating the user's identity.
- The server determines whether the identified user is
permitted to access the requested resource, and if so allows the client to
access it.
With this arrangement, the user must supply a new password for
each server, and the administrator must keep track of the name and password for
each user, typically on separate servers.
As shown in the next section, one of the advantages of
certificate-based authentication is that it can be used to replace the first
three steps in Figure 2 with a mechanism that allows the user to supply just one
password (which is not sent across the network) and allows the administrator to
control user authentication centrally.
Figure
5 shows how client authentication works using certificates and the SSL
Protocol. To authenticate a user to a server, a client digitally signs a
randomly generated piece of data and sends both the certificate and the signed
data across the network. For the purposes of this discussion, the digital
signature associated with some data can be thought of as evidence provided by
the client to the server. The server authenticates the user's identity on the
strength of this evidence.
Like Figure
4, Figure
5 assumes that the user has already decided to trust the server and has
requested a resource, and that the server has requested client authentication in
the process of evaluating whether to grant access to the requested resource.
Figure 5 Using a certificate to
authenticate a client to a server
Unlike the process shown in Figure 4, the process shown in
Figure 5 requires the use of SSL. Figure 5 also assumes that the client has a
valid certificate that can be used to identify the client to the server.
Certificate-based authentication is generally considered preferable to
password-based authentication because it is based on what the user has (the
private key) as well as what the user knows (the password that protects the
private key). However, it's important to note that these two assumptions are
true only if unauthorized personnel have not gained access to the user's machine
or password, the password for the client software's private key database has
been set, and the software is set up to request the password at reasonably
frequent intervals.
Important Neither
password-based authentication nor certificate-based authentication address
security issues related to physical access to individual machines or
passwords. Public- key cryptography can only verify that a private key used to
sign some data corresponds to the public key in a certificate. It is the
user's responsibility to protect a machine's physical security and to keep the
private-key password secret.
These are the steps shown in Figure 3:
- The client software, such as Communicator, maintains a
database of the private keys that correspond to the public keys published in
any certificates issued for that client. The client asks for the password to
this database the first time the client needs to access it during a given
session--for example, the first time the user attempts to access an
SSL-enabled server that requires certificate-based client authentication.
After entering this password once, the user doesn't need to enter it again
for the rest of the session, even when accessing other SSL-enabled servers.
- The client unlocks the private-key database, retrieves
the private key for the user's certificate, and uses that private key to
digitally sign some data that has been randomly generated for this purpose
on the basis of input from both the client and the server. This data and the
digital signature constitute "evidence" of the private key's
validity. The digital signature can be created only with that private key
and can be validated with the corresponding public key against the signed
data, which is unique to the SSL session.
- The client sends both the user's certificate and the
evidence (the randomly generated piece of data that has been digitally
signed) across the network.
- The server uses the certificate and the evidence to
authenticate the user's identity. (For a detailed discussion of the way this
works, see Introduction
to SSL.)
- At this point the server may optionally perform other
authentication tasks, such as checking that the certificate presented by the
client is stored in the user's entry in an LDAP directory. The server then
continues to evaluate whether the identified user is permitted to access the
requested resource. This evaluation process can employ a variety of standard
authorization mechanisms, potentially using additional information in an
LDAP directory, company databases, and so on. If the result of the
evaluation is positive, the server allows the client to access the requested
resource.
As you can see by comparing Figure
5 to Figure
4, certificates replace the authentication portion of the interaction
between the client and the server. Instead of requiring a user to send passwords
across the network throughout the day, single sign-on requires the user to enter
the private-key database password just once, without sending it across the
network. For the rest of the session, the client presents the user's certificate
to authenticate the user to each new server it encounters. Existing
authorization mechanisms based on the authenticated user identity are not
affected.
Types
of Certificates
SSL
Protocol
Signed
and Encrypted Email
Form
Signing
Single
Sign-On
Object
Signing
Five kinds of certificates are commonly used with Netscape
products:
- Client SSL certificates. Used to identify clients
to servers via SSL (client authentication). Typically, the identity of the
client is assumed to be the same as the identity of a human being, such as
an employee in an enterprise. See Certificate-Based
Authentication for a description of the way client SSL certificates are
used for client authentication. Client SSL certificates can also be used for
Form
Signing and as part of a Single
Sign-On solution.
Examples: A bank gives a customer a client SSL
certificate that allows the bank's servers to identify that customer and
authorize access to the customer's accounts. A company might give a new
employee a client SSL certificate that allows the company's servers to
identify that employee and authorize access to the company's servers.
- Server SSL certificates. Used to identify servers
to clients via SSL (server authentication). Server authentication may be
used with or without client authentication. Server authentication is a
requirement for an encrypted SSL session. See SSL
Protocol.
Example: Internet sites that engage in electronic
commerce (commonly known as e-commerce) usually support
certificate-based server authentication, at a minimum, to establish an
encrypted SSL session and to assure customers that they are dealing with a
web site identified with a particular company. The encrypted SSL session
ensures that personal information sent over the network, such as credit
card numbers, cannot easily be intercepted.
- S/MIME certificates. Used for signed and
encrypted email. As with client SSL certificates, the identity of the client
is typically assumed to be the same as the identity of a human being, such
as an employee in an enterprise. A single certificate may be used as both an
S/MIME certificate and an SSL certificate. See Signed
and Encrypted Email. S/MIME certificates can also be used for Form
Signing and as part of a Single
Sign-On solution.
Examples: A company deploys combined S/MIME and
SSL certificates solely for the purpose of authenticating employee
identities, thus permitting signed email and client SSL authentication but
not encrypted email. Another company issues S/MIME certificates solely for
the purpose of both signing and encrypting email that deals with sensitive
financial or legal matters.
- Object-signing certificates. Used to identify
signers of Java code, JavaScript scripts, or other signed files. See Object
Signing.
Example: A software company signs software
distributed over the Internet to provide users with some assurance that
the software is a legitimate product of that company. Using certificates
and digital signatures in this manner can also make it possible for users
to identify and control the kind of access downloaded software has to
their computers.
- CA certificates. Used to identify CAs. Client and
server software use CA certificates to determine what other certificates can
be trusted. See How
CA Certificates Are Used to Establish Trust.
Example: The CA certificates stored in
Communicator determine what other certificates that copy of Communicator
can authenticate. An administrator can implement some aspects of corporate
security policies by controlling the CA certificates stored in each user's
copy of Communicator.
The sections that follow describes how certificates are used
by Netscape products.
The Secure Sockets Layer (SSL) protocol, which was originally
developed by Netscape, is a set of rules governing server authentication, client
authentication, and encrypted communication between servers and clients. SSL is
widely used on the Internet, especially for interactions that involve exchanging
confidential information such as credit card numbers.
SSL requires a server SSL certificate, at a minimum. As part
of the initial "handshake" process, the server presents its
certificate to the client to authenticate the server's identity. The
authentication process uses Public-Key
Encryption and Digital
Signatures to confirm that the server is in fact the server it claims to be.
Once the server has been authenticated, the client and server use techniques of Symmetric-Key
Encryption, which is very fast, to encrypt all the information they exchange
for the remainder of the session and to detect any tampering that may have
occurred.
Servers may optionally be configured to require client
authentication as well as server authentication. In this case, after server
authentication is successfully completed, the client must also present its
certificate to the server to authenticate the client's identity before the
encrypted SSL session can be established.
For an overview of client authentication over SSL and how it
differs from password-based authentication, see Authentication
Confirms an Identity. For more detailed information about SSL, see Introduction
to SSL.
Some email programs (including Messenger, which is part of
Communicator) support digitally signed and encrypted email using a widely
accepted protocol known as Secure Multipurpose Internet Mail Extension (S/MIME).
Using S/MIME to sign or encrypt email messages requires the sender of the
message to have an S/MIME certificate.
An email message that includes a digital signature provides
some assurance that it was in fact sent by the person whose name appears in the
message header, thus providing authentication of the sender. If the digital
signature cannot be validated by the email software on the receiving end, the
user will be alerted.
The digital signature is unique to the message it accompanies.
If the message received differs in any way from the message that was sent--even
by the addition or deletion of a comma--the digital signature cannot be
validated. Therefore, signed email also provides some assurance that the email
has not been tampered with. As discussed at the beginning of this document, this
kind of assurance is known as nonrepudiation. In other words, signed email makes
it very difficult for the sender to deny having sent the message. This is
important for many forms of business communication. (For information about the
way digital signatures work, see Digital
Signatures.)
S/MIME also makes it possible to encrypt email messages. This
is also important for some business users. However, using encryption for email
requires careful planning. If the recipient of encrypted email messages loses
his or her private key and does not have access to a backup copy of the key, for
example, the encrypted messages can never be decrypted.
Network users are frequently required to remember multiple
passwords for the various services they use. For example, a user might have to
type a different password to log into the network, collect email, use directory
services, use the corporate calendar program, and access various servers.
Multiple passwords are an ongoing headache for both users and system
administrators. Users have difficulty keeping track of different passwords, tend
to choose poor ones, and tend to write them down in obvious places.
Administrators must keep track of a separate password database on each server
and deal with potential security problems related to the fact that passwords are
sent over the network routinely and frequently.
Solving this problem requires some way for a user to log in
once, using a single password, and get authenticated access to all network
resources that user is authorized to use--without sending any passwords over the
network. This capability is known as single sign-on.
Both client SSL certificates and S/MIME certificates can play
a significant role in a comprehensive single sign-on solution. For example, one
form of single sign-on supported by Netscape products relies on SSL client
authentication (see Certificate-Based
Authentication). A user can log in once, using a single password to the
local client's private-key database, and get authenticated access to all
SSL-enabled servers that user is authorized to use--without sending any
passwords over the network. This approach simplifies access for users, because
they don't need to enter passwords for each new server. It also simplifies
network management, since administrators can control access by controlling lists
of certificate authorities (CAs) rather than much longer lists of users and
passwords.
In addition to using certificates, a complete single-sign on
solution must address the need to interoperate with enterprise systems, such as
the underlying operating system, that rely on passwords or other forms of
authentication.
For information about the single sign-on support currently
provided by Netscape products, see Single
Sign-On Deployment Guide.
Many kinds of e-commerce require the ability to provide
persistent proof that someone has authorized a transaction. Although SSL
provides transient client authentication for the duration of an SSL connection,
it does not provide persistent authentication for transactions that may occur
during that connection. S/MIME provides persistent authentication for email, but
e-commerce often involves filling in a form on a web page rather than sending an
email.
The Netscape technology known as form signing addresses the
need for persistent authentication of financial transactions. Form signing
allows a user to associate a digital signature with web-based data generated as
the result of a transaction, such as a purchase order or other financial
document. The private key associated with either a client SSL certificate or an
S/MIME certificate may be used for this purpose.
When a user clicks the Submit button on a web-based form that
supports form signing, a dialog box appears that displays the exact text to be
signed. The form designer can either specify the certificate that should be used
or allow the user to select a certificate from among the client SSL and S/MIME
certificates that are installed in Communicator. When the user clicks OK, the
text is signed, and both the text and the digital signature are submitted to the
server. The server can then use a Netscape utility called the Signature
Verification Tool to validate the digital signature.
For more information about support for form signing in
Netscape products, see Netscape
Form Signing.
Communicator and other Netscape products support a set of
tools and technologies called object signing. Object signing uses standard
techniques of public-key cryptography to let users get reliable information
about code they download in much the same way they can get reliable information
about shrink-wrapped software.
Most importantly, object signing helps users and network
administrators implement decisions about software distributed over intranets or
the Internet--for example, whether to allow Java applets signed by a given
entity to use specific computer capabilities on specific users' machines.
The "objects" signed with object signing technology
can be applets or other Java code, JavaScript scripts, plug-ins, or any kind of
file. The "signature" is a digital signature. Signed objects and their
signatures are typically stored in a special file called a JAR file.
Software developers and others who wish to sign files using
object-signing technology must first obtain an object-signing certificate.
For more information about support for object signing in
Netscape products, see Netscape
Object Signing: Establishing Trust for Downloaded Software.
The contents of certificates supported by Netscape and many
other software companies are organized according to the X.509 v3 certificate
specification, which has been recommended by the International
Telecommunications Union (ITU), an international standards body, since 1988.
Users don't usually need to be concerned about the exact
contents of a certificate. However, system administrators working with
certificates may need some familiarity with the information provided here.
An X.509 v3 certificate binds a distinguished name (DN)
to a public key. A DN is a series of name-value pairs, such as uid=doe,
that uniquely identify an entity--that is, the certificate subject.
For example, this might be a typical DN for an employee of
Netscape Communications Corporation:
uid=doe,e=doe@netscape.com,cn=John Doe,o=Netscape Communications Corp.,c=US
The abbreviations
before each equal sign in this example have these meanings:
uid:
user ID
e:
email address
cn:
the user's common name
o:
organization
c:
country
DNs may include a
variety of other name-value pairs. They are used to identify both certificate
subjects and entries in directories that support the Lightweight Directory
Access Protocol (LDAP).
The rules governing the construction of DNs can be quite
complex and are beyond the scope of this document. For comprehensive information
about DNs, see A
String Representation of Distinguished Names.
Every X.509 certificate consists of two sections:
- The data section includes the following information:
- The version number of the X.509 standard supported
by the certificate.
- The certificate's serial number. Every certificate
issued by a CA has a serial number that is unique among the certificates
issued by that CA.
- Information
- Information about the user's public key, including
the algorithm used and a representation of the key itself.
- The DN of the CA that issued the certificate.
- The period during which the certificate is valid
(for example, between 1:00 p.m. on November 15, 1996 and 1:00 p.m.
November 15, 1997)
- The DN of the certificate subject (for example, in a
client SSL certificate this would be the user's DN), also called the
subject name.
- Optional certificate extensions, which may
provide additional data used by the client or server. For example, the
certificate type extension indicates the type of certificate--that is,
whether it is a client SSL certificate, a server SSL certificate, a
certificate for signing email, and so on. Certificate extensions can
also be used for a variety of other purposes.
- The signature section includes the following
information:
Here are the data and signature sections of a certificate in
human-readable format:
Certificate:
Data:
Version: v3 (0x2)
Serial Number: 3 (0x3)
Signature Algorithm: PKCS #1 MD5 With RSA Encryption
Issuer: OU=Ace Certificate Authority, O=Ace Industry, C=US
Validity:
Not Before: Fri Oct 17 18:36:25 1997
Not After: Sun Oct 17 18:36:25 1999
Subject: CN=Jane Doe, OU=Finance, O=Ace Industry, C=US
Subject Public Key Info:
Algorithm: PKCS #1 RSA Encryption
Public Key:
Modulus:
00:ca:fa:79:98:8f:19:f8:d7:de:e4:49:80:48:e6:2a:2a:86:
ed:27:40:4d:86:b3:05:c0:01:bb:50:15:c9:de:dc:85:19:22:
43:7d:45:6d:71:4e:17:3d:f0:36:4b:5b:7f:a8:51:a3:a1:00:
98:ce:7f:47:50:2c:93:36:7c:01:6e:cb:89:06:41:72:b5:e9:
73:49:38:76:ef:b6:8f:ac:49:bb:63:0f:9b:ff:16:2a:e3:0e:
9d:3b:af:ce:9a:3e:48:65:de:96:61:d5:0a:11:2a:a2:80:b0:
7d:d8:99:cb:0c:99:34:c9:ab:25:06:a8:31:ad:8c:4b:aa:54:
91:f4:15
Public Exponent: 65537 (0x10001)
Extensions:
Identifier: Certificate Type
Critical: no
Certified Usage:
SSL Client
Identifier: Authority Key Identifier
Critical: no
Key Identifier:
f2:f2:06:59:90:18:47:51:f5:89:33:5a:31:7a:e6:5c:fb:36:
26:c9
Signature:
Algorithm: PKCS #1 MD5 With RSA Encryption
Signature:
6d:23:af:f3:d3:b6:7a:df:90:df:cd:7e:18:6c:01:69:8e:54:65:fc:06:
30:43:34:d1:63:1f:06:7d:c3:40:a8:2a:82:c1:a4:83:2a:fb:2e:8f:fb:
f0:6d:ff:75:a3:78:f7:52:47:46:62:97:1d:d9:c6:11:0a:02:a2:e0:cc:
2a:75:6c:8b:b6:9b:87:00:7d:7c:84:76:79:ba:f8:b4:d2:62:58:c3:c5:
b6:c1:43:ac:63:44:42:fd:af:c8:0f:2f:38:85:6d:d6:59:e8:41:42:a5:
4a:e5:26:38:ff:32:78:a1:38:f1:ed:dc:0d:31:d1:b0:6d:67:e9:46:a8:
dd:c4
Here is the same
certificate displayed in the 64-byte-encoded form interpreted by software:
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
Certificate authorities (CAs) are entities that validate
identities and issue certificates. They can be either independent third parties
or organizations running their own certificate-issuing server software (such as
the Netscape Certificate Server). A list of third-party certificate authorities
is available at Certificate
Authority Services.
Any client or server software that supports certificates
maintains a collection of trusted CA certificates. These CA certificates
determine which other certificates the software can validate--in other words,
which issuers of certificates the software can trust. In the simplest case, the
software can validate only certificates issued by one of the CAs for which it
has a certificate. It's also possible for a trusted CA certificate to be part of
a chain of CA certificates, each issued by the CA above it in a certificate
hierarchy.
The sections that follow explains how certificate hierarchies
and certificate chains determine what certificates software can trust.
CA
Hierarchies
Certificate
Chains
Verifying
a Certificate Chain
In large organizations, it may be appropriate to delegate the
responsibility for issuing certificates to several different certificate
authorities. For example, the number of certificates required may be too large
for a single CA to maintain; different organizational units may have different
policy requirements; or it may be important for a CA to be physically located in
the same geographic area as the people to whom it is issuing certificates.
It's possible to delegate certificate-issuing responsibilities
to subordinate CAs. The X.509 standard includes a model for setting up a
hierarchy of CAs like that shown in Figure
6.
Figure 6 Example of a hierarchy of
certificate authorities
In this model, the root CA is at the top of the hierarchy. The
root CA's certificate is a self-signed certificate: that is, the
certificate is digitally signed by the same entity--the root CA--that the
certificate identifies. The CAs that are directly subordinate to the root CA
have CA certificates signed by the root CA. CAs under the subordinate CAs in the
hierarchy have their CA certificates signed by the higher-level subordinate CAs.
Organizations have a great deal of flexibility in terms of the
way they set up their CA hierarchies. Figure
6 shows just one example; many other arrangements are possible.
CA hierarchies are reflected in certificate chains. A certificate
chain is series of certificates issued by successive CAs. Figure
7 shows a certificate chain leading from a certificate that identifies some
entity through two subordinate CA certificates to the CA certificate for the
root CA (based on the CA hierarchy shown in Figure
6).
Figure 7 Example of a certificate
chain
A certificate chain traces a path of certificates from a
branch in the hierarchy to the root of the hierarchy. In a certificate chain,
the following occur:
Certificate chain verification is the process of making sure a
given certificate chain is well-formed, valid, properly signed, and trustworthy.
Netscape software uses the following procedure for forming and verifying a
certificate chain, starting with the certificate being presented for
authentication:
- The certificate validity period is checked against the
current time provided by the verifier's system clock.
- The issuer's certificate is located. The source can be
either the verifier's local certificate database (on that client or server)
or the certificate chain provided by the subject (for example, over an SSL
connection).
- The certificate signature is verified using the public
key in the issuer's certificate.
- If the issuer's certificate is trusted by the verifier
in the verifier's certificate database, verification stops successfully
here. Otherwise, the issuer's certificate is checked to make sure it
contains the appropriate subordinate CA indication in the Netscape
certificate type extension, and chain verification returns to step 1 to
start again, but with this new certificate. Figure
8 presents an example of this process.
Figure 8 Verifying a certificate chain
all the way to the root CA
Figure 8 shows what happens when only Root CA is included in
the verifier's local database. If a certificate for one of the intermediate CAs
shown in Figure 8, such as Engineering CA, is found in the verifier's local
database, verification stops with that certificate, as shown in Figure
9.
Figure 9 Verifying a certificate chain
to an intermediate CA
Expired validity dates, an invalid signature, or the absence
of a certificate for the issuing CA at any point in the certificate chain causes
authentication to fail. For example, Figure
10 shows how verification fails if neither the Root CA certificate nor any
of the intermediate CA certificates are included in the verifier's local
database.
Figure 10 A certificate chain that
can't be verified
For general information about the way digital signatures work,
see Digital
Signatures. For a more detailed description of the signature verification
process in the context of SSL client and server authentication, see Introduction
to SSL.
[Top]
The set of standards and services that facilitate the use of
public-key cryptography and X.509 v3 certificates in a networked environment is
called the public key infrastructure (PKI). PKI management is complex
topic beyond the scope of this document. The sections that follow introduce some
of the specific certificate management issues addressed by Netscape products.
Issuing
Certificates
Certificates
and the LDAP Directory
Key
Management
Renewing
and Revoking Certificates
Registration
Authorities
The process for issuing a certificate depends on the
certificate authority that issues it and the purpose for which it will be used.
The process for issuing nondigital forms of identification varies in similar
ways. For example, if you want to get a generic ID card (not a driver's license)
from the Department of Motor Vehicles in California, the requirements are
straightforward: you need to present some evidence of your identity, such as a
utility bill with your address on it and a student identity card. If you want to
get a regular driving license, you also need to take a test--a driving test when
you first get the license, and a written test when you renew it. If you want to
get a commercial license for an eighteen-wheeler, the requirements are much more
stringent. If you live in some other state or country, the requirements for
various kinds of licenses will differ.
Similarly, different CAs have different procedures for issuing
different kinds of certificates. In some cases the only requirement may be your
email address. In other cases, your Unix or NT login and password may be
sufficient. At the other end of the scale, for certificates that identify people
who can authorize large expenditures or make other sensitive decisions, the
issuing process may require notarized documents, a background check, and a
personal interview.
Depending on an organization's policies, the process of
issuing certificates can range from being completely transparent for the user to
requiring significant user participation and complex procedures. In general,
processes for issuing certificates should be highly flexible, so organizations
can tailor them to their changing needs.
The Netscape
Certificate Server, part of the Mission
Control family of products, allows an organization to set up its own
certificate authority and issue certificates.
Issuing certificates is one of several managements tasks that
can be handled by separate Registration
Authorities.
The Lightweight Directory Access Protocol (LDAP) for accessing
directory services supports great flexibility in the management of certificates
within an organization. System administrators can store much of the information
required to manage certificates in an LDAP-compliant directory. For example, a
CA can use information in a directory to prepopulate a certificate with a new
employee's legal name and other information. The CA can leverage directory
information in other ways to issue certificates one at a time or in bulk, using
a range of different identification techniques depending on the security
policies of a given organization. Other routine management tasks, such as Key
Management and Renewing
and Revoking Certificates, can be partially or fully automated with the aid
of the directory.
Information stored in the directory can also be used with
certificates to control access to various network resources by different users
or groups. Issuing certificates and other certificate management tasks can thus
be an integral part of user and group management.
In general, high-performance directory services are an
essential ingredient of any certificate management strategy. The Netscape
Directory Server, part of the Mission
Control family of products, is fully integrated with the Netscape
Certificate Server to provide a comprehensive certificate management
solution.
Before a certificate can be issued, the public key it contains
and the corresponding private key must be generated. Sometimes it may be useful
to issue a single person one certificate and key pair for signing operations,
and another certificate and key pair for encryption operations. Separate signing
and encryption certificates make it possible to keep the private signing key on
the local machine only, thus providing maximum nonrepudiation, and to back up
the private encryption key in some central location where it can be retrieved in
case the user loses the original key or leaves the company.
Keys can be generated by client software or generated
centrally by the CA and distributed to users via an LDAP directory. There are
trade-offs involved in choosing between local and centralized key generation.
For example, local key generation provides maximum nonrepudiation, but may
involve more participation by the user in the issuing process. Flexible key
management capabilities are essential for most organizations.
Key recovery, or the ability to retrieve backups of
encryption keys under carefully defined conditions, can be a crucial part of
certificate management (depending on how an organization uses certificates). Key
recovery schemes usually involve an m of n mechanism: for example, m
of n managers within an organization might have to agree, and each
contribute a special code or key of their own, before a particular person's
encryption key can be recovered. This kind of mechanism ensures that several
authorized personnel must agree before an encryption key can be recovered.
Like a driver's license, a certificate specifies a period of
time during which it is valid. Attempts to use a certificate for authentication
before or after its validity period will fail. Therefore, mechanisms for
managing certificate renewal are essential for any certificate management
strategy. For example, an administrator may wish to be notified automatically
when a certificate is about to expire, so that an appropriate renewal process
can be completed in plenty of time without causing the certificate's subject any
inconvenience. The renewal process may involve reusing the same public-private
key pair or issuing a new one.
A driver's license can be suspended even if it has not
expired--for example, as punishment for a serious driving offense. Similarly,
it's sometimes necessary to revoke a certificate before it has expired--for
example, if an employee leaves a company or moves to a new job within the
company.
Certificate revocation can be handled in several different
ways. For some organizations, it may be sufficient to set up servers so that the
authentication process includes checking the directory for the presence of the
certificate being presented. When an administrator revokes a certificate, the
certificate can be automatically removed from the directory, and subsequent
authentication attempts with that certificate will fail even though the
certificate remains valid in every other respect. Another approach involves
publishing a certificate revocation list (CRL)--that is, a list of
revoked certificates--to the directory at regular intervals and checking the
list as part of the authentication process. For some organizations, it may be
preferable to check directly with the issuing CA each time a certificate is
presented for authentication. This procedure is sometimes called real-time
status checking.
Interactions between entities identified by certificates
(sometimes called end entities) and CAs are an essential part of
certificate management. These interactions include operations such as
registration for certification, certificate retrieval, certificate renewal,
certificate revocation, and key backup and recovery. In general, a CA must be
able to authenticate the identities of end entities before responding to the
requests. In addition, some requests need to be approved by authorized
administrators or managers before being services.
As previously discussed, the means used by different CAs to
verify an identity before issuing a certificate can vary widely, depending on
the organization and the purpose for which the certificate will be used. To
provide maximum operational flexibility, interactions with end entities can be
separated from the other functions of a CA and handled by a separate service
called a Registration Authority (RA).
An RA acts as a front end to a CA by receiving end entity
requests, authenticating them, and forwarding them to the CA. After receiving a
response from the CA, the RA notifies the end entity of the results. RAs can be
helpful in scaling an PKI across different departments, geographical areas, or
other operational units with varying policies and authentication requirements.
Future versions of the Netscape
Certificate Server will support the creation of customizable registration
authorities.
[Top]
Last Updated: 10/09/98 10:35:45
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