Kerberos is a network authentication system for use on physically
insecure networks, based on the key distribution model presented by
Needham and Schroeder (in ``Using Encryption for Authentication in
Large Networks of Computers'', Communications of the ACM, Vol. 21(12),
pp. 993-999, December, 1978). It allows entities communicating over
networks to prove their identity to each other while preventing
eavesdropping or replay attacks. It also provides for data stream
integrity (detection of modification) and secrecy (preventing
unauthorized reading) using cryptography systems such as DES.
Kerberos works by providing principals (users or services) with tickets
that they can use to identify themselves to other principals and secret
cryptographic keys for secure communication with other principals. A
ticket is a sequence of a few hundred bytes. These tickets can then be
embedded in virtually any other network protocol, thereby allowing the
process implementing that protocol to be sure about the identity of the
Practically speaking, Kerberos is mostly used in application-level
protocols (ISO model level 7), such as Telnet or FTP, to provide user
to host security. It is also used, though less frequently, as the
implicit authentication system of data stream (such as
SOCK_STREAM) or RPC mechanisms (ISO model level 6). It could also
be used at a lower level for host to host security, in protocols like
IP, UDP, or TCP (ISO model levels 3 and 4), although such
implementations are currently
rare, if they exist at all.
There are several different versions and distributions of Kerberos.
Most of them are based on an MIT distributions in one form or another,
but the lineage is not always simple. Some of the distributions are
freely available, some are stand-alone commercial products, and others
are part of a larger free or commercial systems.
Several vendors currently sell relevant Kerberos material, and to name
a few: CyberSAFE, Cygnus Support, Digital Equipment Corporation, Emulex
Network Systems, OpenVision Technologies, TGV.
Bones is a system that provides the Kerberos API without using
encryption and without providing any form of security whatsoever. It is
a fake that allows the use of software that expects Kerberos to be
present when it cannot be.
Why does it exist? Kerberos is a network security system which relies
on cryptographic methods for its security. Since Kerberos' encryption
system, DES, is not exportable, Kerberos itself cannot be exported or
used outside of the United States in its original form.
As a partial solution to this problem, the Kerberos source code was
modified by the addition of #ifdef NOENCRYPTION around all calls
to DES functions. Compiling this version with the symbol NOENCRYPTION
defined results in a system that looks like Kerberos from an
application's point of view but that does not require DES libraries
(and, as a result, does not speak the real Kerberos protocol and does
The final piece in this puzzle is a program called ``piranha'' which
takes the Kerberos sources and removes all of the calls to the
encryption routines, replacing it with the code which was under
#ifdef NOENCRYPTION, producing the system known as Bones. Bones has
the property that there is absolutely no question about whether or not
it is legal to transport its sources across national boundaries, since
it neither has any encryption routines nor any calls to encryption
#ifdef NOENCRYPTION was not documented, and it was only intended to be used in the above manner. Someone who tries compiling Kerberos with that #define has in some sense ``voided the warranty'', and will get something which is both not secure and not Kerberos.
PGP is a remarkable phenomenon. Largely the effort of a single person, Philip Zimmermann, it provides a confidentiality and authentication service that can be used for e-mail and file-storage applications. In essence, what Zimmermann has done is the following:
From its beginnings just a few years ago, PGP has grown explosively and is now widely used. A number of reasons can be cited for this growth:
Some people have hesitated to use PGP for several reasons. First, the
developer lives and works in the United States, and because PGP is a
cryptographic product, there is concern about export controls for
organizations that wish to interoperate between U.S. and non-U.S. sites.
However, some recent versions of PGP have initially seen the light of day
outside the United States, which would seem to make U.S.
export controls irrelevant.
Second, the freeware version of PGP was developed and disseminated
without a license for the use of RSA, which is required within the United
States but not outside. For people concerned with this issue, the
commercial version of PGP has an RSA license. In addition, MIT has
issued a new freeware version PGP with a licensed version of RSA.
PGP provides two services: encryption and digital signatures.
Encryption allows a user to encode a file for storage locally or for
transmission as an e-mail message. The local storage option is handy if
you are worried about other people having access to files on your
machine. The e-mail option enables PGP to be used for private exchanges
over a network. PGP encrypts the entire contents of the message in such
a way that only the intended recipient can decode and read the message.
Anyone else who attempts to capture or copy the message en route will
receive meaningless garble.
The digital signature service allows a user to `sign' a document before
transmission in such a way that anyone can verify that the signature is
genuine and belongs with a particular document. If someone alters the
message or substitutes a different message, the signature will no
longer be valid. And any recipient can verify that the message has been
signed by its true creator and not an imposter.
PGP's confidentiality and encoding services use the most popular
public-key encryption scheme, known as RSA. All public-key encryption
systems make use of an encoding and decoding algorithm and a related
pair of keys. The input to the encryption algorithm is the text to be
encrypted -- known as plaintext -- and a key. The algorithm takes the
input and produces scrambled output known as ciphertext. To use the
decryption algorithm, you input the ciphertext plus the key that
matches the one used for encryption,
and the original plaintext is produced as output.
The two keys used in any public-key encryption scheme, including RSA,
are called the public key and private key. The public key, as the name
suggests, is made public. The idea is to make your public key available
to people with whom you correspond. You keep your private key secure,
and it should be known only to you. These two keys can be used to
provide confidentiality and encryption.
Here's how it works. Suppose Alice wants to correspond with Bob. If
Alice prepares a message and encrypts it with Bob's public key, only
Bob can decrypt the message using his private key. If Alice prepares a
message and encrypts it with her private key, then anyone, including
Bob, can decrypt the message. But only Alice could have encrypted the
message, therefore the encrypted message is, in effect, signed by Alice.
It turns out that RSA, and all other known public-key algorithms, are
time-consuming and inefficient. Therefore PGP, like most other
encryption applications, does not use RSA directly to provide
confidentiality and digital signatures.
For confidentiality, PGP encrypts messages with an efficient single-key
or conventional encryption algorithm known as IDEA. It then uses RSA to
encrypt, with the receiver's public key, the IDEA key used to encrypt
the message. The receiver can use RSA to recover the IDEA key and use
that key to recover the message.
For digital signatures, PGP uses an efficient algorithm known as MD5 to
produce a summary code, or hash code, of the message that is, for all
practical purposes, unique to that message. PGP then uses RSA to
encrypt the hash code with the sender's private key. The receiver can
use RSA to recover the hash code and verify that it is the correct hash
code for the message. If it is correct, then only the alleged sender
could have prepared the encrypted hash code.
The most difficult aspect of using PGP, or any public-key application,
is getting your hands on the public keys of the people with whom you
wish to correspond. You must make sure you have the true public key of
each individual in your electronic Rolodex. Suppose I create a linked
pair of public and private keys and send you the public key, declaring
that I am Elvis. How do you know I am the real Elvis and not an
impostor? If I am an impostor, I could send you signed messages and you
would be sure they were from Elvis. If you send an encrypted message to
Elvis, I can capture the message and recover the plain text.
PGP provides a number of tools and recommended procedures for obtaining
public keys in trusted ways. One handy tool is the public-key
fingerprint, which is nothing more than a string of printable
characters based on the MD5 hash code of the key. For all practical
purposes, the fingerprint of a key is unique. So, if Alice knows Bob's
voice, Bob could send his public key to Alice via e-mail. Alice then
could generate the fingerprint of that key, call Bob, and have Bob read
the fingerprint over the phone to make sure there is a match.
Once you have a few trusted keys, you can make use of PGP's signature
capability. If you have Bob's public key and you trust Bob to provide
you with other public keys of other persons, Bob can send you John's
key signed by Bob. That is, Bob takes John's public key and feeds it
through the signature mechanism of PGP. Alice can use Bob's public key
to ensure that John's key was provided by Bob and that the key has not
There also are a number of servers on the Internet that are public-key
repositories. Most of keys are signed by one or more people. You can
obtain someone's public key from the server and if you trust the
signatories to the key, you can have faith that it is genuine. These
public-key servers do not authenticate the keys; they merely serve as
One public-key directory that does attempt to provide authenticated PGP keys is SLED (Stable Large E-mail Database). The public keys in the directory are signed by SLED, indicating that the user's authenticity has been verified.
PEM is an Internet standard for providing security services to
electronic mail. It uses cryptographic techniques to provide message
integrity checking, originator authentication, and confidentiality. It
lets you know that a message hasn't been changed, who it's from, and,
optionally, allows you to keep it secret from all but the intended
There is a set of Proposed Standard RFCs (Internet standards documents)
that specify PEM. The four documents are RFCs 1421 (obsoletes 1113),
1422 (obsoletes 1114), 1423 (obsoletes 1115), and 1424 (new). The
integration of MIME (Multipurpose Internet Mail Extensions) and PEM
will either produce a new RFC for message format or cause RFC 1421 to
An overview of PEM was presented in the August 1993 issue (Volume 38,
Number 8) of ``Communications of the ACM'' in an article entitled
``Internet Privacy Enhanced Mail'' by Stephen T. Kent.
Trusted Information Systems (TIS), under ARPA sponsorship and in
cooperation with RSA Data Security Incorporated (RSADSI), has released
a reference implementation of Privacy Enhanced Mail (TIS/PEM) to the
TIS/PEM is a Unix-based implementation (The authors are currently
pursuing porting it to other operating systems) that has been
integrated with Rand MH 6.8.3 and is easily integrated into other mail
user agents. TIS/PEM is distributed in source form. It is openly
available within the United States and Canada for non-commercial use
(not for resale), but the export from the United States of the
cryptography used in TIS/PEM is controlled by the United States
Riordan's Internet Privacy Enhanced Mail (RIPEM) is an implementation
of PEM. RIPEM allows electronic mail to have the four security
facilities provided by PEM: disclosure protection (optional),
originator authenticity, message integrity measures, and
non-repudiation of origin (always). RIPEM is not really a complete
implementation of PEM, because PEM specifies certificates for
authenticating keys, which RIPEM does not handle at this time (however,
their addition is planned). They have already been added to the
RIPEM was written primarily by Mark Riordan <firstname.lastname@example.org>, but
most of the code is now in the public domain, except for the RSA
routines, which are a library called RSAREF licensed from RSA Data
Security Inc. RIPEM has been ported to almost all platforms.
PEM users have the choice of using bare public keys or public keys embodied in X.509 certificates. If we use X.509 certificates to bind distinguished names to RSA public keys, it is not necessary to join the Internet certification hierarchy or otherwise pay to use PEM.
TIS/PEM is capable of generating the bare keys and certificates that we
may need. Joining the Internet certification hierarchy has the benefit
of making it easier to verify others' mail and for them to verify ours.
To join the Internet certification hierarchy, we must sign up our
Certification Authority (CA) under a Policy-level Certification
A distinguished name is a hierarchical, globally unique name used to
identify something or someone.
A Certification Authority (CA) vouches for the binding between users'
distinguished names and RSA public keys within an organization or
organizational unit. The CA's distinguished name is that of the
organization or organizational unit and users' distinguished names are
created by starting with the CA distinguished name and adding something
to uniquely and unambiguously identify the user, like a common name.
Policy-level Certification Authorities (PCAs) vouch for the binding
between a CA's distinguished name and RSA public key. By joining a PCA,
others can verify our PEM messages by following the certification path
to the Internet PCA certificate without having to have retrieved our
RSA public key using secure, out of band means. PCAs may also make CA
Certificate Revocation Lists (CRLs) and certificates available and
provide other services for its members.
PCAs can be differentiated by the policy that they advertise. The
policy includes the level of effort -- and associated assurance --
that a PCA uses to insure the correctness of the binding and the
requirements they place on CAs which issue certificates under them.
They can also be differentiated by the other services they offer and
Several PCAs exist as part of the Internet certification hierarchy, including PCAs at RSADSI and TIS, and more may come online in the near future. Individual PCAs will have their own price schedules.
Netscape Communications has designed and specified a protocol for
providing data security layered between application protocols (such as
HTTP, Telnet, NNTP, or FTP) and TCP/IP. This security protocol, called
Secure Sockets Layer (SSL), provides data encryption, server
authentication, message integrity, and optional client authentication
for a TCP/IP connection.
SSL is an open, non-proprietary protocol, and has been submitted to the
W3C working group on security for consideration as a standard security
approach for World Wide Web browsers and servers on the Internet.
Netscape Communications is working with the W3C and W3C members
(including EIT) on developing and standardizing common, robust security
mechanisms and protocols for the Internet. They will fully support such
mechanisms and protocols as they become standardized.
The SSL protocol specification is freely available and Netscape
Communications encourage everyone to examine the protocol and/or
implement it themselves.
SSL provides a security ``handshake'' that is used to initiate the
TCP/IP connection. This handshake results in the client and server
agreeing on the level of security they will use, and fulfills any
authentication requirements for the connection. Thereafter, SSL's only
role is to encrypt and decrypt the bytestream of the application
protocol being used (for example, HTTP, NNTP, or Telnet). This means
that all the information in both the HTTP request and the HTTP response
are fully encrypted, including the URL the client is requesting, any
submitted form contents (including things like credit card numbers),
any HTTP access authorization information (usernames and passwords),
and all the data returned from the server to the client.
The Netscape Navigator includes embedded Certificate Authority (CA)
keys for certain CAs, including Netscape Communications ` test CAs. As
new CAs come online, they will embed their keys as well. These embedded
keys allow the browser to verify the legitimacy of arbitrary servers.
The Document Information dialog allow one to inspect both the identity
of a given server as well as the identity of the CA that issued the
server its certificate. SSL requires servers to have certificates
issued by a Certificate Authority; and the Netscape Commerce Server
includes a mechanism to easily acquire such a certificate.
Because HTTP+SSL (or ``https'') and HTTP are different protocols and typically reside on different ports (443 and 80, respectively), the same server system can run both secure and insecure HTTP servers simultaneously. This means a server can provide some information to all users using no security, and other information only securely, if the webmaster so choose. For instance, ``storefront'' and merchandise catalog could be insecure, while ordering and payment documents and forms could be secure.
Secure NCSA Mosaic and Spry SafetyWEB are two browsers that support the S-HTTP protocol.
Because S-HTTP is an application-level protocol, it can provide
non-repudiation of individual requests or responses through digital
signatures. SSL is a lower-level protocol and does not have this
S-HTTP, being an application-level protocol, is able to work with
firewalls. SSL's transport-level encryption, in contrast, hides the
application-level protocol from firewalls. A firewall that relays an
SSL connection has no idea what data is being passed back and forth
over the connection.
S-HTTP is more flexible than SSL, in that an application can configure
the level of security it needs. Encryption and digital signatures can
be expensive to compute, so in principle this flexibility can allow a
server to handle more connections or respond more quickly. The other
side of this trade-off is that SSL, a lower-level protocol, may be
easier to optimize: It encrypts more bytes, but might balance this out
with a lower cost per byte. Since SSL has fewer options it should be
easier to set up and administer than S-HTTP.
Current SSL protocols and implementations are limited in a variety of ways relating to the handling of digital certificates; S-HTTP implementations are considerably more flexible. These SSL limitations are not inherent in the nature of SSL and will likely be removed as proves necessary.
IPSEC is not a general security architecture for the Internet, but is
instead focused on the IP layer. IPSEC working group will develop
mechanisms to protect client protocols of IP. A security protocol in
the network layer will be developed to provide cryptographic security
services that will flexibly support combinations of authentication,
integrity, access control, and confidentiality. The protocol formats
for the IP Security Protocol (IPSP) will be independent of the
cryptographic algorithm. The preliminary goals will specifically pursue
host-to-host security followed by subnet-to-subnet and host-to-subnet
Protocol and cryptographic techniques will also be developed to support
the key management requirements of the network layer security. The key
management will be specified as an application layer protocol that is
independent of the lower layer security protocol. The protocol will
initially support public key-based techniques. Flexibility in the
protocol will allow eventual support of Key Distribution Center (KDC -
such as Kerberos) and manual distribution approaches.
The IP Authentication Header is designed to provide integrity and
authentication without confidentiality to IP datagrams. The lack of
confidentiality ensures that implementations of the Authentication
Header will be widely available on the Internet, even in locations
where the export, import, or use of encryption to provide
confidentiality is regulated. The Authentication Header supports
security between two or more hosts implementing AH, between two or more
gateways implementing AH, and between a host or gateway implementing AH
and a set of hosts or gateways. A security gateway is a system which
acts as the communications gateway between external untrusted systems
and trusted hosts on their own subnetwork. It also provides security
services for the trusted hosts when they communicate with the external
untrusted systems. A trusted subnetwork contains hosts and routers that
trust each other not to engage in active or passive attacks and trust
that the underlying communications channel (e.g., an Ethernet) isn't
The IP Encapsulating Security Payload (ESP) is designed to provide
integrity, authentication, and confidentiality to IP datagrams. The ESP
supports security between two or more hosts implementing ESP, between
two or more gateways implementing ESP, and between a host or gateway
implementing ESP and a set of hosts and/or gateways. A security gateway
is a system which acts as the communications gateway between external
untrusted systems and trusted hosts on their own subnetwork and
provides security services for the trusted hosts when they communicate
with external untrusted systems. A trusted subnetwork contains hosts
and routers that trust each other not to engage in active or passive
attacks and trust that the underlying communications channel (e.g., an
Ethernet) isn't being attacked. Trusted systems always should be
trustworthy, but in practice they often are not trustworthy.
Gateway-to-gateway encryption is most valuable for building private
virtual networks across an untrusted backbone such as the Internet. It
does this by excluding outsiders. As such, it is often not a substitute
for host-to-host encryption, and indeed the two can be and often should
be used together.
You can see the IPSEC Working Group Charter at: ftp://ftp.ripe.net/ietf/ipsec/ipsec-charter.txt with all goals and milestones for this protocol, and/or see the Request For Comments (RFC) on the Security Architecture for the Internet Protocol at: http://www.cis.ohio-state.edu/htbin/rfc/rfc1825 .