Table of Contents
Authentication tools provide the ability to determine
the identity of a party to an interaction and to ensure that a message
came from who it claims to have come from. Authentication is seldom used
in isolation. Authentication is used as the basis for authorization
(determining whether a privilege will be granted to a particular user or
process), privacy (keeping information from becoming known to non-participants),
and non-repudiation (not being able to deny having done something
that was authorized to be done based on the authentication).
All authentication schemes are based on the possession of some secret information
known only to the user and possibly (but not necessarily) to the authentication
system itself. Interactions with other parties use this secret in a way
that allows the recipient to verify that the user possesses the secret,
but that does not divulge the secret itself. This means that the secret
itself cannot be shared, since to do so would allow the recipient to impersonate
the user on subsequent interactions with other parties. One of the major
differences between authentication systems lies in how to prove you know
the secret without telling it. This is reflected in the basic algorithm
used for authentication.
Authentication system also provide differing levels of functionality. At
minimum, they allow a recipient to verify that a message originated with
a particular user (or user's agent; e.g., a program). More powerful systems
ensure that messages cannot be copied and replayed in the future, that
a client can prove to a third party that a message originated with a particular
user (non-repudiation), and that require multiple users to validate a message
(the equivalent to requiring multiple signatures on a checking account).
A final difference in authentication systems lies in who is trusted with
the user's secret.
There are three main algorithms for authentication: passwords, Needham
and Schroeder protocol (used in Kerberos), and public key encryption. In
all of them, the central issue is to never allow the secret information
outside a secured environment, while at the same time allowing the recipient
to verify that the secret was used.
The descriptions that appear below only give a flavor of the algorithms
and discuss their advantages and disadvantages. For a more complete description
of the algorithms and their variants, see the references.
Passwords are simply secrets that are provided by the user upon request
by a recipient. Passwords are often stored on a server in an encrypted
form so that a penetration of the file system does not reveal password
lists. The problem with password-based systems is that the password becomes
known to the recipient, who can then impersonate the user. Even if the
recipient is trusted not to do this, passwords are dangerous in network
environments since they are susceptible to interception during transmission.
In general, passwords are unacceptable security in a network environment.
Needham and Schroeder Protocol
In the Needham/Schroeder protocol used in Kerberos, the secret information
used for verification is never transmitted in the clear and is never seen
by a recipient. Instead, an "authentication server" creates a collection
of "session secrets" (derived from its knowledge of the secrets of the
sender and receiver in a particular interchange) that are used by the sender
and receiver for authentication of messages during a particular interaction.
Session information is good only between session participants, and can
be timestamped to protect against replaying of messages. New interactions
(even between the same client and server) require new session keys. The
basic algorithm is given below; variants exist.
Each participant in an authentication realm possesses a secret encryption
key know only to itself and an authentication server. This is its secret
information. This key is used only for communication with the authentication
server, which is presumed to be trusted and secure (i.e., it will neither
misuse nor divulge the keys). An interaction between a client and a server
begins with a client request to an authentication server for a "session
encryption key" and an "authentication ticket" that will be used for client/server
interactions. The session key will be used to encrypt messages between
the client and server to protect the communications from eavesdroppers.
The authentication ticket, which is encrypted using the server's secret
encryption key, is handled, but is not readable by the client. The authentication
ticket is shipped along with client request to the server in its encrypted
form. Thus, a communication between a client and a server consists of a
request, encrypted using the session key, and a ticket, encrypted (by the
authentication server) using the server's key. Upon receipt of an encrypted
message, the server decrypts the ticket. Inside, it finds the session key,
which it can use to decrypt the message, and also authentication information
put there by the authentication server verifying that the ticket is actually
valid for a session with the particular client. Timestamps are also contained
in the ticket to limit the time during which the session key will be considered
Because session keys are known only to the client and server involved in
a particular session, conventional encryption can be used (see Encryption).
This is advantageous, since conventional encryption is typically much faster
than public key encryption, which is necessary if an authentication server
is not used to generate verifiable session keys. The disadvantage of the
Needham/Schroeder protocol is the need for interactions with an authentication
server, and the need to trust the authentication server.
Extensions exist to support hierarchical authentication servers across
domain. Basically what happens is that if an authentication server does
not know the secret key of a server for which a ticket has been requested,
it communicates via the same algorithm with a higher level authentication
server in much the same way that name servers do. Eventually, the collection
of authentication servers produce a session key and ticket, which are then
used in the session as if they were obtained from a single, global authentication
server. Thus, the algorithm scales well and matches to name server domains
Security of messages during a session requires them to be encrypted with
the session key, which in turn requires that the applications be "Kerberos
aware". For a list of such software, see Kerberos
Resources. Of course, all parties to a session must be Kerberos aware,
else they cannot interpret the encrypted messages.
An Authetication System for Computer Networks.
At least some projects to integrate DCE with the WWW use authentication
based on Kerberos, see OSF's
Public Key Encryption
Public key encryption (see Encryption)
can also be used for authentication using "digital signatures". In public
key encryption, each user, i, has both a public key, Ei, which is made
publicly available, and a private key, Di, which only user i knows. The
keys are mathematically related, and both are generated by the user. Thus,
there is no need for anyone else to hold the private key, which enhances
Public and private keys are inverses and are symmetrical, in the sense
that for a given message m, Ei(Di(m)) = Di(Ei(m)) = m. To preserve privacy,
a user X will obtain the public key Ey for user Y and compute Ey(m)). Since
only Y knows Dy, only Y can decrypt. A checksum or some other identifying
pattern is embedded into m so that a valid decryption can be verified.
Digital signatures work similarly, except that when X wants to sign a message
to Y, X uses his/her private key Dx and computes Dx(m). Upon receipt, Y
computes Ex(Dx(m)) = m. Since only X had knowledge of Dx, only X could
have signed the message. Privacy encryption can be combined with digital
signatures by computing Ey(Dx(m)), which is decrypted as Ex(Dy(Ey(Dx(m))))
The public key register of the Ei need not be read secure, since the Ei
are given away freely. The registry must be protected against corruption,
since that would allow fraudulent keys to be given out. The channel to
the registry must be secure to prevent "spoofing" attacks, but this can
be done using public key encryption.
The disadvantage of public key encryption is that it is several orders
of magnitude slower than conventional encryption because of the nature
of the encryption algorithms. Thus, for instances where a session involves
many messages or where high performance is required, a Kerberos-based system
may be more appropriate.
Both Needham/Schroeder and public key encryption can support different
levels of functionality. Without going into the details, more functionality
or security requires more messages/encryptions or more frequent messages.
Thus it is important to balance the security required with the costs of
obtaining it for a given application.
Timestamps can be embedded in authentication tickets or encrypted messages
to limit the time during which the message is valid. The recipient can
determine how old a message it is willing to accept. This time must be
selected with regard to propagation time, clock skew, and server response
time. Shorter intervals increase security at the cost of the increased
likelihood that valid messages will be discarded and the additional costs
of obtaining new tickets or re-encrypting. Kerberos uses 5 minutes as its
With non-repudiation, a client cannot claim that it did not send a particular
message. Given a message, the server can prove to a third party that the
message could only have originated with the client; specifically, the server
itself could not have forged the message. Basic Kerberos does not provide
this, but extensions do. Digital signatures provide non-repudiation.
Multiple User Signatures
Extensions provide the ability for m of n parties to sign or otherwise
validate a message in order for it to be authenticated. These algorithms
are considerably more expensive than single signers.
There are really two dominant systems for authentication, one based on
Needham/Schroeder, and the other on public key encryption. These are:
This research is sponsored by the Defense Advanced Research
Projects Agency and managed by the U.S. Army Research Laboratory under
contract DAAL01-95-C-0112. The views and conclusions contained in this
document are those of the authors and should not be interpreted as necessarily
representing the official policies, either expressed or implied of the
Defense Advanced Research Projects Agency, U.S. Army Research Laboratory,
or the United States Government.
© Copyright 1996 Object Services and Consulting, Inc.
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This page was written by David Wells. Send questions and
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Last updated: 04/22/96 7:37 PM
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