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Chapter: Cryptography and Network Security Principles and Practice : Mutual Trust : User Authentication

Remote User-Authentication Using Symmetric Encryption

Secret keys Ka and Kb are shared between A and the KDC and B and the KDC, respectively. The purpose of the protocol is to distribute securely a session key Ks to A and B.



Mutual Authentication

As was discussed in Chapter 14, a two-level hierarchy of symmetric encryption keys can be used to provide confidentiality for communication in a distributed environment. In general, this strategy involves the use of a trusted key distribution center (KDC). Each party in the network shares a secret key, known as a master key, with the KDC. The KDC is responsible for generating keys to be used for a short time over a connection between two parties, known as session keys, and for distributing those keys using the master keys to protect the distribution. This approach is quite common. As an example, we look at the Kerberos system in Section 15.3. The discussion in this subsection is relevant to an understanding of the Kerberos mechanisms.

Figure 14.3 illustrates a proposal initially put forth by Needham and Schroeder  for  secret  key  distribution  using  a  KDC  that, as was mentioned in Chapter 14, includes authentication features. The protocol can be summarized as follows.

Secret keys Ka and Kb are shared between A and the KDC  and  B  and  the KDC, respectively. The purpose of the protocol is to distribute securely a session key Ks to A and B. A securely acquires a new session key in step 2. The message in step 3 can be decrypted, and hence understood, only by B. Step 4 reflects B’s knowledge of Ks, and step 5 assures B of A’s knowledge of Ks and assures B that this is a fresh mes- sage because of the use of the nonce N2. Recall from our discussion in Chapter 14 that the purpose of steps 4 and 5 is to prevent a certain type of replay attack. In par- ticular, if an opponent is able to capture the message in step 3 and replay it, this might in some fashion disrupt operations at  B.

Despite the handshake of steps 4 and 5, the protocol is still vulnerable to a form of replay attack. Suppose that an opponent, X, has been able to compromise an old ses- sion key. Admittedly, this is a much more unlikely occurrence than that an opponent has simply observed and recorded step 3. Nevertheless, it is a potential security risk. X can impersonate A and trick B into using the old key by simply replaying step 3. Unless B remembers indefinitely all previous session keys used with A, B will be unable to determine that this is a replay. If X can intercept the handshake message in step 4, then it can impersonate A’s response in step 5. From this point on, X can send bogus mes- sages to B that appear to B to come from A using an authenticated session key.

Denning proposes to overcome this weakness by a mod- ification to the Needham/Schroeder protocol that includes the addition of a time- stamp to steps 2 and 3. Her proposal assumes that the master keys, Ka and Kb, are secure, and it consists of the following steps.

T is a timestamp that assures A and B that the session key has only just been generated. Thus, both A and B know that the key distribution is a fresh exchange. A and B can verify timeliness by checking that

where Δt1 is the estimated normal discrepancy between the KDC’s clock and the local clock (at A or B) and Δt2 is the expected network delay time. Each node can set its clock against some standard reference source. Because the timestamp T is encrypted using the secure master keys, an opponent, even with knowledge of an old session key, cannot succeed because a replay of step 3 will be detected by B as untimely.

A final point: Steps 4 and 5 were not included in the original presentation [DENN81] but were added later [DENN82]. These steps confirm the receipt of the session key at B.

The Denning protocol seems to provide an increased degree of security compared to the Needham/Schroeder protocol. However, a new concern is raised: namely, that this new scheme requires reliance on clocks that are synchro- nized throughout the network. [GONG92] points out a risk involved. The risk is

based on the fact that the distributed clocks can become unsynchronized as a result of sabotage on or faults in the clocks or the synchronization mechanism.2 The problem occurs when a sender’s clock is ahead of the intended recipient’s clock. In this case, an opponent can intercept a message from the sender   and

replay it later when the timestamp in the message becomes current at the recipi- ent’s site. This replay could cause unexpected results. Gong refers to such attacks as suppress-replay attacks.

One way to counter suppress-replay attacks is to enforce the requirement that parties regularly check their clocks against the KDC’s clock. The other alternative, which avoids the need for clock synchronization, is to rely on handshaking protocols using nonces. This latter alternative is not vulnerable to a suppress-replay attack, because the nonces the recipient will choose in the future are unpredictable to the sender. The Needham/Schroeder protocol relies on nonces only but, as we have seen, has other vulnerabilities.

In [KEHN92], an attempt is made to respond to the concerns about suppress- replay attacks and at the same time fix the problems in the Needham/Schroeder protocol. Subsequently, an inconsistency in this latter protocol was noted and an improved strategy was presented in . The protocol is

Let us follow this exchange step by step.

1.                                       A initiates the authentication exchange by generating a nonce, Na, and sending that plus its identifier to B in plaintext. This nonce will be returned to A in an encrypted message that includes the session key, assuring A of its timeliness.

B alerts the KDC that a session key is needed. Its message to the KDC includes its identifier and a nonce, Nb. This nonce will be returned to B in an encrypted message that includes the session key, assuring B of its timeliness. B’s message to the KDC also includes a block encrypted with the secret key shared by B and the KDC. This block is used to instruct the KDC to issue credentials to A; the block specifies the intended recipient of the credentials, a suggested expiration time for the credentials, and the nonce received from A.

2.                        The KDC passes on to A B’s nonce and a block encrypted with the secret key that B shares with the KDC. The block serves as a “ticket” that can be used by A for subsequent authentications, as will be seen. The KDC also sends to A a block encrypted with the secret key shared by A and the KDC. This block verifies that B has received A’s initial message (IDB) and that this is a timely message and not a replay (Na), and it provides A with a session key (Ks) and the time limit on its use (Tb).

3.                        A transmits the ticket to B, together with the B’s nonce, the latter encrypted with the session key. The ticket provides B with the secret key that is used to decrypt E(Ks, Nb) to recover the nonce. The fact that B’s nonce is encrypted with the session key authenticates that the message came from A and is not a replay.

This protocol provides an effective, secure means for A and B to establish a session with a secure session key. Furthermore, the protocol leaves A in possession of a key that can be used for subsequent authentication to B, avoiding the need to contact the authentication server repeatedly. Suppose that A and B establish a ses- sion using the aforementioned protocol and then conclude that session. Subsequently, but within the time limit established by the protocol, A desires a new session with B. The following protocol ensues:

When B receives the message in step 1, it verifies that the ticket has not expired. The newly generated nonces N¿a and N¿b assure each party that there is no replay attack. In all the foregoing, the time specified in Tb is a time relative to B’s clock.

Thus, this timestamp does not require synchronized clocks, because B checks only self-generated  timestamps.


One-Way Authentication

Using symmetric encryption, the decentralized key distribution scenario illustrated in Figure 14.5 is impractical. This scheme requires the sender to issue a request to the intended recipient, await a response that includes a session key, and only then send the message.

With some refinement, the KDC strategy illustrated in Figure 14.3 is a candi- date for encrypted electronic mail. Because we wish to avoid requiring that the recipient (B) be on line at the same time as the sender (A), steps 4 and 5 must be eliminated. For a message with content M, the sequence is as follows:


This approach guarantees that only the intended recipient of a message will be able to read it. It also provides a level of authentication that the sender is A. As specified, the protocol does not protect against replays. Some measure of defense could be provided by including a timestamp with the message. However, because of the potential delays in the e-mail process, such timestamps may have limited usefulness.

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