Digital signatures are considered the most important development in public key cryptography. Sun Developer Network states: “A digital signature is a string of bits that is computed from some data (the data being “signed”) and the private key of an entity. The signature can be used to verify that the data came from of the entity. and was not modified in transit” (The Java Tutorial, nd). Digital signatures must have the properties of verifying the author, verifying the date and time of the signature, authenticating the content at the time of signing, as well as being verifiable by a third party to resolve disputes. Based on these properties, there are several requirements for a digital signature. The first of these requirements is that the signature must be a bit pattern that depends on the message being signed. The following requirement is stated to prevent forgery and denial. It states that the signature must use some information that is unique to the sender. The third requirement is that it must be fairly easy to generate the digital signature. Being relatively easy to recognize and verify the digital signature is another requirement. The fifth requirement states that it must be computationally infeasible to forge a digital signature, either by constructing a new message for an existing digital signature or by constructing a fraudulent digital signature for a given message. The last requirement is that it must be practical to store a copy of the digital signature. Many approaches to implementing digital signatures have been proposed, and they fall under the direct and arbitrated digital signature approaches (Stallings, 2003).
Direct digital signature involves only communication between the source and destination parties, and arbitrated digital signature schemes include the use of an arbitrator. The direct digital signature is created by encrypting the entire message or a hash code of the message with the sender’s private key. Further confidentiality can be provided by encrypting the entire message and adding a signature using the recipient’s public key or a shared secret key between sender and recipient. One weakness in the direct signature scheme is that a sender can later deny having sent a message. Another weakness is the threat of a private key being stolen and a message being sent using the signature. Both of these weaknesses are the main reason for the arbitrated digital signature scheme. In the arbitrated scheme, a sender’s message must first pass through an arbitrator who runs a series of tests to verify origin and content before sending it to the receiver. Because the referee plays such a crucial role, the sender and receiver must have great confidence in this referee. This trust in the arbitrator assures the sender that no one can forge his signature and assures the receiver that the sender cannot be unaware of his signature (Stallings, 2003).
The issue of replay attacks is a primary concern when it comes to mutual authentication when both parties confirm each other’s identity and exchange session keys. The main problems with mutual authentication lie in the exchange of keys: confidentiality and deadlines. Schedules are susceptible to replay attacks that disrupt operations by presenting parties with messages that appear to be genuine but are not. One type of replay attack is the suppress-response attack that can occur in the Denning protocol. The Denning protocol uses timestamps to increase security. The problem here revolves around the dependency on clocks being synchronized across the network. It is stated, “…that distributed clocks can become out of sync as a result of sabotage or failures in the clocks or in the timing mechanism” (Stallings, 2003 p. 387). Li Gong states: “…the recipient remains vulnerable to accepting the message as a current one, even after the sender has detected its clock error and resynchronized the clock, unless the postdated message has been invalidated in some way.” “, which is unlikely. If the sender’s clock is ahead of the receivers and the message is intercepted, the opponent can replay the message when the timestamp is updated. This type of attack is known as a suppression-reproduction attack.
To address the suppression and replay attack concern, an improved protocol has been introduced. Here are the detailed steps.
1. “A initiates the authentication exchange by generating a nonce, Na, and sending it plus its identifier to B in plain text. This nonce will be returned to A in an encrypted message that includes the session key, assuring A of its lines of weather.
2. B alerts the KDC that a session key is needed. Your message to the KDC includes your 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 an encrypted block with the secret key shared by B and the KDC. This block is used to tell the KDC to issue credentials for A; the block specifies the intended recipient of the credentials, a suggested expiration time for the credentials, and the nonce received from A.
3. The KDC passes the nonce of AB and a block encrypted with the secret key by A for subsequent authentications, as will be seen. The KDC also sends A a block encrypted with the secret key shared by A and the KDC. This block verifies that B has received the initial message (IDB) from A and that this is a timely message and not a repeat (Na), and provides A with a session key (KS) and the time limit for its use ( Tb).
4. A transmits the ticket to B, along with B’s nonce, the latter encrypted with the session key. The ticket provides B with the secret key that is used to decrypt EKS[Nb] to retrieve 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” (Stallings, 2003 pp. 387-388).
This protocol is not vulnerable to suppress-replay attacks due to the fact that the nonces that the recipient will choose in the future are unpredictable to the sender (Gong, nd).
In conclusion, digital signatures are considered the most important development in public key cryptography and include both direct and arbitrated digital signature approaches. Direct digital signature involves only communication between the source and destination parties, and arbitrated digital signature schemes include the use of an arbitrator. Replay suppression attacks can occur if the sender’s clock is ahead of the receiver’s and the message is intercepted. This allows the opponent to play the message when the timestamp is updated. This problem is solved by implementing a protocol that uses timestamps that do not require synchronized clocks because receiver B checks only self-generated timestamps (Stallings, 2003).
Works Cited
Gong, Li (nd). A security risk of relying on synchronized clocks. ORA Corporation and Cornell University. Retrieved November 5, 2005, from https://portal.acm.org
Stallings, William. (2003). Cryptography and network security: principles and practices. New Jersey: Pearson Education, Inc.
The Java Tutorial (nd). Sun Developer Network. Retrieved November 5, 2005, from http://java.sun.com/docs/books/tutorial/index.html
