Obviousness

Based on the provided analysis of prior art cited during prosecution, here is an analysis of the obviousness of US Patent 5,412,730 under 35 U.S.C. § 103.

Definition of a Person Having Ordinary Skill in the Art (PHOSITA)

A person having ordinary skill in the art (PHOSITA) in early 1992 would have possessed a bachelor's degree in electrical engineering or computer science, along with practical experience in the fields of data communications, modem technology, and applied cryptography. Such a person would be familiar with standard encryption algorithms like the Data Encryption Standard (DES), the concept of stream ciphers, and common methods for generating pseudo-random number sequences for cryptographic purposes, such as using linear-feedback shift registers (LFSRs) initialized with a secret seed.

Obviousness Analysis of Independent Claims 1, 6, and 8

The core invention recited in claims 1 (transmitting method), 6 (receiving method), and 8 (modem apparatus) is a system where encryption keys are changed in synchrony at two ends of a communication link. This is achieved by:
a) Pre-sharing a secret "seed value."
b) Using this seed to initialize identical pseudo-random number generators (PRNGs) at both ends.
c) Advancing the PRNGs to the next key in their sequence based on monitoring a predetermined characteristic of the data stream itself (e.g., counting a number of data blocks).

A strong case for obviousness can be made by combining U.S. Patent 4,850,017 ('017) with the general knowledge of a PHOSITA regarding cryptographic key generation.

Primary Reference: U.S. Patent 4,850,017 ('017)

The '017 patent discloses the foundational elements of the '730 patent's method. Specifically, it teaches:

• A secure communication system with a master (transmitter) and remote (receiver) station.
• A shared encryption key that is periodically and synchronously changed.
• The key change is triggered after "a specific number of data transmissions have occurred."
• Both the master and remote stations contain counters that are incremented with each transmission to maintain synchronization for the key change.

This directly teaches the core concept of monitoring a characteristic of the data flow (a count of transmissions) and using that count to trigger a synchronized key change at both ends of a link, without transmitting the new key or a separate synchronization signal.

Motivation to Combine '017 with General Knowledge

The only arguable difference between the disclosure of '017 and the claims of '730 is the specific mechanism for generating the sequence of new keys. Claim 1 of '730 recites using a "pseudo-random number seed value" to generate a sequence of "pseudo-random numbers" which serve as keys.

A PHOSITA in 1992, tasked with implementing the key-changing system described in '017, would have found it obvious to use a standard, seeded PRNG to generate the succession of keys. The use of a PRNG initialized with a secret seed was a fundamental and widely known technique for creating a cryptographic key stream. This was a standard building block in the art of cryptography.

Therefore, the motivation would be to implement the "key changing" function of '017 using a well-known, efficient, and standard component: a seeded PRNG. Doing so would be an obvious design choice, not an inventive step. The system in '017 requires a new key upon the counter reaching a certain value; a PRNG provides an immediate and logical source for that new key. The initial shared secret required by the system would naturally serve as the seed for the PRNG.

Conclusion for Claims 1, 6, and 8

Because '017 teaches synchronized key changes based on counting transmissions, and the use of a seeded PRNG to generate a sequence of keys was a well-known and conventional technique in the art, a PHOSITA would have found it obvious to combine these elements. Thus, claims 1, 6, and 8, which cover the method and apparatus for this process, would be rendered obvious by U.S. Patent 4,850,017 in view of the general knowledge of one of ordinary skill in the art of cryptography.

Alternative Obviousness Combination: U.S. Patent 5,052,040

Similarly, U.S. Patent 5,052,040 ('040) teaches using a shared master key (analogous to a seed) and a synchronized, incrementing "message sequence number" (a counter) to generate a unique key for each message. This also teaches the core concept of synchronized key generation based on a shared secret and a counter. A PHOSITA would find it an obvious design choice to substitute the key derivation method of '040 with a standard PRNG key-stream generator, advanced by the same message sequence number, to achieve a similar result.

Obviousness Analysis of Independent Claim 9

Claim 9 expands the core method to a network environment with a central control station and multiple remote terminals, where the central station provides keys to authorize communications between specific terminals.

A strong case for the obviousness of Claim 9 can be made by combining the teachings of the primary obviousness argument above with U.S. Patent 4,423,287 ('287).

Base Combination: U.S. Patent 4,850,017 (or 5,052,040) in view of general cryptographic knowledge.

Secondary Reference: U.S. Patent 4,423,287 ('287)

The '287 patent explicitly teaches a key management system for a network. Its disclosure includes:

• A network with multiple users or terminals.
• A "key notarizing" scheme that functions as a central authority.
• A system for securely distributing keys to intended users for a specific context, thereby managing which terminals are authorized to communicate securely.

Motivation to Combine

A PHOSITA looking to enhance the security of the network key management system described in '287 would be motivated to incorporate a method for dynamic key alteration. The system in '287 manages the distribution of what are essentially static session keys. A known vulnerability in such systems is that if a session key is compromised, the entire session is compromised.

The method taught in '017 (or '040) of continuously changing the key based on the data flow provides a direct solution to this problem. A PHOSITA would be motivated to combine these teachings to build a more robust network. The central controller from '287 would be used to securely distribute the initial "seed values" (from '730's claims) or shared secrets (from '017), thereby authorizing a communication link. Once the link is established, the keys would begin to change dynamically according to the data-counting method of '017. This combination leverages the network control of '287 with the enhanced link security of '017, an obvious improvement that would have been a predictable design goal for anyone skilled in the art of network security.

Conclusion for Claim 9

Claim 9 is obvious over the combination of U.S. Patent 4,850,017 and U.S. Patent 4,423,287. The former teaches the dynamic, data-dependent key-changing mechanism, while the latter teaches the network architecture and central key management role. Combining them would be a straightforward and logical step to improve the security of the managed network.