# US 12,120,231 B2 · Digital watermark system, digital watermark method and program

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- Number: US 12,120,231 B2
- Document ID: US12120231B2
- Status: Granted
- Kind: B2
- Application number: 17/774,828
- Filed: 2019-11-11
- Published: 2022-12-22
- Granted: 2024-10-15
- Earliest claimed: 2019-11-11
- Assignees: Nippon Telegraph and Telephone Corporation
- Applicants: Nippon Telegraph and Telephone Corporation
- Inventors: Ryo Nishimaki
- Web page: https://patentgrep.com/patent/US12120231B2

## CPC

- H04L 9/0894: {Escrow, recovery or storing of secret information, e.g. secret key escrow or cryptographic key storage}
- H04L 9/32: including means for verifying the identity or authority of a user of the system {or for message authentication, e.g. authorization, entity authentication, data integrity or data verification, non-repudiation, key authentication or verification of credentials}
- G09C 1/00: Apparatus or methods whereby a given sequence of signs, e.g. an intelligible text, is transformed into an unintelligible sequence of signs by transposing the signs or groups of signs or by replacing them by others according to a predetermined system (cryptographic typewriters G09C3/00)
- H04L 9/0825: {using asymmetric-key encryption or public key infrastructure [PKI], e.g. key signature or public key certificates}
- H04N 1/32: Circuits or arrangements for control or supervision between transmitter and receiver {or between image input and image output device, e.g. between a still-image camera and its memory or between a still-image camera and a printer device (H04N1/38, H04N1/387 take precedence)}
- H04L 2209/608: Watermarking
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)
- G09C: CIPHERING OR DECIPHERING APPARATUS FOR CRYPTOGRAPHIC OR OTHER PURPOSES INVOLVING THE NEED FOR SECRECY
- H04N: PICTORIAL COMMUNICATION, e.g. TELEVISION

## Abstract

A digital watermark system that realizes digital watermark for a cryptographic function, including a memory and a processor configured to take as input a security parameter and generate a watermark embedding key for embedding information representative of a watermark in a circuit that realizes the cryptographic function, and a watermark extraction key for extracting information representative of the watermark from a circuit in which a watermark is embedded; take as input a master private key used for the cryptographic function, the watermark embedding key, and information representative of a watermark, and generate a circuit in which the watermark is embedded and that realizes the cryptographic function; and take as input the watermark extraction key and the circuit in which the watermark has been embedded, and extract information representative of a watermark from the circuit.

## Claims

### Claim 1

A digital watermark system that realizes digital watermark for a cryptographic function, comprising:
a memory; and
a processor configured to:
take as input a security parameter and generate a watermark embedding key for embedding information representative of a watermark in a circuit that realizes the cryptographic function, and a watermark extraction key for extracting information representative of the watermark from a circuit in which a watermark is embedded;
take as input a master private key used for the cryptographic function, the watermark embedding key, and information representative of a watermark, and generate a circuit, wherein the circuit comprises the watermark and a program code to perform the cryptographic function, and the cryptographic function comprises encryption of data, decryption of data, and generation of a signature of the data; and
take as input the watermark extraction key and the circuit, extract information representative of a watermark from the circuit for authenticating the program code.

### Claim 2 (depends on 1)

The digital watermark system according to claim 1, wherein the cryptographic function is a function provided by an encryption scheme that is secure based on a computationally difficult problem n and has an all-but-N security proof.

### Claim 3 (depends on 2)

The digital watermark system according to claim 2, wherein the processor
simulates the problem π from the master private key, and
generates a circuit that takes as input the problem I, the watermark embedding key, a query in security proof, and a random number used when generating an answer to the query, and simulates the answer, as the circuit with the watermark embedded therein and realizing the cryptographic function.

### Claim 4 (depends on 3)

The digital watermark system according to claim 3, wherein the processor
verifies whether or not an answer output by inputting the query and the random number to the circuit is correct, using a master public key used for the cryptographic function, and
extracts information representative of the watermark, using a total number of the answers that have been verified as correct.

### Claim 5

A digital watermark method executed by a digital watermark system that realizes digital watermark for a cryptographic function, the method comprising the steps of:
a key generating process in which a security parameter is taken as input and a watermark embedding key for embedding information representative of a watermark in a circuit that realizes the cryptographic function, and a watermark extraction key for extracting information representative of the watermark from a circuit in which a watermark is embedded are generated;
an embedding process in which a master private key used for the cryptographic function, the watermark embedding key, and information representative of a watermark are taken as input, and generating a circuit, wherein the circuit comprises the watermark and a program code to perform the cryptographic function, and the cryptographic function comprises encryption of data, decryption of data, and generation of a signature of the data; and
an extraction process in which the watermark extraction key and the circuit, are taken as input, and information representative of a watermark is extracted from the circuit for authenticating the program code.

### Claim 6 (depends on 5)

A non-transitory computer-readable recording medium having a program causing a computer to execute the digital watermark method of claim 5.