# US 5,353,351 A · Secure teleconferencing > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,353,351 A - Document ID: US5353351A - Status: Granted - Kind: A - Application number: 07/896,520 - Filed: 1992-06-09 - Granted: 1994-10-04 - Earliest claimed: 1992-06-09 - Assignees: AT&T Bell Laboratories - Inventors: Paul D. Bartoli; Bruce E. McNair - Web page: https://patentgrep.com/patent/US5353351A ## CPC - H04K 1/00: Secret communication - H04M 3/56: Arrangements for connecting several subscribers to a common circuit, i.e. affording conference facilities (video conference systems H04N7/15) - H04M 2203/609: Secret communication - H04K: SECRET COMMUNICATION; JAMMING OF COMMUNICATION - H04M: TELEPHONIC COMMUNICATION (circuits for controlling other apparatus via a telephone cable and not involving telephone switching apparatus G08) ## Abstract Secure teleconferencing between three or more parties is achieved by receiving encrypted information streams from each of the parties to the conference. The encrypted information streams are received in decryption/re-encryption units co-located with a conference bridge established in a telecommunication switching system in a public switched telephone network central office. The decryption/re-encryption units decrypt the information streams and then re-encrypt those information streams in accordance with a common conference encryption algorithm. The re-encryption processes for the information streams are synchronized so that the re-encryption circuitry for all information streams are in the same cryprostate at all times during the re-encryption procedure. The decryption/re-encryption units are located in secure surroundings to protect the unencrypted information streams present in those units. A gating circuit permits only one of the re-encrypted information streams to be input to the conference bridge at a given time. A circuit which simulates the operation of a silent conferee prevents noise bursts from being sent to the conferees during periods of time when all are silent. ## Claims ### Claim 1 A secure teleconferencing apparatus, comprising: a decryption circuit for decrypting at least three encrypted information streams; a re-encryption circuit for re-encrypting the decrypted information streams; and a conference bridge for combining the re-encrypted information streams. ### Claim 2 (depends on 1) The secure teleconferencing apparatus of claim 1, in which the decryption circuit decrypts at least three encrypted information streams in accordance with one or more respective conferee encryption algorithms. ### Claim 3 (depends on 1) The secure teleconferencing apparatus of claim 1, in which the re-encryption circuit re-encrypts at least three information streams in accordance with a single conference encryption algorithm. ### Claim 4 (depends on 2) The secure teleconferencing apparatus of claim 2, in which the re-encryption circuit re-encrypts at least three information streams in accordance with a single conference encryption algorithm. ### Claim 5 (depends on 3) The secure teleconferencing apparatus of claim 3, further comprising a means for synchronizing the re-encryption of the at least three information streams in accordance with the conference encryption algorithm. ### Claim 6 (depends on 5) The secure teleconferencing apparatus of claim 5, in which the re-encryption circuit comprises at least first, second, and third encryption circuits, each for encrypting a respective information stream in accordance with the conference encryption algorithm. ### Claim 7 (depends on 6) The secure teleconferencing apparatus of claim 6, in which the means for synchronizing comprises a common crypto state generator and a common key variable generator connected to the first, second, and third encryption circuits in the re-encryption circuit. ### Claim 8 (depends on 1) The secure teleconferencing apparatus of claim 1, in which at least part of the decryption circuit and the re-encryption circuit are located in a secure, tamper proof housing. ### Claim 9 (depends on 1) The secure teleconferencing apparatus of claim 1, in which the decryption circuit and the encryption circuit are co-located with the conference bridge. ### Claim 10 (depends on 1) The secure teleconferencing apparatus of claim 1, further comprising a gating circuit which denies access to the conference bridge by each information stream when one of the other information streams has been granted access to the conference bridge. ### Claim 11 (depends on 1) The secure teleconferencing apparatus of claim 1, further comprising a gating circuit which directs only one of the three or more re-encrypted information streams to the conference bridge at a time. ### Claim 12 (depends on 1) The secure teleconferencing apparatus of claim 1, further comprising a decryption circuit for decrypting the combined re-encrypted information streams in accordance with a single conference encryption algorithm. ### Claim 13 (depends on 12) The secure teleconferencing apparatus of claim 12, further comprising a re-encryption circuit for re-encrypting in accordance with a respective conferee encryption algorithm the information streams decrypted in accordance with the conference encryption algorithm. ### Claim 14 (depends on 1) The secure teleconferencing apparatus of claim 1, further comprising a silent conferee circuit for directing encrypted silence to the conference bridge when all conferees are silent. ### Claim 15 A method of obtaining a secure teleconference between three or more conferees, comprising the steps of: decrypting at least three encrypted information streams from the three or more conferees; re-encrypting the decrypted information streams; and combining the re-encrypted information streams in a conference bridge co-located with circuits performing the decrypting and re-encrypting steps. ### Claim 16 (depends on 15) The method of claim 15, further comprising the step of gating only one of the re-encrypted information streams to the conference bridge at a time. ### Claim 17 (depends on 15) The method of claim 15, further comprising the step of directing encrypted silence to the conference bridge when all conferees are silent. ### Claim 18 (depends on 15) The method of claim 15, in which the re-encrypting step comprises the step of re-encrypting each of the three information streams in accordance with a common conference encryption algorithm. ### Claim 19 A secure teleconferencing apparatus, comprising: a conference bridge; a gating circuit for directing only one of a plurality of encrypted information streams to the conference bridge at a time; and a generator of encrypted silence directed to the conference bridge when all of the plurality of encrypted information streams represent silence. ### Claim 20 (depends on 19) The secure teleconferencing apparatus of claim 19, in which the encrypted information streams are each encrypted in accordance with a common encryption algorithm. ### Claim 21 A secure teleconferencing apparatus, comprising: a conference bridge for combining encrypted information streams from a plurality of conferees; and a generator of encrypted silence to be directed to the conference bridge when all the conferees are silent. ### Claim 22 (depends on 21) The secure teleconferencing apparatus of claim 21, in which the encrypted information streams are encrypted in accordance with a common encryption algorithm. ### Claim 23 A public switched telephone network, comprising: at least one central office switching system capable of being configured as a teleconference bridge; at least first, second, and third decryption/re-encryption units co-located with the central office switching system for receiving at least first, second, and third information streams from at least first, second, and third conferees, the information streams received from the conferees being encrypted in accordance with respective conferee encryption algorithms; the at least first, second, and third decryption/re-encryption units each comprising a means for decrypting the information streams received from the conferees in accordance with respective conferee encryption algorithms and a means for re-encrypting the decrypted information streams in accordance with a single conference encryption algorithm; a means for gating the re-encrypted information streams to inputs of a conference bridge formed in the switching system, the conference bridge producing a combined conference output and directing that conference output back to the conferees.