Secure electronic messaging system requiring key retrieval for deriving decryption keys

A method for secure communication of a message M between a sender and a recipient, with the assistance of a key server, the method comprising the steps of:

• the sender obtaining a symmetric key Ks and encrypting the message M with said symmetric key Ks to thereby form an encrypted message Me;
• the sender encrypting first information comprising the symmetric key Ks with a public encryption key Ke associated with the recipient to thereby form an encrypted symmetric key Kse;
• the sender forming first and second split-key fragments Kse1 and Kse2, respectively, from said encrypted symmetric key Kse;
• the sender sending the second split-key fragment Kse2 to the key server;
• the key server storing decryption key information needed by the recipient to form a decryption key suitable for decrypting the encrypted message, the decryption key information being formed from the second split-key fragment Kse2;
• the key server transmitting to the sender, key retrieval information Kr that indexes the decryption key information;
• the sender transmitting to the recipient, the encrypted message Me, the first split-key fragment Kse1 and the key retrieval information Kr;
• the recipient transmitting the key retrieval information Kr to the key server and receiving said decryption key information in response thereto;
• the recipient forming the encrypted symmetric key Kse from the first split-key fragment Kse1 and Kse2, Kse2 being derived from the decryption key information;
• the recipient decrypting the encrypted symmetric key Kse with a private decryption key Kd of the recipient to thereby form the symmetric key Ks;
• and
• the recipient decrypting the encrypted message Me with the symmetric key Ks to read the original message M.

A method for secure communication of a message M between a sender and a recipient, with the assistance of a key server, the method comprising the steps of:

• the sender obtaining a symmetric key Ks and encrypting the message M with said symmetric key Ks to thereby form an encrypted message Me;
• the sender encrypting first information comprising the symmetric key Ks with a public encryption key Ke associated with the recipient to thereby form an encrypted symmetric key Kse;
• the sender sending the encrypted symmetric key Kse to the key server;
• the key server storing decryption key information needed by the recipient to form a decryption key suitable for decrypting the encrypted message, the decryption key information being formed from the encrypted symmetric key Kse;
• the key server transmitting to the sender, key retrieval information Kr that indexes the decryption key information;
• the sender transmitting to the recipient, the encrypted message and the key retrieval information Kr;
• the recipient transmitting the key retrieval information to the key server and receiving said decryption key information in response thereto;
• the recipient deriving the encrypted symmetric key Kse from said decryption key information;
• the recipient decrypting the encrypted symmetric key Kse with a private decryption key Kd of the recipient to thereby form the symmetric key Ks;
• and
• the recipient decrypting the encrypted message Me with the symmetric key Ks to read the original message M.

A method for secure communication of a message M between a sender and a recipient, with the assistance of a key server, the method comprising the steps of:

• the key server obtaining first and second split-key fragments Ks1 and Ks2, respectively, corresponding to a symmetric key Ks;
• the key server storing decryption key information needed by the recipient to form a decryption key suitable for decrypting the encrypted message, the decryption key information being formed from the second split-key fragment Ks2;
• the key server transmitting to the sender, a public encryption key Ke associated with the recipient, symmetric key information sufficient to form Ks, and key retrieval information Kr that indexes the decryption key information;
• the sender encrypting the message M with Ks to thereby form an encrypted message Me, the sender also encrypting first information comprising the first split-key fragment Ks1 with the recipient's public encryption key Ke to form an encrypted first split-key fragment Kse1;
• the sender transmitting to the recipient, the encrypted message Me, the key retrieval information Kr, and the encrypted first split-key fragment Kse1;
• the recipient transmitting the key retrieval information Kr to the key server and receiving said decryption key information in response thereto;
• the recipient decrypting the encrypted first split-key fragment Kse1 with a private decryption key Kd of the recipient to thereby form the first split-key fragment Ks1;
• the recipient forming Ks from the first and second split-key fragments Ks1 and Ks2, respective, Ks2 being derived from the decryption key information;
• and
• the recipient decrypting the encrypted message Me with the symmetric key Ks to read the original message M.

A method of operating a key server in a secure communication system to facilitate secure transmission of a message M between a first entity and a second entity, the method comprising the steps of:

• during a first sequence of exchanges:
• receiving from a first entity, a request to provide a public encryption key Ke corresponding to a second entity specified by the first entity;
• transmitting to the first entity, said public encryption key Ke;
• storing decryption key information needed by the second entity to form a decryption key suitable for decrypting an encrypted version of message M;
• transmitting to the first entity, key retrieval information Kr which indexes said decryption key information;
• and
• during a second sequence of exchanges:
• receiving from the second entity, the key retrieval information Kr;
• and
• transmitting to the second entity, said decryption key information, without ever receiving the message or the encrypted version of the message.

A key server computer in a secure communication system configured to facilitate secure transmission of a message M between a sender and a recipient, the key server computer being programmed to:

• during a first sequence of exchanges:
• receive from a first entity, a request to provide a public encryption key Ke corresponding to a second entity specified by the first entity;
• transmit to the first entity, said public encryption key Ke;
• store decryption key information needed by the second entity to form a decryption key suitable for decrypting an encrypted version of message M;
• transmit to the first entity, key retrieval information Kr which indexes said decryption key information;
• and
• during a second sequence of exchanges:
• receive from the second entity, the key retrieval information Kr;
• and
• transmit to the second entity, said decryption key information, without having to receive the message or the encrypted version of the message.

A computer-readable memory having executable code stored thereon, the code including:

• a first set of code comprising:
• code to receive from a first entity, a request to provide a public encryption key Ke corresponding to a second entity specified by the first entity;
• code to transmit to the first entity, said public encryption key Ke;
• code to store decryption key information needed by the second entity to form a decryption key suitable for decrypting an encrypted version of a message M;
• code to transmit to the first entity, key retrieval information Kr which indexes said decryption key information;
• and
• a second set of code comprising:
• code to receive from the second entity, the key retrieval information Kr;
• and
• code to transmit to the second entity, said decryption key information.

A method for secure communication of a message M between a sender and a recipient, with the assistance of a key server, the method comprising the steps of:

• the sender encrypting the message M with a message encryption key Kme to form an encrypted message;
• the sender encrypting first information derived from a message decryption key Kmd corresponding to the message encryption key Kme, using a public encryption key Ke of the recipient;
• the key server storing decryption key information needed by the recipient to form said message decryption key Kmd suitable for decrypting the encrypted message;
• the sender receiving key retrieval information Kr which indexes the decryption key information;
• the sender sending to the recipient, at least the encrypted message and the key retrieval information Kr;
• the recipient sending the key retrieval information Kr to the key server and receiving the decryption key information in response thereto;
• the recipient forming the message decryption key Kmd from at least the decryption key information and a private decryption key Kd of the recipient;
• and
• the recipient decrypting the encrypted message with the message decryption key Kmd;
• without the message or the encrypted message being sent to the key server.

A method for secure communication of a message M between a sender and a recipient, with the assistance of a key server, the method comprising the steps of:

• the sender encrypting the message M with a message encryption key Kme to form an encrypted message;
• the sender encrypting first information derived from a message decryption key Kmd corresponding to the message encryption key Kme, using a public encryption key Ke of the recipient;
• the key server storing decryption key information needed by the recipient to form said message decryption key Kmd suitable for decrypting the encrypted message;
• the sender sending to the recipient, at least the encrypted message;
• the recipient sending key retrieval information Kr to the key server which indexes the decryption key information, and receiving the decryption key information in response thereto;
• the recipient forming the message decryption key Kmd from at least the decryption key information and a private decryption key Kd of the recipient;
• and
• the recipient decrypting the encrypted message with the message decryption key Kmd;
• without the message or the encrypted message being sent to the key server.