Public/private key system with decreased public key size

A private-key decryption device comprising:

• a communication interface circuit, wherein the communication interface circuit is arranged to communicate with a public-key encryption device;
• and
• a processor circuit,
• wherein the processor circuit is arranged to generate a first private-key matrix,
• wherein the processor circuit is arranged to compute a first public-key matrix from the first private-key matrix,
• wherein the processor circuit is arranged to allow transfer of the first public-key matrix to the public-key encryption device,
• wherein the processor circuit is arranged to receive from the public-key encryption device an indication of a key size and/or matrix size, a second public-key matrix and a masked transmission key,
• wherein the second public-key matrix has fewer matrix elements than the first public-key matrix,
• wherein the processor circuit is arranged to compute a first raw shared key from the second public-key matrix and the first private-key matrix,
• wherein the processor circuit is arranged to compute the first raw shared key,
• wherein the computing comprises a matrix multiplication between the second public-key matrix and the first private-key matrix,
• wherein the processor circuit is arranged to unmask the masked transmission key in dependence on the received size using at least part of the first raw shared key to obtain a transmission key,
• wherein the processor circuit is arranged to perform a handshake with the public-key encryption device, the handshape comprising:
• sending to the public-key encryption device at least two indications of at least two first public-key matrices of the private-key decryption device, wherein the two first public-key matrices have different sizes;
• receiving an indication of a selected first public-key matrix from the public-key encryption device;
• sending the selected first public-key matrix, when a request for the selected first public-key matrix is received, wherein the transmission key is decapsulated with the selected first-public key.

A public-key encryption method comprising:

• establishing communication with a private-key decryption device;
• obtaining a first public-key matrix, wherein the first public-key matrix is associated with the private-key decryption device;
• obtaining a key size for the transmission key;
• generating a transmission key according to the key size;
• determining a matrix size for the second private-key matrix, wherein the matrix size is arranged to accommodate masking of the transmission key;
• generating a second private-key matrix according to the matrix size;
• computing a second public-key matrix from the second private-key matrix, wherein the second public-key matrix has fewer matrix elements than the first public-key matrix;
• computing a second raw shared key from the second private-key matrix and the first public-key matrix;
• computing the second raw shared key, wherein the second raw shared key comprises a matrix multiplication between the second private-key matrix and the first public-key matrix;
• masking the transmission key with at least part of the second raw shared key;
• and
• transferring an indication of the key size and/or matrix size, the second public-key matrix, and the masked transmission key to the private-key decryption device;
• and
• performing a handshake with the private-key decryption device, wherein the handshaking comprises:
• receiving at least two indications of at least two first public-key matrices of the private-key decryption device from the private-key decryption device,
• wherein the two first public-key matrices have different sizes;
• selecting the larger of the first public-key matrix and the second public-key matrix;
• sending an indication of the selected first public-key matrix to the private-key decrypting device;
• and
• requesting the first public-key matrix when the first public-key matrix is not available in a key-cache, wherein the transmission key is encapsulated with the selected first-public key.

A private-key decryption method comprising:

• establishing communication with a public-key encryption device;
• generating a first private-key matrix;
• computing a first public-key matrix from the first private-key matrix, and allow transfer of the first public-key matrix to the public-key encryption device;
• receiving from the public-key encryption device an indication of a key size and/or matrix size;
• receiving a second public-key matrix and a masked transmission key, wherein the second public-key matrix has fewer matrix elements than the first public-key matrix;
• computing a first raw shared key from the second public-key matrix and the first private-key matrix;
• computing the first raw shared key comprising a matrix multiplication between the second public-key matrix and the first private-key matrix;
• and
• unmasking the masked transmission key in dependence on the received size using at least part of the first raw shared key to obtain a transmission key wherein the processor circuit is arranged to perform a handshake with the public-key encryption device, the handshape comprising:
• sending to the public-key encryption device at least two indications of at least two first public-key matrices of the private-key decryption device, wherein the two first public-key matrices have different sizes;
• receiving an indication of a selected first public-key matrix from the public-key encryption device;
• sending the selected first public-key matrix, when a request for the selected first public-key matrix is received, wherein the transmission key is decapsulated with the selected first-public key.

A computer program stored on a non-transitory medium, wherein the computer program when executed on a processor:

• establishes communication with a public-key encryption device;
• generates a first private-key matrix;
• computes a first public-key matrix from the first private-key matrix, and allow transfer of the first public-key matrix to the public-key encryption device;
• receives from the public-key encryption device an indication of a key size and/or matrix size;
• receives a second public-key matrix and a masked transmission key, wherein the second public-key matrix has fewer matrix elements than the first public-key matrix;
• computes a first raw shared key from the second public-key matrix and the first private-key matrix;
• computes the first raw shared key comprising a matrix multiplication between the second public-key matrix and the first private-key matrix;
• and
• unmasks the masked transmission key in dependence on the received size using at least part of the first raw shared key to obtain a transmission key;
• and
• performing a handshake with the private-key decryption device, wherein the handshaking comprises:
• receiving at least two indications of at least two first public-key matrices of the private-key decryption device from the private-key decryption device,
• wherein the two first public-key matrices have different sizes;
• selecting the larger of the first public-key matrix and the second public-key matrix;
• sending an indication of the selected first public-key matrix to the private-key decrypting device;
• and
• requesting the first public-key matrix when the first public-key matrix is not available in a key-cache, wherein the transmission key is encapsulated with the selected first-public key.

A computer program stored on a non-transitory medium, wherein the computer program when executed on a processor:

• establishes communication with a public-key encryption device;
• generates a first private-key matrix;
• computes a first public-key matrix from the first private-key matrix, and allow transfer of the first public-key matrix to the public-key encryption device;
• receives from the public-key encryption device an indication of a key size and/or matrix size;
• receives a second public-key matrix and a masked ansmission key, wherein the second public-key matrix has fewer matrix elements than the first public-key matrix;
• computes a first raw shared key from the second public-key matrix and the first private-key matrix;
• computes the first raw shared key comprising a matrix multiplication between the second public-key matrix and the first private-key matrix;
• and
• unmasks the masked transmission key in dependence on the received size using at least part of the first raw shared key to obtain a transmission key,
• wherein the processor circuit is arranged to perform a handshake with the public-key encryption device, the handshape comprising:
• sending to the public-key encryption device at least two indications of at least two first public-key matrices of the private-key decryption device, wherein the two first public-key matrices have different sizes;
• receiving an indication of a selected first public-key matrix from the public-key encryption device;
• sending the selected first public-key matrix, when a request for the selected first public-key matrix is received, wherein the transmission key is decapsulated with the selected first-public key.