Block cryptographic method for encrypting/decrypting messages and cryptographic devices for implementing this method

A block decipherment method using round repetition for providing, from a ciphertext message, a plaintext message and a global tag for authenticating said plaintext message, said ciphertext message being converted into ordered ciphertext blocks having a same bit length, said ciphertext blocks being successively processed in an orderly manner as a current ciphertext block during a round, said method comprising the following actions:

• loading a first data into a register;
• running said round by performing the following steps:
• (1) inputting input data into a block decipherment algorithm, said input data comprising data of said register and a reproducible data;
• (2) for computing, within said block decipherment algorithm, a first output by decrypting the input data using a single cryptographic key;
• said first output comprising a first segment and a second segment, the latter being defined as an authentication local tag;
• (3) computing a plaintext block by performing a first operation using, as operands, said first segment and said current ciphertext block;
• as long as all said ciphertext blocks have not been processed in a round, reloading the register with said current ciphertext block and applying an updating function to said reproducible data before running a new round by starting at step (1);
• determining said plaintext message by concatenating the plaintext blocks of all of the rounds in said orderly manner;
• reloading the register with the current ciphertext block and applying said updating function to said reproducible data before performing only steps (1) and (2) of said round;
• and
• computing said global tag by a second operation using all the computed authentication local tags as operands;
• wherein step (3) of said round further comprises said second segment as an operand.

A cryptographic device for implementing a block cipher method using round repetition for providing, from a plaintext message, a ciphertext message and a global tag for authenticating said ciphertext message, comprising:

• a register configured to receive a first data;
• an updater configured to update a reproducible data according to an updating function;
• an interface configured to receive at least said plaintext message;
• a converting unit configured to convert and store said plaintext message into ordered plaintext blocks having a same bit length;
• a processing unit configured to successively process said plaintext blocks in an orderly manner as a current plaintext block during a round, said processing unit further comprising a ciphering unit, hosting a block cipher algorithm, and a memory storing a single cryptographic key;
• the processing unit being configured to perform the following steps during each round:
• (1) input data into the block cipher algorithm, said input data comprising data of said register and a reproducible data;
• (2) compute, within said block cipher algorithm, a cryptogram by encrypting the input data using said single cryptographic key, said cryptogram comprising a first segment and a second segment, the latter being defined as an authentication local tag and being stored in said memory;
• (3) compute and store in said memory a ciphertext block by performing a first operation using, as operands, said first segment and said current plaintext block;
• said processing unit being further configured to reload the register with said ciphertext block and to update the reproducible data before running a new round starting with (1), as long as all said plaintext blocks have not been processed in a round;
• said converting unit being further configured to determine said ciphertext message by concatenating the ciphertext blocks of all the rounds in said orderly manner, and
• said processing unit being further configured to reload the register with the last computed ciphertext block and to apply the updating function to said reproducible data in order to update it before performing only steps (1) and (2) of said round, and then computing said global tag by a second operation using, from said memory, all the computed authentication local tags as operands;
• and
• said interface being further configured to output said ciphertext message and said global tag;
• wherein said processing unit is further configured to also take said second segment as an operand in step (3) of said round.

A cryptographic device for implementing a block decipherment method using round repetition for providing, from a ciphertext message, a plaintext message and a global tag for authenticating said plaintext message, comprising:

• a register configured to receive a first data;
• an updater configured to update a reproducible data according to an updating function;
• an interface configured to receive at least said ciphertext message;
• a converting unit configured to connect and store said ciphertext message into a ordered ciphertext blocks having a same bit length;
• a processing unit configured to successively process said ciphertext blocks in an orderly manner as a current ciphertext block during a round, said processing unit further comprising a deciphering unit, hosting a block decipherment algorithm, and a memory comprising a single cryptographic key;
• the processing unit being configured to perform the following steps during each round:
• (1) input data into the block decipherment algorithm, said input data comprising data of said register and a reproducible data;
• (2) compute, within said block cipherment algorithm, a first output by processing the input data using said single cryptographic key, said first output comprising a first segment and a second segment, the latter being defined as an authentication local tag and being stored in said memory;
• (3) compute and store in said memory a plaintext block by performing a first operation using, as operands, said first segment and said current ciphertext block.
• said processing unit being further configured to reload the register with said current ciphertext block and to update the reproducible data before running a new round starting with step (1), as long as all said ciphertext blocks have not been processed in a round;
• said converting unit being further configured to determine said plaintext message by concatenating the plaintext blocks of all the rounds in said orderly manner;
• and
• said processing unit being further configured to reload the register with the current ciphertext block and to apply the updating function to said reproducible data in order to update it before performing only steps (1) and (2) of said round, and then computing said global tag by a second operation using, from said memory, all the computed authentication local tags as operands;
• and
• said interface being further configured to output said plaintext message;
• wherein said processing unit is further configured to also take said second segment as an operand in step (3) of said round.