Digital Signature System

A client device comprising:

• a processor;
• and
• a memory to store data used by the processor, wherein the processor is operative to:
• receive a security field SN from an authority, the security field SN being signed and/or encrypted by the authority, the security field SN resulting from successively applying a one-way function to a seed S0 N times yielding a chain having a plurality of values Si including SN, i being an integer from 1 to N inclusive;
• decrypt the seed SN if the seed SN is received in an encrypted format from the authority;
• receive up to N messages and up to N digital signatures from a message signing system, each one of the digital signatures signing a different one of the messages;
• and
• authenticate a first one of the messages against a first one of the digital signatures signing the first message, the authentication including:
• evaluating a decryption function with the first digital signature and a MAC of the first message as input to the decryption function yielding a first result, the MAC being a keyed hash function;
• applying the one-way function to the first result, or successively applying the one-way function a number of times to the first result, yielding a first value;
• and
• checking if the first value is equal to the security field SN.

A message signing method comprising:

• receiving a seed S0 and a number N from an authority providing permission to digitally sign up to N messages for a client device, the seed S0 being received from the authority in an encrypted format;
• decrypting the seed S0;
• successively applying a one-way function to the seed S0 yielding a chain having a plurality of values Si, i being greater than zero such that the value Si is a result of successively applying the one-way function to the seed S0 i times;
• creating up to N digital signatures, creation of each of the digital signatures including evaluating an encryption function with one of the values Si and a MAC of one of the messages as input to the encryption function, the MAC being a keyed hash function, each of the N digital signatures being based on a different one of the values Si and a different one of the messages;
• and
• sending the created digital signatures and the messages signed by the created digital signatures to the client device.

A method comprising:

• receiving a security field SN from an authority, the security field SN being signed and/or encrypted by the authority, the security field SN resulting from successively applying a one-way function to a seed S0 N times yielding a chain having a plurality of values Si including SN, i being an integer from 1 to N;
• decrypting the seed SN if the seed SN is received in an encrypted format from the authority;
• receiving up to N messages and up to N digital signatures from a message signing system, each one of the digital signatures signing a different one of the messages;
• authenticating a first one of the messages against a first one of the digital signatures signing the first message, the authenticating including:
• evaluating a decryption function with the first digital signature and a MAC of the first message as input to the decryption function yielding a first result, the MAC being a keyed hash function;
• applying the one-way function to the first result, or successively apply the one-way function a number of times to the first result, yielding a first value;
• and
• checking if the first value is equal to the security field SN.