Fair cryptosystems and methods of use

A method, using a cryptosystem, for enabling a predetermined entity to monitor communications of users suspected of unlawful activities while protecting the privacy of law-abiding users, comprising the steps of:

• providing trustees pieces of information that are guaranteed to include shares of at least a secret decryption key;
• and
• upon a predetermined request, having the trustees reveal the shares of the secret decryption key to enable the entity to attempt to monitor communications to the suspected user during a time period specified in the predetermined request.

A method, using a cryptosystem, for enabling a predetermined entity to monitor communications of users suspected of unlawful activities while protecting the privacy of law-abiding users, comprising the steps of:

• having trustees hold pieces of information, wherein a piece of information is guaranteed to include a share of secret decryption key;
• and
• upon a predetermined request, having a given number of trustees each reveal the piece of information that includes the share of at least one secret decryption key to enable the entity to monitor communications to the suspected user.

A method, using a cryptosystem, for enabling a predetermined entity to monitor communications of users suspected of unlawful activities while protecting the privacy of law-abiding users, wherein one user has at least a secret decryption key, comprising the steps of:

• having trustees hold pieces of information that are guaranteed to include shares of a secret decryption key;
• and
• upon a predetermined request, having a given number of trustees each reveal the piece of information that includes the share of the secret decryption key to enable the entity to attempt to monitor communications to the user suspected of unlawful activities.

A method for revealing a user's secret value, comprising the steps of:

• having trustees hold pieces of information, wherein a piece of information includes a share of secret value;
• and
• upon a predetermined request, having a given number of trustees each reveal the piece of information that includes the share of the secret value to enable the entity to reconstruct the secret value at a prescribed time specified in the predetermined request.

A method, using a cryptosystem, for enabling a predetermined entity to verify that a user is sending messages encrypted by means of a secure device implementing the given cryptosystem, wherein the secure device contains a secret key known to the entity, comprising the steps of:

• having the secure device use the given cryptosystem to generate a first string, the first string being an encryption of a message;
• having the secure device use the secret key to generate a second string that guarantees to the entity that the first string was generated with the given cryptosystem.