An ABE scheme

[tilenmarc]

ABE schemes provide a functionality that an entity can decrypt a ciphertext if and only if he or she possesses sufficient set of attributes. This PR implements a key policy (KP) ABE scheme presented in this paper. It is based on a monotone span programs (MSP) providing a policy for the decryption. It includes the following functionalities:

• encryption of a message with only specifying which set of attributes will be important for decryption policy
• a policy convertor that transforms a boolean formula expressing the policy which attributes are needed for the decryption into an equivalent MSP structure
• key derivation that given given a MSP structure creates a set of keys corresponding to attributes
• key delegation that delegates the above keys to entities with some attributes
• decryption that based on given keys decrypts or denies the user

As a part of the decryption process a helping function solving a matrix vector equation Ax = v is implemented using Gaussian elimination.

[tilenmarc]

Thanks for the review Jan. I included all the suggestions.

[tilenmarc]

Thanks for a careful review. Now all the points should be included if not commented.

[tilenmarc]

I included the suggestions and I run goimports which also improved a bit the code from the last pull request. Thanks.

[janhartman]

Excellent work, thank you!