The proposed reputation algorithm reduces significantly the vulnerability to the Sybil attack described above because most of the score of the reputation algorithm is preceded by confidence in the degrees of friendship that binds each user to another user. Thus, if a user does not know (at all) another user, very high ratings of the latter in the system are not enough for guaranteeing reliability for the first user because, in the proposed system, the weight of the average rating is 37.5% of the total trust rate, while the weight of the degree of friendship is 62.5%.