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Proxy re-encryption (PRE) allows a semi-trust proxy with a re-encryption key to transform a ciphertext encrypted under one key to an encryption of the same message under another key. Attribute-based proxy re-encryption (AB-PRE) is a generalization of PRE which enables fine-grained access control and delegation of encrypted data. However, traditional AB-PRE suffers from the single point of failure as it relies on a single proxy to perform ciphertext transformations. To resolve this problem, this paper introduces a new primitive called attribute-based threshold proxy re-encryption (AB-TPRE), that utilizes multiple (N) proxies for the transformations. In AB-TPRE, the re-encryption key is split into N shares, with each proxy receiving one share to generate a transformed ciphertext share. Only when a threshold (t) number of transformed ciphertext shares are combined can the transformed ciphertext be correctly generated …