Paper 2023/534

Group Oblivious Message Retrieval

Zeyu Liu, Yale University
Eran Tromer, Columbia University
Yunhao Wang, Columbia University

Anonymous message delivery, as in private communication and privacy-preserving blockchain applications, ought to protect recipient metadata: a message should not be inadvertently linkable to its destination. But how can messages then be delivered to each recipient, without each recipient scanning all messages? Recent work constructed Oblivious Message Retrieval (OMR) protocols that outsource this job to untrusted servers in a privacy-preserving manner. We consider the case of group messaging, where each message may have multiple recipients (e.g., in a group chat or blockchain transaction). Direct use of prior OMR protocols in the group setting increases the servers' work linearly in the group size, rendering it prohibitively costly for large groups. We thus devise new protocols where the servers' cost grows very slowly with the group size, while recipients' cost is low and independent of the group size. Our approach uses Fully Homomorphic Encryption and other lattice-based techniques, building on and improving on prior work. The efficient handling of groups is attained by encoding multiple recipient-specific clues into a single polynomial or multilinear function that can be efficiently evaluated under FHE, and via preprocessing and amortization techniques. We formally study several variants of Group Oblivious Message Retrieval (GOMR) and describe corresponding GOMR protocols. Our implementation and benchmarks show, for parameters of interest, cost reductions of orders of magnitude compared to prior schemes. For example, the servers' cost is ~3.36 per million messages scanned, where each message may address up to 15 recipients.

Note: Minor editorial improvements, and added link to open-source implementation.

Available format(s)
Cryptographic protocols
Publication info
Anonymous message deliveryFully homomorphic encryption
Contact author(s)
zeyu liu @ yale edu
eprint2eran @ tromer org
yw3736 @ columbia edu
2023-05-22: revised
2023-04-13: received
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Creative Commons Attribution


      author = {Zeyu Liu and Eran Tromer and Yunhao Wang},
      title = {Group Oblivious Message Retrieval},
      howpublished = {Cryptology ePrint Archive, Paper 2023/534},
      year = {2023},
      note = {\url{}},
      url = {}
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