How to Back Up High-Value Secret Keys

Sanjam Garg; Noemi Glaeser; Abhishek Jain; Michael Lodder; Hart Montgomery

Paper 2026/1565

How to Back Up High-Value Secret Keys

Sanjam Garg

, University of California, Berkeley

Noemi Glaeser

, TU Darmstadt

Abhishek Jain

, Johns Hopkins University, NTT Research

Michael Lodder

, Tectonic Labs

Hart Montgomery

, Linux Foundation

Abstract

Consider a cryptocurrency exchange that secures the bulk of its reserves under a small set of keys, each of which is only used to transfer cryptocurrency once a year; or the backup codes for an account login or a password manager, which are again rarely used but provide access to crucial systems or information. Securing such infrequently-used high-value secrets is crucial, but existing solutions, such as threshold wallets and 'cold' (offline) wallets, are unsatisfactory. In this work, we envision a system that allows users to conveniently back up their rarely-used, high-value keys. This new setting necessitates a novel set of design requirements. Specifically: - We allow user keys to be threshold secret-shared among a large number of custodians where each custodian wallet comprises of a hot (i.e., online) and a cold (i.e., offline) portion. The cold part of the wallet is not touched during the backup process (thus, it is independent of the number of system users) but must be accessed for recovery. - We provide a mechanism to continually assure users that their keys are safely stored. This feature is critical because our system is not designed for frequent key use. We also enable proactive key refresh. - Finally, in our approach, restoring a backed-up key is equivalent to generating a signature. Thus, signatures made by users of this system should look the same as "normal" signatures to avoid exposing holders of high-value keys to targeted attacks. Based on these requirements, we develop new security definitions and a UC-secure protocol that implements threshold BLS signatures in our new model. Our protocol is practically efficient for the envisioned large numbers of custodians: for a 67-out-of-100 threshold configuration, creating a new backup takes 10s, while recovery takes less than 2ms.

Note: Full version

Metadata

Available format(s): PDF
Category: Public-key cryptography
Publication info: Published elsewhere. Major revision. ACM CCS 2026
DOI: 10.1145/3830454.3832658
Keywords: Backups Threshold signature BLS signature secret sharing
Contact author(s): sanjamg @ berkeley edu
noemi glaeser @ tu-darmstadt de
abhishek @ cs jhu edu
redmike7 @ gmail com
hart montgomery @ gmail com
History: 2026-08-03: approved; 2026-07-30: received; See all versions
Short URL: https://ia.cr/2026/1565
License: CC BY

BibTeX

@misc{cryptoeprint:2026/1565,
      author = {Sanjam Garg and Noemi Glaeser and Abhishek Jain and Michael Lodder and Hart Montgomery},
      title = {How to Back Up High-Value Secret Keys},
      howpublished = {Cryptology {ePrint} Archive, Paper 2026/1565},
      year = {2026},
      doi = {10.1145/3830454.3832658},
      url = {https://eprint.iacr.org/2026/1565}
}