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Convergent encryption

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This is an old revision of this page, as edited by Unique Nitrogen (talk | contribs) at 16:17, 1 February 2013 (Mega has responded they don't: https://mega.co.nz/#blog_3 mega-blog Jan 22nd 2013, deduplication is only applied to same content encrypted using same key, therefore they can't distinguish the same file from two different users, so no convergent encryption). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.
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Convergent encryption, also known as content hash keying, is a cryptosystem that produces identical ciphertext from identical plaintext files. This has applications in cloud computing to remove duplicate files from storage without the provider having access to the encryption keys [1]. The system was first mentioned by John Pettitt on the cypherpunk's mailing list in 1996 [2] and has been used by Farsite[3], Freenet, MojoNation, GNUnet, flud, and the Tahoe Least-Authority Filesystem [4].

The system gained additional visibility in 2011 when cloud storage provider Bitcasa announced they were using convergent encryption [5].

Overview

  1. The plaintext is hashed using a cryptographic hash
  2. The hash is then used to encrypt the plaintext
  3. The ciphertext is then uploaded to the cloud provider

Known Attacks

Convergent encryption is open to a "confirmation of a file attack" since if the attacker knows the plaintext of the file, they can then check if a user has this file. This attack may only be a problem for a user storing information that is also publicly available - e.g. banned books or files that cause copyright infringement.

There is also a "learn the remaining information attack" described by Drew Perttula in 2008 [6].

References

  1. ^ Secure Data Deduplication, Mark W. Storer Kevin Greenan Darrell D. E. Long Ethan L. Miller http://www.ssrc.ucsc.edu/Papers/storer-storagess08.pdf
  2. ^ "Re: Hash of plaintext as key?", Cypherpunks Mailing List, http://web.archive.org/web/20061103171849/http://cypherpunks.venona.com/date/1996/02/msg02013.html
  3. ^ Reclaiming Space from Duplicate Files in a Serverless Distributed File System, MSR-TR-2002-30, http://research.microsoft.com/apps/pubs/default.aspx?id=69954
  4. ^ Drew Perttula and Attacks on Convergent Encryption https://tahoe-lafs.org/hacktahoelafs/drew_perttula.html
  5. ^ Finally! Bitcasa CEO Explains How The Encryption Works, September 18th, 2011, http://techcrunch.com/2011/09/18/bitcasa-explains-encryption/
  6. ^ https://tahoe-lafs.org/hacktahoelafs/drew_perttula.html
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