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Subject: RE: [office] The problem of visible hashes for protection keys
Right. We use unsalted SHA1 hashes (but not in the encryption methods) in our password verifiers for sheet protection, write protection, etc. Since folks want to *keep* using their memorable passwords and have been frightened about stories concerning hash collisions, we agreed to recommend SHA256 instead of SHA1. That pandering has it appear that we have accomplished something concerning the confidentiality of those passwords. (Elsewhere, I equate this to security theater.) - Dennis PS: The more-complicated worries about the ODF 1.x encryption method is that, along with its complexity (always a warning sign) is the fact that information about the plaintext is leaked in a way that can reveal high-value documents and also exposes known-plaintext attacks on those document, especially considering that the document is durable and an attacker may have a significant amount of time to carry out an attack. This is a speculative risk, but I assume prudence dictates finding a method that does not have such fragility as a concern. -----Original Message----- From: David LeBlanc [mailto:dleblanc@exchange.microsoft.com] Sent: Saturday, October 30, 2010 12:15 To: dennis.hamilton@acm.org; ODF TC List Subject: RE: [office] The problem of visible hashes for protection keys [ ... ] Any time you want to store a password verifier, for example to be used with sheet protection, write protection, etc, PBKDF2 should be used with a good hash, a large iteration count, and a large, random salt. Not using salt is what leads to the attacks listed in the URL below. ________________________________________ From: Dennis E. Hamilton [dennis.hamilton@acm.org] Sent: Saturday, October 30, 2010 11:48 AM To: ODF TC List Subject: [office] The problem of visible hashes for protection keys We know that, using XML documents, it is easy to subvert a lock on text or spreadsheet documents that involves locking material against changes (but not reading). When the lock is itself locked against removal using a password hash, this exposes the password used for the hash to compromise. The problem is the desire by folks to reuse familiar, memorable passwords for this purpose when the memorable password is also used to protect something that is of serious high value. (That is to say, the password is more valuable than the trivial-to-break lock in the document.) Some bad news: < http://www.ciozone.com/index.php/Security/Cracking-14-Character-Complex-Pass words-in-5-Seconds.html> Note that memorable passwords are not the hard ones to crack, and increasing the strength of the hash will not do much to protect memorable passwords from discovery against this kind of computational power. Note: This attack does not work against the PBKDF2 methods used for ODF 1.2 encryption, because the start password is never revealed. On the other hand, the techniques by which the password-hash attack were accelerated so much is probably a reason for concern that the various vulnerabilities of the ODF 1.2 encryption will be too-soon exploitable as a practical matter. My personal assumption is that no well-informed government body or commercial entity that is concerned about document confidentiality will allow use of the ODF 1.x encryption and would require very strong whole-package encryption techniques, whether defined for ODF documents or not. In that regard, the ETSI draft that we have been asked to comment on has moved ahead of us in the level of confidentiality-by-encryption that it attains for Zip-packaged documents. - Dennis Dennis E. Hamilton ------------------ NuovoDoc: Design for Document System Interoperability mailto:Dennis.Hamilton@acm.org | gsm:+1-206.779.9430 http://NuovoDoc.com http://ODMA.info/dev/ http://nfoWorks.org --------------------------------------------------------------------- To unsubscribe from this mail list, you must leave the OASIS TC that generates this mail. Follow this link to all your TCs in OASIS at: https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php --------------------------------------------------------------------- To unsubscribe from this mail list, you must leave the OASIS TC that generates this mail. Follow this link to all your TCs in OASIS at: https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php
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