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Bug 1229472

Summary: RFC: linux-pam hardcodes sha512 salt length to 8 bytes
Product: Red Hat Enterprise Linux 7 Reporter: Paulo Andrade <pandrade>
Component: pamAssignee: Tomas Mraz <tmraz>
Status: CLOSED WONTFIX QA Contact: BaseOS QE Security Team <qe-baseos-security>
Severity: low Docs Contact:
Priority: low    
Version: 7.0CC: cww, pdwyer, pkis
Target Milestone: rcKeywords: FutureFeature
Target Release: ---   
Hardware: All   
OS: All   
Whiteboard:
Fixed In Version: Doc Type: Enhancement
Doc Text:
Cause: Consequence: Fix: Result:
 Story Points: ---
Clone Of: Environment:
Last Closed: 2016-01-19 14:56:02 UTC Type: Bug
Regression: --- Mount Type: ---
Documentation: --- CRM:
Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Embargoed:
Bug Depends On:    
Bug Blocks: 1203710, 1229474    

Description Paulo Andrade 2015-06-08 21:16:50 UTC 
Looking at linux-pam git, it appears the salt is
always hardcoded at 8 bytes. This commit was the
last change in the related code:

https://git.fedorahosted.org/cgit/linux-pam.git/commit/modules/pam_unix/passverify.c?id=459e97431e99fa2c32e30e957993f95794b98dd0

---8<---
+	crypt_make_salt(sp, 8);
+	/* For now be conservative so the resulting hashes
+	 * are not too long. 8 bytes of salt prevents dictionary
+	 * attacks well enough. */
+	sp = crypt(password, salt);
---8<---

The salt can be up to 16 bytes.

Since the code was last touched in 2008, and was mostly
written before it, is it still a good enough value?

As a note, anaconda, in a clean install, creates
entries in /etc/shadow with 16 byte salt, but if one
changes the password, it becomes a 8 byte salt.
Comment 2 Tomas Mraz 2015-06-09 08:23:17 UTC 
8 character salt provides the search space of 2^48 * number-of-precomputed-passwords. That's 281 TB * length of password hash per single password. That is still completely impractical value for creation of rainbow tables.

So there is no hurry to change this.

However I'll report it upstream and we'll probably change it to a higher value in the next upstream release.
Comment 3 Paulo Andrade 2015-06-09 13:01:50 UTC 
(In reply to Tomas Mraz from comment #2)
> 8 character salt provides the search space of 2^48 *
> number-of-precomputed-passwords. That's 281 TB * length of password hash per
> single password. That is still completely impractical value for creation of
> rainbow tables.
> 
> So there is no hurry to change this.
> 
> However I'll report it upstream and we'll probably change it to a higher
> value in the next upstream release.

Could it be made configurable?
The user would like to be able to change the length.
It could be a pam_unix option, to choose from 8 to
16. Or a number larger than 8 if the default minimum
becomes larger.
Comment 4 Tomas Mraz 2015-06-09 13:08:13 UTC 
I do not think it is worth adding an option for this. We would just raise the length if the current one is decided to be too short for foreseeable future.
Comment 8 RHEL Program Management 2016-01-19 14:56:02 UTC 
Development Management has reviewed and declined this request.
You may appeal this decision by reopening this request.