Changes

== Certificate Verification in Firefox Before Version 31 Background ==

Gecko (and therefore Firefox ) relies on NSS to implement various cryptographic functions. NSS consists of a collection of loosely-coupled libraries. libssl, for example, is the TLS implementation. NSS is a Mozilla project, but its development differs significantly from the rest of the tree. In fact, it has its own tree that is periodically imported wholesale into mozilla-central. The component that uses the NSS libraries in Firefox is a layer called PSM ("Personal Security Manager" or "Privacy and Security Module").

To enable secure TLS connections to the best of our ability, PSM implements a certificate verification callback. It performs a number of checks, but ultimately it must determine if it trusts a certificate presented by a peer. In versions before 31The approach PSM takes is to repeatedly build a potential path to a trust anchor, validate the potential path, and either return that path if it does this by calling one of two certificate verification libraries in NSS: "classic" validates correctly or libpkixfind another potential path. The classic mozilla::pkix is a C++ library handles DV ("domain validation") certificates while libpkix handles EV ("extended validation") certificatesthat provides a framework to implement this approach.

=== "classic" verification =mozilla::pkix ==

mozilla::pkix was originally implemented as part of mozilla-central (i.e. gecko) but has since been moved into NSS. However, it is not part of NSS' stable C API. As a library, mozilla::pkix uses the notion of a "trust domain" provided by the application to build a trusted chain from an end-entity certificate to a root. The classic trust domain is responsible for saying what trust level a certificate verification algorithm performs has, finding potential issuers of a certificate, and checking the revocation for a certificate. A certificate can be a trust anchor, it can inherit its trust, or it can be actively distrusted. Given an end-entity certificate and a trust domain, the library will perform issuer-independent checks on the given that certificate(e.g. expiration, appropriate key usages), finds get a list of potential issuer, verifies that the signature matchesissuers, and recursesperform a depth-first traversal. If multiple issuers are foundit encounters a distrusted certificate, it attempts to use the "best" oneabandons searching that path. HoweverIf it finds a trust anchor, it queries the trust domain again to see if that path is acceptable (this is a heuristic, and as it does where gecko implements checks that are specific to the platform at not perform backtracking, it can fail to verify the abstract problem of building a valid trusted certificatechain). This is spectacularly apparent in If so, the case of keyend-pinning if the algorithm chooses to not traverse a entity certificate path that contains a necessary keyhas successfully been verified.

Because this library Unlike the other NSS libraries, mozilla::pkix is written in C ++ and because NSS makes strong guarantees about API backwards-compatibility, it would require significant work to fix. This would be on par with writing a new verification librarycan take advantage of more modern language features.

=== libpkix ===libpkix was auto-translated from Java to C. It attempts to implement Java's exception semantics The platform looks for trust anchors in Ca few locations. It makes liberal use First, Mozilla ships a list of unclear macros trust anchors with the platform corresponding to the root Certificate Authorities (eCAs) in the Mozilla Root CA Program.gAdditionally, the user may import their own trust anchors. https://mxrThese are stored in the profile's cert9.mozilladb file.org/mozillaThe user may also import third-central/source/security/nss/lib/libpkix/pkix/util/pkix_tools.hparty PKCS#67 )11 modules that provide trust anchors. A source-line-counting tool clocks it in at 45The enterprise roots feature, if enabled,000 lines of code (may collect trust anchors provided by the code is here: https://mxr.mozilla.org/mozilla-central/source/security/nss/lib/libpkix ). There are known bugs in the implementation. No one who works on it wants to continue working on itoperating system.

== Certificate Verification in Firefox As part of Version 31 ==At the time of the decision to work on mozilla::pkixMozilla's Root CA Program, we had there is a number list of options:# We could have fixed the classic verification implementation. As already stated, this would have required considerable work. # We could have fixed and maintained libpkix ourselvesroot certificates that are trusted to issue Extended Validation (EV) certificates. This was undesirable for the aforementioned reasons. Furthermore, as Google moved away from NSS, we would have had less and less help working on this library. # We could have used whatever verification implementation Google develops. We would have had to wait a year or more for this, and we would be depending on Google to share their work. # We could have used OpenSSL's certificate verification routine. Apparently it list is buggy as wellavailable in code form at https://hg.# We could have started over from scratch and written another entirely new verification librarymozilla. This would have set us back a yearorg/mozilla-central/annotate/80374044414da9f5b3634c91345d07612754fcda/security/certverifier/ExtendedValidation.cpp# Finally, we could have used the new verification library known as "mozilla::pkix" (formerly known as "insanity::pkix"). This is what we decided to do.l90

==Other Verification Routines in NSS = mozilla::pkix Design ===As a library, mozilla::pkix uses the notion of a "trust domain" provided by the application to build a trusted chain from an end-entity certificate to a root. The trust domain is responsible for saying what trust level a certificate has, finding potential issuers of a certificate, and checking the revocation for a certificate. A certificate can be a trust anchor, it can inherit its trust, or it can be actively distrusted. Given an end-entity certificate and a trust domain, the library will perform issuer-independent checks on that certificate (e.g. expiration, appropriate key usages), get a list of potential issuers, and perform a depth-first traversal. If it encounters a distrusted certificate, it abandons searching that path. If it finds a trust anchor, it queries the trust domain again to see if that path is acceptable (here is where we check key pinning). If so, the end-entity certificate has successfully been verified.

Unlike the NSS libraries, exposes other certificate verification functions that are not yet implemented using mozilla::pkix is written in C++. As a result, we can use scoped data types that automatically clean up after themselves rather than having to manually manage memory. This reduces memory-safety bugs as well as error-handling bugs.

=== Progress "classic" verification ===We have been working on this project for a long time. Last year, progress was slower than everyone would have liked. However, starting in late January, development picked up considerably to the point where we had landed a working implementation (albeit with no OCSP checking) within a month. Since then, we have landed OCSP checking and test improvements, as well as an OCSP cache. In fact, we managed to have a sufficiently complete and interoperable implementation for it to be released in Firefox 31 on July 22nd, 2014.The library code is here: https://mxr.mozilla.org/mozilla-central/source/security/pkix/ and the trust domain is here: https://mxr.mozilla.org/mozilla-central/source/security/certverifier/

=== Tests ===Due to The so-called "classic" certificate verification algorithm performs issuer-independent checks on the sensitive nature of this codegiven certificate, finds a potential issuer, we want to ensure proper testing. To verifies that endthe signature matches, we first made sure the new implementation passed the same tests as the current implementationand recurses. We then added more testsIf multiple issuers are found, finding some bugs in both implementations in it attempts to use the process"best" one. At However, this pointis a heuristic, while we will add still more testsand as it does not perform backtracking, we believe it would be beneficial for the community at large can fail to inspect verify a valid certificate. This is spectacularly apparent in the design and implementation case of key-pinning if the code. Note that this stems algorithm chooses to not from traverse a lack of confidence in code quality but rather the understanding certificate path that the privacy of our users depends on the correctness of this codecontains a necessary key.

Matt Wobensmith just completed compatibility-testing of 200k HTTPS sites and found 16 with issuesThe code is here: https://hg. These were investigated and resolved to our satisfactionmozilla.org/projects/nss/file/tip/lib/certhigh

=== Implementation status libpkix === Released libpkix was auto-translated from Java to C. It attempts to implement Java's exception semantics in FF 31C. Remaining bugs:# Enforce consistent handling It makes liberal use of isCA bit and certSign/crlSign key usages: {{bug|970196}} unclear macros (e.g. https:briansmith)# Enable all PSM xpcshell tests on Android/B2G: {{bug|676972}} (:keeler)# Add SHA-2 support to the OCSP implementation: {{bug|966856}} (:keeler)# Document functions exported from the library: {{bug|968451}} (:briansmith) For more details, see the dependency trees for {{bug|915930}} and {{bug|976961}}, respectively/hg. == Choosing your Verification Library == mozilla::pkix has been enabled by default in Firefox 31. The boolean pref "security.use_mozillapkix_verification" controls this. There is an additional hidden boolean pref "security.use_libpkix_verification" that can be used to enable org/projects/nss/annotate/5d9f8b809e6f7020529ba1345b64e36f61994c8d/lib/libpkix instead. If both of these prefs are false, classic verification is used. mozilla::/pkix takes precedence over libpkix/util/pkix_tools. See the following table: {|border="1" cellpadding="5" cellspacing="0" align="center"! colspan="2" rowspan=2|! colspan="2"|security.use_libpkix_verification|----| false (defaulth#l67 )| true|----|rowspan=2 | '''security.use_mozillapkix_verification'''| false| Classic| libpkix|A source-line---|true counting tool clocks it in at 45,000 lines of code (default)| mozillathe code is here:https:pkix| //hg.mozilla::pkix|----|} As of Firefox 33, mozilla::pkix is .org/projects/nss/file/tip/lib/libpkix ). There are known bugs in the only verification library availableimplementation. No one who works on it wants to continue working on it.