Verifiable Builds Comparsion Table
|Whonix ™||Tails||Tor Browser||Qubes OS TorVM||corridor|
|Deterministic builds ||No||No (planned) ||Yes ||No||Not applicable |
|Based on a deterministically built  operating system||No ||No ||Not applicable||No ||No |
|Verifiably no backdoor in the project's own source code||Invalid ||Invalid ||Invalid ||Invalid ||Invalid |
|Verifiably vulnerability-free [archive]||No ||No ||No ||No ||No |
|Verifiably no hidden source code  in upstream distribution / binaries ||No ||No ||No ||No ||No |
|Project's binary builds are verifiably created from project's own source code (no hidden source code  in the project's own source code)||No (deprecated) ||No||Yes||No||Not applicable |
Open Source software does not automatically prevent backdoors [archive], unless the user creates their own binaries directly from the source code. People who compile, upload and distribute binaries (including the webhost) could add hidden code, without publishing the backdoor. Anybody can claim that a certain binary was built cleanly from source code, when it was in fact built using the source code with a hidden component. Those deciding to infect the build machine with a backdoor are in a privileged position; the distributor is unlikely to become aware of the subterfuge.
Deterministic builds can help to detect backdoors, since it can reproduce identical binary packages (byte-for-byte) from a given source. For more information on deterministic builds and why this is important, see:
- liberationtech mailing list: Deterministic builds and software trust [archive].
- gitian.org [archive]
- As Mike Perry has observed: Deterministic Builds Part One: Cyberwar and Global Compromise [archive]. See:
- The Debian wiki tracking progress / development efforts to implement Reproducible Builds for all packages [archive].
- See Tails Roadmap [archive].
- See Deterministic Builds Part One: Cyberwar and Global Compromise [archive] and Deterministic Builds Part Two: Technical Details [archive].
- corridor only uses shell scripts.
- To be fair, there are no deterministically built operating systems yet. It is a difficult process and takes a lot of effort to complete. While Debian has around 22,000 reproducible packages [archive] in mid-2018, this work has been ongoing since 2013 and is far from done.
The first form of backdoor [archive] is a vulnerability [archive] (bug) in the source code. Vulnerabilities are introduced either purposefully or accidentally due to human error. Following software deployment, an attacker may discover the vulnerability and use an exploit [archive] to gain unauthorized access. Such vulnerabilities can be cleverly planted in plain sight [archive] in open source code, while being very difficult to spot by code auditors. Examples of this type of backdoor include:
- An attempt to backdoor the kernel [archive].
- The Debian SSL debacle [archive]; many argued that this wasn't a bug but in fact a backdoor, as it hadn't been spotted for several years.
It is therefore impossible to claim that non-trivial source code is backdoor-free, because backdoors can be hidden as vulnerabilities. Auditors scrutinizing the source code can only state an opinion about the quality of the source code, and eventually report vulnerabilities if/when they are identified. Assertions that source code is free of computer viruses (like trojan horses) is the only reasonable assertion that can be made.
- Although theoretically possible, there are no mathematically proven bug-free [archive] operating systems yet.
- The upstream distribution is the distribution on which the project is based. Whonix and Tails are based on Debian, thus Debian is their upstream distribution. QubesOS TorVM is based on Qubes OS, which is itself based on Fedora and Xen.
- See verifiable builds.