[New-bugs-announce] [issue25529] Provide access to the validated certificate chain in ssl module

[Cory Benfield]

New submission from Cory Benfield:

I’m currently working on adding support for HPKP to the Requests and urllib3 modules. HPKP (HTTP Public Key Pinning), specified in RFC 7469, is an extension to HTTP that allows a web server to specify a whitelist of public keys that are valid for TLS certificates on that domain. This prevents a rogue certificate authority from issuing a certificate that would be trusted by a browser and would allow a man-in-the-middle attack on a domain (as happened to Google in 2013[0]).

Right now, the draft version of the support I have will only work when you use PyOpenSSL for your TLS needs, not the standard library. This is because to get HPKP to work I need access to the validated certificate chain: that is, the certificate chain that OpenSSL has built and validated for the TLS connection. I also need to be able to work with those certificates in order to extract their public keys. The standard library’s ssl module does not expose any of this functionality.

To get this to work with the standard library, I would require the following things from the standard library:

1. The ability to access the validated certificate chain. This requires saving off the certificate each time the OpenSSL verify callback is called. This is an easy enough change to make.
2. The ability to extract the public key from the saved certificates. This could be done by extending the logic used for getpeercert() to provide a DER-encoded ASN.1 representation of the public key in the dictionary, and then using that representation for each cert in the peer cert chain.

The motivation for making this available in the standard library would be pip. Right now python.org and all its subdomains (including pypi.python.org) are HPKP-enabled. Making this support available in the standard library would ensure that all pip installations are safe from man-in-the-middle attacks on its packaging infrastructure. Without it, a number of third-party packages would be required to add this security. In particular, pip could distribute a HPKP preload value for pypi.python.org, which would ensure that pip is truly invulnerable to MITM TLS attacks via malicious attackers coercing a CA to provide TLS certificates for *.python.org.

I’m happy to do the work required to provide this functionality, but I’d only like to start work if people believe there’s a likelihood of getting it merged.

[0]: https://nakedsecurity.sophos.com/2013/01/08/the-turktrust-ssl-certificate-fiasco-what-happened-and-what-happens-next/

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components: Library (Lib)
messages: 253864
nosy: Lukasa
priority: normal
severity: normal
status: open
title: Provide access to the validated certificate chain in ssl module
type: enhancement
versions: Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue25529>
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