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OCSP blog posts, some minor spell fixes.

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Dhananjay Balan 2019-08-07 17:35:53 +02:00
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21
blog/2019-08-06-trouble-with-ocsp.markdown
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@ -5,14 +5,13 @@ date: 2019-08-06
comments: true
tags: ocsp, tor, security, privacy
---
# Trouble with OCSP
> This is a post about side channel information leakage that was present in OnionBrowser in some OCSP flows. This is not a exhaustive documentation of OCSP protocol.
> This is a post about side channel information leakage that was present in OnionBrowser in some OCSP flows. This post omits a lot of details about OCSP protocol.
Digital certificates are issued for a longer timespan. It reduces maintaince overhead. Most of the cases CAs issue a certificate and they are valid untill the expiration time.
Digital certificates are issued for a longer timespan. It reduces maintenance overhead. Most of the cases CAs issue a certificate and they are valid until the expiration time.
But what are we to do when a certificate is compromised? We can
re-issue a certificate, but the old one is still in the wind. Thats
re-issue a certificate, but the old one is still in the wind. That's
where Online Certificate Status Protocol comes in. It defines a way to
check validity of a certificate in a timely[^1] manner.
@ -30,7 +29,7 @@ the certificate itself, removing the need for another seperate
request.
## Dissecting an OCSP Request
If the request is <255 bytes, OCSP allows it to be passed as a GET path. So a typical request looks like this
If the request is <255 bytes, OCSP allows it to be passed as a GET path. A typical request looks like this
```bash
GET http://ocsp.int-x3.letsencrypt.org/MFgwVqADAgEAME8wTTBLMAkGBSsOAwIaBQAEFH7maudymrP8%2BKIgZGwWoS1gcQhdBBSoSmpjBH3duubRObemRWXv86jsoQISA6D%2BPqgUVCy3wtolHIxq%2Bk0e
@ -77,7 +76,7 @@ curl https://crt.sh/?serial=03a0fe3ea814542cb7c2da251c8c6afa4d1e
## Privacy Takes a Backseat.
Careful examination of above workflow will reveal that the OCSP flow
is happening over HTTP. Most issuers seems to stick to http; possibly
is happening over HTTP. Most issuers seem to stick to http; possibly
to avoid cyclical dependencies. This means man-in-the-middle leakage
of certificates a user is validating is happening, and by extension
leakage of websites user is accessing.
@ -97,11 +96,11 @@ Whenever Onion Browser accesses a website with EV cert, (for e.g
I stumbled upon this accidently while inspecting requests from my
iPhone with [mitmproxy](https://mitmproxy.org/). The bug was reported
to Onion Browser team and [they have a better write up of the
situation](https://github.com/OnionBrowser/OnionBrowser/wiki/2019-178:-sites-with-EV-HTTPS-certificates-leak-information-via-OCSP). Unfortunatly
it is really hard to fix. :-(
to Onion Browser team and [they have a nice write up of the
situation](https://github.com/OnionBrowser/OnionBrowser/wiki/2019-178:-sites-with-EV-HTTPS-certificates-leak-information-via-OCSP). Unfortunately,
it is hard to fix. :-(
[^1]: Opposed to checking againist a [Certificate Revocation List](https://en.wikipedia.org/wiki/Certificate_revocation_list).
[^2]: Familier readers will note that this is plain OCSP, the non-stapling kind.
[^1]: Opposed to checking against a [Certificate Revocation List](https://en.wikipedia.org/wiki/Certificate_revocation_list).
[^2]: Familiar readers will note that this is plain OCSP, the non-stapling kind.

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text-align: left;
}
blockquote {
font-style: italic;
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video, code {
font-weight: 500;
max-width: 100%;