gen-revoke: extending revocation certificates to subkeys

Traditionally, OpenPGP revocation certificates are used as a last resort. You are expected to generate one for your primary key and keep it in a secure location. If you ever lose the secret portion of the key and are unable to revoke it any other way, you import the revocation certificate and submit the updated key to keyservers. However, there is another interesting use for revocation certificates — revoking shared organization keys.

Let’s take Gentoo, for example. We are using a few keys needed to perform automated signatures on servers. For this reason, the key is especially exposed to attacks and we want to be able to revoke it quickly if the need arises. Now, we really do not want to have every single Infra member hold a copy of the secret primary key. However, we can give Infra members revocation certificates instead. This way, they maintain the possibility of revoking the key without unnecessarily increasing its exposure.

The problem with traditional revocation certificates is that they are supported for the purpose of revoking the primary key only. In our security model, the primary key is well protected, compared to subkeys that are totally exposed. Therefore, it is superfluous to revoke the complete key when only a subkey is compromised. To resolve this limitation, gen-revoke tool was created that can create exported revocation signatures for both the primary key and subkeys.

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Identity with OpenPGP trust model

Let’s say you want to send a confidential message to me, and possibly receive a reply. Through employing asymmetric encryption, you can prevent a third party from reading its contents, even if it can intercept the ciphertext. Through signatures, you can verify the authenticity of the message, and therefore detect any possible tampering. But for all this to work, you need to be able to verify the authenticity of the public keys first. In other words, we need to be able to prevent the aforementioned third party — possibly capable of intercepting your communications and publishing a forged key with my credentials on it — from tricking you into using the wrong key.

This renders key authenticity the fundamental problem of asymmetric cryptography. But before we start discussing how key certification is implemented, we need to cover another fundamental issue — identity. After all, who am I — who is the person you are writing to? Are you writing to a person you’ve met? Or to a specific Gentoo developer? Author of some project? Before you can distinguish my authentic key from a forged key, you need to be able to clearly distinguish me from an impostor.

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Portability of tar features

The tar format is one of the oldest archive formats in use. It comes as no surprise that it is ugly — built as layers of hacks on the older format versions to overcome their limitations. However, given the POSIX standarization in late 80s and the popularity of GNU tar, you would expect the interoperability problems to be mostly resolved nowadays.

This article is directly inspired by my proof-of-concept work on new binary package format for Gentoo. My original proposal used volume label to provide user- and file(1)-friendly way of distinguish our binary packages. While it is a GNU tar extension, it falls within POSIX ustar implementation-defined file format and you would expect that non-compliant implementations would extract it as regular files. What I did not anticipate is that some implementation reject the whole archive instead.

This naturally raised more questions on how portable various tar formats actually are. To verify that, I have decided to analyze the standards for possible incompatibility dangers and build a suite of test inputs that could be used to check how various implementations cope with that. This article describes those points and provides test results for a number of implementations.

Please note that this article is focused merely on read-wise format compatibility. In other words, it establishes how tar files should be written in order to achieve best probability that it will be read correctly afterwards. It does not investigate what formats the listed tools can write and whether they can correctly create archives using specific features.

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