A Security Audit of the OpenPGP Format

For over two decades the OpenPGP format has provided the mainstay of email confidentiality and authenticity, and is currently being relied upon to provide authenticated package distributions in open source Unix systems. In this work, we provide the first language theoretical analysis of the OpenPGP format, classifying it as a deterministic context free language and establishing that an automatically generated parser can in principle be defined. However, we show that the number of rules required to describe it with a deterministic context free grammar is prohibitively high, and we identify security vulnerabilities in the OpenPGP format specification. We identify possible attacks aimed at tampering with messages and certificates while retaining their syntactical and semantic validity. We evaluate the effectiveness of these attacks against the two OpenPGP implementations covering the overwhelming majority of uses, i.e., the GNU Privacy Guard (GPG) and Symantec PGP. The results of the evaluation show that both implementations turn out not to be vulnerable due to conservative choices in dealing with malicious input data. Finally, we provide guidelines to improve the OpenPGP specification.