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International audience; The SHA-1 hash function was designed in 1995 and has been widely used during two decades. A theoretical collision attack was first proposed in 2004 [29], but due to its high complexity it was only implemented in practice in 2017, using a large GPU cluster [23]. More recently, an almost practical chosen-prefix collision attack against SHA-1 has been proposed [12]. This more powerful attack allows to build colliding messages with two arbitrary prefixes, which is much more threatening for real protocols. In this paper, we report the first practical implementation of this attack, and its impact on real-world security with a PGP/GnuPG impersonation attack. We managed to significantly reduce the complexity of collision attacks against SHA-1: on an Nvidia GTX 970, identical-prefix collisions can now be computed with a complexity (expressed in terms of SHA-1 equivalents on this GPU) of 2 61.2 rather than 2 64.7 , and chosen-prefix collisions with a complexity of 2 63.4 rather than 2 67.1. When renting cheap GPUs, this translates to a cost of US$ 11k for a collision, and US$ 45k for a chosen-prefix collision, within the means of academic researchers. Our actual attack required two months of computations using 900 Nvidia GTX 1060 GPUs (we paid US$ 75k because GPU prices were higher, and we wasted some time preparing the attack). Therefore, the same attacks that have been practical on MD5 since 2009 are now practical on SHA-1. In particular, chosen-prefix collisions can break signature schemes and handshake security in secure channel protocols (TLS, SSH), if generated extremely quickly. We strongly advise to remove SHA-1 from those type of applications as soon as possible. We exemplify our cryptanalysis by creating a pair of PGP/GnuPG keys with different identities, but colliding SHA-1 certificates. A SHA-1 certification of the first key can therefore be transferred to the second key, leading to an impersonation attack. This proves that SHA-1 signatures now offer virtually no security in practice. The legacy branch of GnuPG still uses SHA-1 by default for identity certifications, but after notifying the authors, the modern branch now rejects SHA-1 signatures (the issue is tracked as CVE-2019-14855).

International audience; The SHA-1 hash function was designed in 1995 and has been widely used during two decades. A theoretical collision attack was first proposed in 2004 [29], but due to its high complexity it was only implemented in practice in 2017, using a large GPU cluster [23]. More recently, an almost practical chosen-prefix collision attack against SHA-1 has been proposed [12]. This more powerful attack allows to build colliding messages with two arbitrary prefixes, which is much more threatening for real protocols. In this paper, we report the first practical implementation of this attack, and its impact on real-world security with a PGP/GnuPG impersonation attack. We managed to significantly reduce the complexity of collision attacks against SHA-1: on an Nvidia GTX 970, identical-prefix collisions can now be computed with a complexity (expressed in terms of SHA-1 equivalents on this GPU) of 2 61.2 rather than 2 64.7 , and chosen-prefix collisions with a complexity of 2 63.4 rather than 2 67.1. When renting cheap GPUs, this translates to a cost of US$ 11k for a collision, and US$ 45k for a chosen-prefix collision, within the means of academic researchers. Our actual attack required two months of computations using 900 Nvidia GTX 1060 GPUs (we paid US$ 75k because GPU prices were higher, and we wasted some time preparing the attack). Therefore, the same attacks that have been practical on MD5 since 2009 are now practical on SHA-1. In particular, chosen-prefix collisions can break signature schemes and handshake security in secure channel protocols (TLS, SSH), if generated extremely quickly. We strongly advise to remove SHA-1 from those type of applications as soon as possible. We exemplify our cryptanalysis by creating a pair of PGP/GnuPG keys with different identities, but colliding SHA-1 certificates. A SHA-1 certification of the first key can therefore be transferred to the second key, leading to an impersonation attack. This proves that SHA-1 signatures now offer virtually no security in practice. The legacy branch of GnuPG still uses SHA-1 by default for identity certifications, but after notifying the authors, the modern branch now rejects SHA-1 signatures (the issue is tracked as CVE-2019-14855).

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 semantical 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 conser- vative choices in dealing with malicious input data. Finally, we provide guidelines to improve the OpenPGP specification