Legacy Encryption Downgrade Attacks against LibrePGP and CMS

Falko Strenzke; Johannes Roth

doi:10.62056/ayl86chdj

Legacy Encryption Downgrade Attacks against LibrePGP and CMS

Authors

Falko Strenzke, Johannes Roth

Falko Strenzke

MTG AG, Darmstadt, Germany
falko dot strenzke at mtg dot de

Johannes Roth

MTG AG, Darmstadt, Germany
johannes dot roth at mtg dot de

Keywords: AEAD downgrade CMS LibrePGP decryption-oracle

Abstract

This work describes vulnerabilities in the specification of AEAD modes and Key Wrap in two cryptographic message formats. Firstly, this applies to AEAD packets as introduced in the novel LibrePGP specification that is implemented by the widely used GnuPG application. Secondly, we describe vulnerabilities in the AES-based AEAD schemes as well as the Key Wrap Algorithm specified in the Cryptographic Message Syntax (CMS). These new attacks exploit the possibility to downgrade AEAD or AES Key Wrap ciphertexts to valid legacy CFB- or CBC-encrypted related ciphertexts and require that the attacker learns the content of the legacy decryption result. This can happen in two principal ways: either due to the human recipient returning the decryption output to the attacker as a quote or due to a programmatic decryption oracle in the receiving system that reveals information about the plaintext. The attacks effect the decryption of low-entropy plaintext blocks in AEAD ciphertexts and, in the case of LibrePGP, also the manipulation of existing AEAD ciphertexts. For AES Key Wrap in CMS, full key decryption is possible. Some of the attacks require multiple successful oracle queries. The attacks thus demonstrate that CCA2 security is not achieved by the LibrePGP and CMS AEAD or Key Wrap encryption in the presence of a legacy cipher mode decryption oracle. The proper countermeasure to thwart the attacks is a key derivation that ensures the use of unrelated block cipher keys for the different encryption modes.

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PDF Open access

DOI: https://doi.org/10.62056/ayl86chdj

History

Submitted: 2025-01-10
Accepted: 2025-03-11
Published: 2025-04-08

How to cite

Falko Strenzke and Johannes Roth, Legacy Encryption Downgrade Attacks against LibrePGP and CMS. IACR Communications in Cryptology, vol. 2, no. 1, Apr 08, 2025, doi: 10.62056/ayl86chdj.

License

Copyright is held by the author(s)

This work is licensed under a Creative Commons Attribution (CC BY) license.

@article{10.62056/ayl86chdj,
         author={Falko Strenzke and Johannes Roth},
         title={Legacy Encryption Downgrade Attacks against Libre{PGP} and {CMS}},
         volume={2},
         number={1},
         year={2025},
         date={2025-04-08},
         issn={3006-5496},
         doi={10.62056/ayl86chdj},
         journal={{IACR} Communications in Cryptology},
         publisher={International Association for Cryptologic Research}
}

TY - JOUR
AU - Falko Strenzke
AU - Johannes Roth
PY - 2025
TI - Legacy Encryption Downgrade Attacks against LibrePGP and CMS
JF - IACR Communications in Cryptology
JA - CIC
VL - 2
IS - 1
DO - 10.62056/ayl86chdj
UR - https://doi.org/10.62056/ayl86chdj
AB - <p> This work describes vulnerabilities in the specification of AEAD modes and Key Wrap in two cryptographic message formats. Firstly, this applies to AEAD packets as introduced in the novel LibrePGP specification that is implemented by the widely used GnuPG application. Secondly, we describe vulnerabilities in the AES-based AEAD schemes as well as the Key Wrap Algorithm specified in the Cryptographic Message Syntax (CMS). These new attacks exploit the possibility to downgrade AEAD or AES Key Wrap ciphertexts to valid legacy CFB- or CBC-encrypted related ciphertexts and require that the attacker learns the content of the legacy decryption result. This can happen in two principal ways: either due to the human recipient returning the decryption output to the attacker as a quote or due to a programmatic decryption oracle in the receiving system that reveals information about the plaintext. The attacks effect the decryption of low-entropy plaintext blocks in AEAD ciphertexts and, in the case of LibrePGP, also the manipulation of existing AEAD ciphertexts. For AES Key Wrap in CMS, full key decryption is possible. Some of the attacks require multiple successful oracle queries. The attacks thus demonstrate that CCA2 security is not achieved by the LibrePGP and CMS AEAD or Key Wrap encryption in the presence of a legacy cipher mode decryption oracle. The proper countermeasure to thwart the attacks is a key derivation that ensures the use of unrelated block cipher keys for the different encryption modes. </p>
ER -

Falko Strenzke and Johannes Roth, Legacy Encryption Downgrade Attacks against LibrePGP and CMS. IACR Communications in Cryptology, vol. 2, no. 1, Apr 08, 2025, doi: 10.62056/ayl86chdj.