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30 lines
2.1 KiB
Markdown
30 lines
2.1 KiB
Markdown
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SPDX-FileCopyrightText: 2023 The "Notes on OpenPGP" project
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SPDX-License-Identifier: CC-BY-SA-4.0
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-->
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# Advanced material: Decryption
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(decryption-seipd-quick-check)=
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## Verify successful session key decryption
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SEIPDv1 packets might make use of a "quick check" mechanism to quickly verify that the correct session key was used without the need to decrypt the whole SEIPD packet.
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This check consists of 16 random bytes, followed by a copy of the two last bytes, which are prefixed to the plaintext.
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During decrypting, these 2 bytes can be compared to the 15th and 16th random byte to detect use of the wrong session key.
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Since the chance to accidentally end up with matching quick check bytes albeit the use of the wrong session key is 1:65536, some implementations validate further contents of the plaintext, such as the packet headers.
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The standard [warns against](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-risks-of-a-quick-check-orac) using the quick check mechanism, as it introduces the risk of a decryption oracle. Instead, the use of SEIPDv2 is recommended, as the AEAD mechanism automatically detects use of the wrong session key early on after the first chunk has been decrypted.
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(decryption-anonymous-recipient)=
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## Anonymous recipients
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Having all recipients keys listed as part of the PKESK packets presents a metadata leakage. An observer can easily enumerate recipients of a message by comparing the PKESKs with certificates of potential recipients.
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To prevent this issue, the sender can decide to add individual recipients as anonymous recipients using a wildcard key-ID / fingerprint.
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This is done by creating a normal PKESK packet for the recipient, but setting the recipient key field to `0` (as well as omitting the version number of the key for v6 PKESKs).
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A recipient of such a message that does not find a PKESK addressed specifically to any of their keys, can then try to decrypt any anonymous PKESK packets using any of their encryption subkeys.
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To reduce the number of keys to try, the recipient can skip all secret keys which do not share the public-key algorithm stated in the PKESK packet.
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