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Improve section on symmetric ciphers used for SKESKv4+SEIPDv1

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Paul Schaub 2023-12-08 15:52:54 +01:00 • committed by Heiko Schaefer
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book/source/11-decryption.md
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@ -55,7 +55,11 @@ Decrypting the session-key from a version 4 SKESK packet.
```
With version 4 SKESK packets, which are only used with version 1 SEIPD packets, the *session-key* is used as *message-key* without an intermediate derivation.
The symmetric cipher algorithm tag of the SKESK packet dictates the cipher algorithm used to decrypt the plaintext from the SEIPD packet.
When the direct method is used, meaning no encrypted session-key was contained in the SKESK packet, the symmetric cipher algorithm ID of the SKESK packet dictates the cipher algorithm used to decrypt the plaintext from the SEIPD packet.
Otherwise, the cipher algorithm ID to decrypt the SEIPD packet was prefixed to the decrypted session key.
Sanitizing this first byte acts as a very early quick check to verify that the used passphrase was correct. For further validation of the session-key, see [](decryption_seipd_quick_check).
### SKESK v6
@ -186,6 +190,7 @@ Legacy mode, may be decrypted, but not produced.
## Advanced topics
(decryption_seipd_quick_check)=
### Verify successful session-key decryption
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.