explicitly define backward & forward secrecy
as it repeatedly trips people up, including me
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@ -267,8 +267,16 @@ future research.
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### Lack of Backward Secrecy
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Once the key to a Megolm session is compromised, the attacker can decrypt any
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future messages sent via that session.
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[Backward secrecy](https://intensecrypto.org/public/lec_08_hash_functions_part2.html#sec-forward-and-backward-secrecy)
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(also called 'future secrecy' or 'post-compromise security') is the property
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that if current private keys are compromised, an attacker cannot decrypt
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future messages in a given session. In other words, when looking
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**backwards** into the past at a compromise, messages sent since the compromise
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will be secret.
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By itself, Megolm does not posess this property: once the key to a Megolm
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session is compromised, the attacker can decrypt any future messages sent via
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that session.
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In order to mitigate this, the application should ensure that Megolm sessions
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are not used indefinitely. Instead it should periodically start a new session,
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@ -279,7 +287,15 @@ with new keys shared over a secure channel.
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### Partial Forward Secrecy
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Each recipient maintains a record of the ratchet value which allows them to
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[Forward secrecy](https://intensecrypto.org/public/lec_08_hash_functions_part2.html#sec-forward-and-backward-secrecy)
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is the property that if the current private keys are compromised, an attacker
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cannot decrypt *past* messages in a given session (unless past private keys
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are retained). 'Perfect forward secrecy' means that no past keys are retained.
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'Partial forward secrecy' means that some past key data may be retained. In
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other words, when looking **forwards** into the future at a potential
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compromise, messages sent prior to the compromise will be secret.
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In Megolm, each recipient maintains a record of the ratchet value which allows them to
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decrypt any messages sent in the session after the corresponding point in the
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conversation. If this value is compromised, an attacker can similarly decrypt
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those past messages.
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