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| -rw-r--r-- | docs/megolm.md | 29 |
1 files changed, 23 insertions, 6 deletions
diff --git a/docs/megolm.md b/docs/megolm.md index b9eedec..ec98001 100644 --- a/docs/megolm.md +++ b/docs/megolm.md @@ -267,8 +267,17 @@ future research. ### Lack of Backward Secrecy -Once the key to a Megolm session is compromised, the attacker can decrypt any -future messages sent via that session. +[Backward secrecy](https://intensecrypto.org/public/lec_08_hash_functions_part2.html#sec-forward-and-backward-secrecy) +(also called 'future secrecy' or 'post-compromise security') is the property +that if current private keys are compromised, an attacker cannot decrypt +future messages in a given session. In other words, when looking +**backwards** in time at a compromise which has already happened, **current** +messages are still secret. + +By itself, Megolm does not possess this property: once the key to a Megolm +session is compromised, the attacker can decrypt any message that was +encrypted using a key derived from the compromised or subsequent ratchet +values. In order to mitigate this, the application should ensure that Megolm sessions are not used indefinitely. Instead it should periodically start a new session, @@ -279,10 +288,18 @@ with new keys shared over a secure channel. ### Partial Forward Secrecy -Each recipient maintains a record of the ratchet value which allows them to -decrypt any messages sent in the session after the corresponding point in the -conversation. If this value is compromised, an attacker can similarly decrypt -those past messages. +[Forward secrecy](https://intensecrypto.org/public/lec_08_hash_functions_part2.html#sec-forward-and-backward-secrecy) +(also called 'perfect forward secrecy') is the property that if the current +private keys are compromised, an attacker cannot decrypt *past* messages in +a given session. In other words, when looking **forwards** in time towards a +potential future compromise, **current** messages will be secret. + +In Megolm, each recipient maintains a record of the ratchet value which allows +them to decrypt any messages sent in the session after the corresponding point +in the conversation. If this value is compromised, an attacker can similarly +decrypt past messages which were encrypted by a key derived from the +compromised or subsequent ratchet values. This gives 'partial' forward +secrecy. To mitigate this issue, the application should offer the user the option to discard historical conversations, by winding forward any stored ratchet values, |