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Merge branch 'matthew/define-secrecy' into 'master'

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explicitly define backward & forward secrecy

See merge request matrix-org/olm!3
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Matthew Hodgson 2019-06-19 23:25:56 +00:00
commit 25662564d4
1 changed files with 23 additions and 6 deletions
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docs/megolm.md
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@ -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,