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Breaking message franking

sjoerdlangkemper.nl · 575 words · saved by 1 readers

Message franking is a mechanism to facilitate abuse reporting when using end-to-end encryption. This article describes how it works and how it can fail. Facebook Messenger provides end-to-end encryption. This means that Facebook is not aware of the contents of messages. This introduces a problem when a user reports an abusive message to Facebook: Alice says Bob is sending dick pics to her, but Facebook can’t know whether this is true since it can’t view the message contents. Message franking solves this by providing verifiable abuse reporting. When Bob sends a message to Alice, they first create a binding tag, which is a HMAC using a random key. The HMAC is readable by Facebook, but the HMAC key and the message are encrypted with Alice’s key. Facebook stores this HMAC before sending it and the encrypted message along to Alice. When Alice reports the message, she provides the message and the key used for the HMAC to Facebook. Facebook can verify that it indeed seen this HMAC and that th

Message franking is a mechanism to facilitate abuse reporting when using end-to-end encryption. This article describes how it works and how it can fail. The problem Facebook Messenger provides end-to-end encryption. This means that Facebook is not aware of the contents of messages. This introduces a problem when a user reports an abusive message to Facebook: Alice says Bob is sending dick pics to her, but Facebook can’t know whether this is true since it can’t view the message contents. Message franking Message franking solves this by providing verifiable abuse reporting. When Bob sends a mess

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