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Security Education Companion
A free resource for digital security educators

Forward Secrecy

A property of a secure messaging system which ensures that your past communications can remain secure even if one of the private keys is stolen later.  For HTTPS websites, forward secrecy is an important protection against adversaries like intelligence agencies which may record large amounts of traffic and use a stolen key to decrypt it.  For instant messaging and chat systems, forward secrecy is necessary to ensure that deleted messages are really deleted, but you will also need to either disable logging or securely delete past messages.