# Formal definition of (perfect) forward security/secrecy

In recent weeks the concept of (perfect) forward security/secrecy has been mentioned a lot, primarily in the context of the shocking revelations about NSA eavesdropping. As far as I'm aware, this concept was introduced by C. G. Günther in [1]. Unfortunately [1] only sketches the concept but does not give a security definition. The survey [2] gives a formal security definition only for forward secure pseudo-random generators. The textbooks [3, 4, 5, 6] all don't even mention the concept, and the relevant Wikipedia article is near unreadable.

Could somebody please point me towards a formal definition of (perfect) forward security/secrecy?

1. C. G. Günther, "An Identity-based Key-exchange Protocol".
2. G. Itkis, "Forward Security: Adaptive Cryptography: Time Evolution".
3. J. Katz, Y. Lindell, "Introductin to Modern Cryptography".
4. O. Goldreich, "Foundations of Cryptography".
5. J. Hoffstein, J. Pipher, J. H. Silverma, "An Introduction to Mathematical Cryptography".
6. D. R. Stinson, "Cryptography: Theory and Practise".
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