Digital signature system
{"Image DocumentName": "Digital signature system", "Image ImageDescription": "Security is asymmetric, too. usually: unconditionally secure for recipient only cryptographically secure for signer. message domain signature domain. new: signer is absolutely secure against breaking his signatures provable. only cryptographically secure for recipient. proof of forgery. \uf0b7 s\u2018(x) x s s(x) \uf0b7 \uf0b7 digital sinatures system with special propoerties: remeber digital signautre (slide 82) / difference to symmetric authentication\u2026 digital signature: asymmetric system \u2013 but now security property are asymmetric as well\u2026 \u201eunconditional for recipient means: recipient can test; if ok it stays ok\u2026. exermination question: why no unconditonal secure signastures? practical problems: software failures; loos of key; hacking of machnine; smart card still not 100% secure; new algorithms for breaking the system; factoring; dis log etc.; too small keys\u2026 security of signer decreaes! impossible: if key is broken \uf0e0 signatures are invalid: would tunr the whole situation \u2013 but no security for recipient at all!! remeber: key revocation: from now on key is invalid for signing\u2026 (CRL, OCSP) \uf0e8 heise news\u2026 certificate haked\u2026 comodo case. how to distribut risk if crypto is broken\u2026 possible messages / possible signatures / signin key s. idea: make signature domain larger (should be large anyway\u2026) \uf0e0 many signatures should be possible seems to be more insecure (guessing\u2026) ratio stay the same\u2026 first idea \u2013 useless \uf04a main idea: if recipient has to different signatures: proof of forgery\u2026 because signer will always generate the same signature\u2026 attacker: could calculate  all  signatuers: but does not know which is the right one\u2026. (remeber prrof for no info theroretic secure-..) note: also signer can not break the system!!! draw picture on blackboard.. options for risk taking: \u2026. this is the important question \u2013 and here we have design options!!! note: does not help agains loosing the secret key\u2026 t true. distribution of risks if signature is forged: 1. recipient 2. insurance or system operator 3. signer.", "Image XResolution": "37", "Image YResolution": "37", "Image ResolutionUnit": "Pixels/Centimeter", "Image Software": "SlidePlayer", "Image YCbCrPositioning": "Centered"}