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welcome to the second week of the exciting topic digital signatures what we did last week we gave an introduction to digital signatures and we did actually more with we started talking about security services which is an important topic blah blah blah blah blah and then algorithmically we did our say digital signature and we today's lecture is a direct continuation offset so kind of the most important thing the most important basic elements we had last week what I would like to do with you today is mainly two things which we split into three chapters today I want to show you an attack against ours a digital signature and it's what we did to at the end of last week and I couldn't squeeze it in last week and then maybe the main part of today's lecture deals with another signature scheme el-gamal a digital signature and then after introducing digital signature which it's not all that complicated I want to talk about things you have to watch out for you nor things we can make mistakes weaknesses slash attacks okay so the attack against ours a and it's not like an attack like factoring which is you know it's pretty easy to protect against you just choose really large modulae and you're fine this is kind of built in and it's building in a lot of digital signatures and it's kind of interesting to see what can happen we see with the construction we had and it has a really fancy name it's called exists tensho I don't know what the drum translation is maybe accents elo but it doesn't really fit what it is it's existential forgery forgery special attack against RS a digital signature so let's look at the protocol again it's kind of fun so what I'm going to do now I just draw the protocol from last week LS Bop and then we add Oscar and we look what Oscar can do as he is you know if he wants to do bad things so we have LS we have synced lose another next board you need at least one sort of a page if you're copying you know if it were at the end flip over half a page would be better so we start with Alice and Bob so if you look up what how hours a digital signature works it works quite similar to to a regular RSA encryption scheme so Bob computes a public key which is the modulus N and the public exponent E and he keeps the private exponent then he distributes his key openly over the channel and then what he normally does and now you can copy that then in order to sign we simply take the message X we raise it it erases to his private his secret exponent and that's it and then he sends over the channel x comma signature right ok and then what popped and what else does upon...