Digital Signature Legitimateness for Communications & Media in United Kingdom

How to eSign a document: digital signature legitimateness for Communications & Media in United Kingdom

[Music] the digital signature algorithm it's an asymmetric encryption algorithm but it's different from the other asymmetric encryption arguments we discussed recall that when we discussed asymmetric encryption we discussed that asymmetric encryption can be used for three things encryption signatures and a key exchange earlier we talked about the rsa algorithm and we talked about how the rsa algorithm creates a pair of communitive keys that's that set of keys where you can encrypt with one and decrypt with the other the rsa algorithm can be used for all three of these asymmetric encryption functions we can use those public and private keys for encryption by encrypting a message with the public key and sending it across the wire and the other side can decrypt it safely with their private key we can use rsa for signatures by doing sort of the inverse we can hash the message we want to sign encrypt it with our own private key and send it across the wire and the other side can validate the signature with my public key we can also use rsa to do a key exchange by using the encryption mechanism we just discussed to send across a seed value to generate keys so rsa can be used for all three of these asymmetric encryption functions now we also talked about the diffie-hellman protocol we discussed that diffie-hellman can be used to establish a shared secret over an unsecured medium well if we look at this list over here that means that diffie-hellman can only be used for this function we can't do encryption for diffie-hellman and we can't do signatures with diffie-hellman which finally brings us to dsa dsa is another asymmetric encryption algorithm but it can only be used for signatures it can't do encryption and it can't do key exchange that is the digital signature algorithm and that's how it's different from the other algorithms we've discussed so the digital signature algorithm is an asymmetric description algorithm with only two operations signature generation and signature verification the signature generation operation involves four things the message you're trying to sign the private key you're going to sign it with a random number and these dsa parameters these dsa parameters are values which are required for the dsa math much like the prime number and the generator are values are required in diffie-hellman either way if you feed these four things into the signature generation operation dsa is going to spit out a signature correlating to that message then the signature verification operation also requires four things the message you're trying to verify the public key which correlates to the private key which was used to create the signature the signature itself which was the output of the signature generation operation and then finally those same dsa parameters if you run these four things through the signature verification operation dsa will spit out a one or a zero indicating true or false meaning is the signature valid or invalid that's it notice there's no encryption there's no ciphertext there's no plaintext all you can do with dsa is generate signatures and verify signatures notice that's sort of baked into the name it is simply a signature algorithm now i want to talk in more detail about something important about the dsa algorithm it has to do with that random number that random number is very important it must be unique for every single message that you're trying to sign or dsa will fail catastrophically what i mean by that is if you happen to reuse the same random number on two different messages you can compare the signatures of both of those messages and extract the private key that's really bad that's why dsa can fail catastrophically if you don't use unique random numbers one option is to use a random number that is so large that you never run the risk of using the same random number twice another option is to use rfc 6979 this rrc allows you to deterministically generate a random number based upon the message you're trying to sign what that means is if you use the formula outlined in this rfc the only way you'll ever have duplicate random numbers is if you're trying to sign the same message and if that's the case you don't have this failure point because either way you're signing the same message so that's the digital signature algorithm now we're not going to actually go through the math for dsa the math is actually pretty complicated much more so than it was for diffie-hellman and rsa which we did do the math for the main things to understand about dsa is that it is an asymmetric encryption protocol that can be used for only two things signature generation and signature verification and understanding how it is different from the other asymmetrical encryption protocols that we've already discussed so that's it for this lesson the main takeaways about dsa are on your screen right now i hope you enjoyed this video i want to thank you for watching and i'll see you in the next one hey 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