Certified Digital Signature

How to eSign a document: how to create a digital signature

digital signatures rely on a symmetric key cryptography so before we talk about digital signatures let's quickly remind ourselves how a symmetric key cryptography works with asymmetric key cryptography there's a pair of mathematically related keys if you encrypt a message with one of the keys than the other key and only the other key can be used to decrypt it this is Albert and this is Sheila if Sheila wants Albert to send her a secret message she needs an asymmetric a pair of keys a computer program can generate these for her she sends Albert a copy of one of the keys and she keeps the other key to herself instead of sending Albert a key she could put a key in a public place for him to go and fetch himself it doesn't matter if someone else gets a copy of the public key Albert uses the public key to encrypt his message when Albert has encrypted the message he sends a Sheila the ciphertext only Sheila can decrypt the ciphertext because only Sheila has the matching key the private key truth be told most people don't need to worry about what's really going on because the whole process is taken care of behind the scenes my programs built into web browsers or email applications but what do asymmetric keys have to do with digital signatures well it so happens that it doesn't matter which one of the pair of keys is made public and which one is kept private if you encrypt a message with either one than the other and only the other can be used to decrypt it as long as she doesn't change her mind later Sheila can decide which one of the keys will be private and which one will be public this is the crucial feature of asymmetric key cryptography that makes digital signatures possible now let's think about why we even need digital signatures this is Carol and this is Bob Bob's builder carol has a leaky roof and she asks Bob to fix it Carol agrees to pay Bob a thousand pounds to fix her roof Bob agrees to start work on Monday and have it finished by Wednesday Carol writes details of the agreement on a piece of paper and she signs it and she sends it to Bob in the post no computers here Bob signs the piece of paper and sends a copy back to Carol in the post Carol sends Bob the money bob writes a note to carol to say he's got the money he signs this receipt and he sends it to Carol in the post on Monday Bob doesn't turn up he's gone on holiday for a week to spend the money on Thursday it rains all day and all of Carol's furniture is destroyed poor Carol Carol says it's Bob's fault but Bob denies ever having agreed to do the work Carol takes Bob to court the judge looks at the...