Cryptography

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Secret Key Cryptography


Single key used to encrypt and decrypt.
Key must be known by both parties.
Assuming we live in a hostile environment (otherwise - why
the need for cryptography?), it may be hard to share a secret key.

Bob’s Dilemma

Nobody can read the message from Alice, but anyone could produce it.
How does Bob know that the message was really sent from Alice?

Bob may be comforted to know that only Alice can read his reply.


Alice can sign her message!


Alice can create a digital signature and prove she sent the message (or someone with knowledge of her private key).
The signature can be a message digest encrypted with Aprivate.


Alice’s Signature


Alice feeds her original message through a hash function and encrypts the message digest with Aprivate.
Bob can decrypt the message digest using Apublic.
Bob can compute the message digest himself.
If the 2 message digests are identical, Bob knows Alice sent the message.