dissertacao.pdf

messages, Alice and Bob needed a secure channel (that is, a channel to send
messages without the risk of Marvin intercepting any message) to exchange the
secret key. But suppose now that Alice and Bob never met, will never have
the chance to meet, and have at their disposition only an insecure channel to
exchange messages. Can they exchange encrypted messages between them, with
Marvin knowing all these cypher texts but without being able to decrypt them?
Until we got to 1976, this question seemed to have no answer. It was then
that Diffie and Hellman, predicting a coming revolution in cryptography[13],
proposed a new concept of cryptosystem which would lead to the creation of
RSA.
1.2 Public Key Cryptography
Public key cryptography consists in cryptosystems where there is no need for a
secure channel to exchange any prior information, like the secret key used by
private key cryptosystems. A common analogy to explain the concept of public
key cryptography is that of a simple mailbox. Everyone can put a letter in
Alice’s mailbox but only Alice, who has the key for her mailbox, can open it
and read her letters. The same is true for Bob and his mailbox. In a technical
way, this can be described as follows: Alice creates a pair of public and private
keys. Then she reveals her public key, allowing anyone to encrypt messages with
this key and send them to her. When receiving them, she decrypts them using
her private key. To clear any doubts, should Bob wish to receive encrypted
messages he should also create a pair of public/private keys and follow Alice’s
procedure.
This way, there is no need for a safe channel to agree on a key, as there
are no common keys which need to be changed between the several users. The
conditions such a cryptosystem should satisfy, which were initially proposed by
Diffie and Hellman in 1976, are presented below:
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