TCP/IP Tutorial and Technical Overview - IBM Redbooks

e and d are called the public and private exponents, respectively. The public key
is the pair (n,e); the private key is d. The factors p and q must be kept secret or
destroyed.
A simplified example of RSA encryption is:
1. Suppose Alice wants to send a private message, m, to Bob. Alice creates the
ciphertext c by exponentiating:
c = me mod n
Where e and n are Bob's public key.
2. Alice sends c to Bob.
3. To decrypt, Bob exponentiates:
m = c d mod n
And recovers the original message; the relationship between e and d ensures
that Bob correctly recovers m. Because only Bob knows d, only Bob can
decrypt the ciphertext.
A simplified example of RSA authentication is:
1. Suppose Alice wants to send a signed message, m, to Bob. Alice creates a
digital signature s by exponentiating:
s = md mod n
Where d and n belong to Alice's private key.
2. She sends s and m to Bob.
3. To verify the signature, Bob exponentiates and checks if the result, compares
to m:
m = s e mod n
Where e and n belong to Alice's public key.
Diffie-Hellman key exchange
The Diffie-Hellman key exchange is a crucial component of the ISAKMP/Oakley
framework. In the earliest phase of a key negotiation session, there is no secure
channel in place. The parties derive shared secret keys using the Diffie-Hellman
algorithm. These keys will be used in the next steps of the key negotiation
protocol.
The following steps outline the algorithm:
1. The parties (Alice and Bob) share two public values, a modulus m and an
integer g. m is a large prime number.
784 TCP/IP Tutorial and Technical Overview