TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Alice's
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Figure 22-4 Authentication by encrypting with a private key
Going a step further, encrypting with the private key gives non-repudiation, too.
The mere existence of such an encrypted message testifies that the originator
has really sent it, because only he or she could have used the private key to
generate the message. Additionally, if a time stamp is included, the exact date
and time can also be proven. There are protocols involving trusted third parties
that prevent the sender from using phony time stamps.
Examples of public key algorithms
Algorithms based on public keys can be used for a variety of purposes. Two
common applications are:
► Encryption (see “RSA public key algorithm” on page 783).
► Generation of shared keys for use with symmetric key algorithms (see
“Diffie-Hellman key exchange” on page 784).
The most popular public key algorithm is the de facto standard RSA, named after
its three inventors: Ron Rivest, Adi Shamir, and Leonard Adleman. The security
of RSA relies on the difficult problem of factoring large numbers. The public and
private keys are functions of two very large (200 digits or even more) prime
numbers. Given the public key and the ciphertext, an attack is successful if it can
factor the product of the two primes. RSA has resisted many years of extensive
attacks. As computing power grows, keeping RSA secure is a matter of
increasing the key length (unlike DES, where the key length is fixed).
782 TCP/IP Tutorial and Technical Overview
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Alices's
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Encryption Decryption &
Ciphertext
authentication
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Alice Bob