Public Key
 Data Encryption Standard (DES)
o US Govt. standard for encrypting large blocks of data;
o Both Sender and Receiver must have the same key to encrypt
and decrypt.
o Difficult to break computationally
o Unsuitable over the Internet; since the keys must be transmitted
over the insecure Internet
o Only useful for Bilateral communication
 RSA Public-Key Cryptography
o Each person has a private and a public key
o Private key is never transmitted over the network
o Sender uses his or her private key to create a digital signature
o The encrypted message is sent along with the sender’s public
key
o The recipient can verify the digital signature using the sender’s
public key
o Inefficient for large blocks of data like videos
 Pretty Good Privacy (PGP)
o Phil Zimmerman 1991
o PGP creates a random session key for the message
o IDEA algorithm to encrypt the message with the session key.
o RSA algorithm to encrypt the session key with recipient’s public
key
o Bundles the message and the session key together for
transmission
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 Kerberos
o Embedded in any network protocol
o Mainly used by APP like telnet & ftp to provide host security
o Maintains a central database of secret keys
o Each user or service shares a secret key with Kerberos key
distribution center (KDC)
o All kerberos messages contains a checksum, this is checked on
decryption by the recipient.
o Difficult to secure the kerberos server itself.
 Solution to Public Key encryption overhead:
o Use public key encryption to distribute a randomly generated,
one-time “session” key that is used for all communications
 -> Secure Session Layer Solution (SSL)
o Client (Browser) and Server both support 1. Public Key
Encryption & 2. Single Key Encryption.
o When Browser connects to a “secure” service, a random bit
string is generated and communicated to the server using public
key encryption. From then on, all messages are
encrypted/decrypted using this SESSION “key”.
 EXAPLE
o Can John “stamp” a document so that only Mary can use it?
 Encrypt with Mary’s public key, no one else can decrypt it
except Mary
o How does Mary know the “stamped” doc. came from John?
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 Encrypt it twice, first with Mary’s public key, then with
John’s Private Key. Mary can decrypt it with her private
key and John’s Public Key
 Other:
o In almost all cases, the use of the keys is hidden from the users
and built-in to the application software, i.e. https:, ssh sftp, …
o However, there is still the chance that a breakthrough in
computer speed (i.e. quantum computers) or algorithms could
change the landscape. We are already using more and more bits
in the keys as computers get faster.
 How can I be sure a digital document hasn’t been altered?
o Use a identifier like hash key -> encrypt it and send along with
message.
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