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Network Security
Network Security
• Information secrecy-only specified parties
know the information exchanged. Provided by
• Information integrity-the information is
unaltered received by the specified party.
Provided by digital signatures.
• Authenticaton-user is communicating with the
user with whom he/she thinks is
The encryption model (for a symmetric-key cipher).
Symmetric-Key Algorithms
• Transmitting and receiving users have the same key
that they agreed on somehow.
• Plain text, P, is encrypted by the transmitting user
based on key, K, and becomes EK(P). Receiver is
decrypting EK(P) using the same key, and obtains
plaintext P=DK(EK(P)). Function P is impossible (or
very hard) to guess from EK(P).
• DES – Data Encryption Standard
Key has 64 bits
• AES – Advanced Encryption Standard
Key has 128 bits
Public-Key Algorithms
• Each user has a public and a private key.
• Plain text, P, is encrypted by the transmitting
user based on the public key of the receiving
user, Bpub, and becomes EBpub(P).
• Receiving user is decrypting the message using
its private key Bpri and obtains plaintext
Public-Key Algorithms
• RSA (Rivest,Shamir, Adleman)
1. Choose two large prime numbers p and q (typically
1024 bits)
2. Compute n=pxq and z=(p-1)x(q-1)
3. Choose a number relatively prime to z and call it d.
4. Find e such that exd=1mod z
5. Public key is (n,e), private key is (n,d)
6. Encryption is C=Pemod n
7. Decryption is P=Cd mod n
Learning Shared Keys
• Encryption using a symmetric shared key is much
faster. Users can exchange the shared key, either by
using public keys or key distribution centers (KDC).
• Transmitting user encrypts the shared-key using
public key of the receiving user. Receiving user
decrypts the message and learns the shared key.
• KDC has a key for each user. User A sends encrypted
request that it wants to communicate with user B.
KDC sends the shared key, S, and encrypted pair
Digital Signatures
• Ensure to the receiving party that it is
receive the exact information that was sent
my the transmitting party.
• For example, transmitting user adds to the
plaintext the signature which is the
encrypted plaintext using its private key.
Receiving user decrypts the signature
using the public key of the transmitting
user, and compares with the plaintext.
Public-Key Signatures
Digital signatures using public-key cryptography.
Digital Signatures Using
Message Digests (MD5,SHA-1)
• Forming the signature by encrypting the entire plaintext
is time consuming, and incurs an inefficient
• For this reason, a message digest is derived from the
plaintext (message). Message digest is a function of the
plaintext such that it is computationaly infeasible to
find two messages with the same message digest
(hash). Singature is encrypted hash.
• Since the message digest is much shorter than the
message itself, the signature takes less time to compute
and less bandwidth to transfer.
Authentication Protocol
(Shared Key)
• User A sends to user B “I am A”, and nonce RA.
• B sends a nonce RB and HASH(RA,RB,A,B,S) to
• A sends HASH(RA,RB,S) and sends it to B
Authentication Protocol
(Public Key)
• User A sends to user B “I am A”, and RA.
• B sends a nonce RB and EBpriv(RA,RB,A,B) to A.
• A decrypts encrypted part of the message to
check if it is B and sends encrypted nonces
using its private key EApriv(RA,RB) and sends it to
• B decrypts encrypted message and checks if this
is really A.
Management of Public Keys
• How users learn real public keys of
other users.
• Certificate comprises the public key
and basic data about some user, and is
signed by the certificate authority (CA).
• X.509 defines certificate’s format.
• Public key infrastructures comprises
CAs that are organized hierarchicaly.
+ Alg for hashing,
Certificate Authority,
Time validity
A possible certificate and its signed hash.
Public-Key Infrastructures
(a) A hierarchical PKI. (b) A chain of certificates.
Network Security in Practice
Virtual Private Networks (VPNs)
E-mail security (Pretty Good
• DNS security
• WWW security (Secure Sockets
IPsec based on
Authentication Header Protocol
Determines security association
with IP source address
and security protocol
Using encrypted message disgest
based on a shared key
The IPsec authentication header in transport mode for IPv4.
IPsec based on
Encapsulation Security Payload Protocol
(a) ESP in transport mode. (b) ESP in tunnel mode.
• Two types: packet filtering, and
application gateways.
• Packet filtering is done based on IP
addresses, TCP or UDP ports, ICMP
message type, TCP SYN or ACK bits
(first segment has ACK=0 so this
prevents outside users to open TCP
connections with inside servers).
• Application gateways may restrict
certain applications to certain users.
Virtual Private Networks
(a) A leased-line private network. (b) A virtual private network.
E-mail Security
PGP – Pretty Good Privacy
International Data
Encryption Alg
PGP in operation for sending a message.
Secure DNS
An example RRSet for The KEY record is Bob's
public key. The SIG record is the top-level com server's signed
hash of A and KEY records to verify their authenticity.
A simplified version of the SSL connection establishment subprotocol.