The use of mathematical operations to protect messages traveling between parties or stored on a computer.
Confidentiality:
Ensures that intercepted communications cannot be read.
Encryption for confidentiality:
The original purpose of cryptography.
Encryption and Ciphertext:
Encryption: A cryptographic process that turns plaintext into ciphertext (a seemingly random stream of bits).
Sender sends ciphertext to the receiver.
Eavesdroppers cannot understand the ciphertext.
Receiver decrypts the ciphertext back into plaintext.
Cipher:
A specific mathematical process used in encryption and decryption.
Key:
A random string of 40 to 4,000 bits.
Kerckhoffs’ Law:
Keeping keys secret is crucial for secure encryption.
Cryptanalysis:
The art of cracking encryption.
Simplest type: brute-force key cracking (trying all possible keys).
Types of Ciphers:
Substitution Ciphers:
Substitute one letter (or bit) for another.
Example: The cipher in Figure 3-2.
Transposition Ciphers:
Change the order of letters or bits without changing the letters/bits themselves.
Real-world ciphers use both substitution and transposition.
Ciphers vs. Codes:
Real-world ciphers mix transposition and substitution for randomness.
Ciphers can encrypt any message expressed in binary (1s and 0s).
Flexibility and speed make ciphers dominant for encryption today.
Codes:
More specialized, substituting one thing for another (e.g., word for word).
Symmetric Key Encryption:
Both parties encrypt and decrypt with the same key.
Key length:
Only the key needs to be kept secret for confidentiality.
Exhaustive search: trying all possible keys until success.
3.2: Symmetric Key Encryption Ciphers
Common well-tested ciphers:
RC4, DES, 3DES, and AES.
Other symmetric key encryption ciphers:
IDEA (especially in Europe).
SEED (in South Korea).
GOST (in Russia).
Camellia (in Japan).
Only a few have survived years of cryptanalysis.
3.3: Cryptographic System Standards
Cryptographic Systems:
A packaged set of cryptographic countermeasures for protecting dialogues.
Parties communicating need to use a specific standard.
3.4: The Negotiation Stage
Cipher Suite Options:
A specific set of security methods and options for a particular cryptographic system standard (e.g., SSL/TLS).
Includes methods and options for initial authentication, key exchange, and ongoing message confidentiality, authentication, and integrity.
3.5: Authentication: Supplicant, Verifier, and Credentials
Supplicant:
Wishes to prove its identity.
Credentials are proofs of identity (passwords, etc.).
Verifier:
Tests the credentials; accepts or rejects the supplicant.
Hashing:
A hashing algorithm is applied to a bit string of any length.
The result of the calculation is called the hash.
For a given hashing algorithm, all hashes are the same short length.
Hashing vs. Encryption:
Encryption:
Result length: About the same length as the plaintext.
Reversible: Yes. Decryption.
Hashing:
Result length: Short fixed length regardless of message length.
Reversible: No. There is no way to get from the short hash back to the long original message.
Hashing Algorithms:
MD5 (128-bit hashes) (should not be used because it has been shown to be unsecure).
SHA-1 (160-bit hashes) (should not be used because it has been shown to be unsecure).
SHA-2 (Name gives hash length in bits: SHA-224, SHA-256, SHA-384, and SHA-512).
3.6: The Keying Stage
Two types of ciphers used for confidentiality:
Symmetric key encryption:
The two sides use the same key.
For each dialogue (session), a new symmetric key is generated: the symmetric session key.
Public key encryption:
Each party has a public key and a private key that are never changed.
A person’s public key is available to anyone.
A person keeps his or her private key secret.
Diffie-Hellman key exchange:
Fast public key encryption.
Named for the two creators of public key encryption, who also created this keying method.
3.7: Message-by-Message Authentication
Message-by-message authentication can protect messages against man-in-the-middle and replay attacks.
Man-in-the-middle (MITM) attack:
Form of attack in which an attacker intercepts messages being sent between two parties and forward them on.
Replay attack:
Occurs when an attacker intercepts an encrypted message and transmits it again later.
Electronic signature:
Provides both authentication and message integrity.
Two common types:
Digital signatures.
Key-hashed message authentication codes (HMACs).
True party:
The person the supplicant claims to be.
Cannot use the sender’s public key:
It would always “validate” the sender’s digital signature.
Normally requires a digital certificate:
File provided by a certificate authority (CA).
The certificate authority must be trustworthy.
Digital certificate provides the subject’s (True Party’s) name and public key.
Don’t confuse digital signatures and the digital certificates used to test digital signatures!
Testing the Digital Signature:
The digital certificate has a digital signature of its own.
Signed with the Certificate Authority’s (CA’s) private key.
Must be tested with the CA’s well-known public key.
If the test works, the certificate is authentic and unmodified.
Checking the Valid Period:
Certificate is valid only during the valid period in the digital certificate (not shown in the figure).
If the current time is not within the valid period, reject the digital certificate.
Checking for Revocation:
Certificates may be revoked for improper behavior or other reasons.
Revocation must be tested.
Cannot be done by looking at fields within the certificate.
Receiver must check with the CA.
Verifier may download the entire certificate revocation list from the CA.
See if the serial number is on the certificate revocation list.
If so, do not accept the certificate.
Or the verifier may send a query to the CA.
Requires the CA to support the Online Certificate Status Protocol.
Also Brings Message Integrity:
If the message has been altered, the authentication method will fail automatically.
Digital Signature Authentication:
Uses public key encryption for authentication.
Very strong but expensive.
Key-Hashed Message Authentication Codes
An alternate authentication method using hashing.
Much less expensive than digital signature authentication.
Much more widely used.
Nonrepudiation:
Means that the sender cannot deny that he or she sent a message.
With digital signatures, the sender must use his or her private key.
It is difficult to repudiate that you sent something if you use your private key.
With HMACs, both parties know the key used to create the HMAC.
The sender can repudiate the message, claiming that the receiver created it.
3.8: Quantum Security
Quantum Mechanics:
Describes the behavior of fundamental particles.
Complex and even weird results.
Quantum Key Distribution:
Transmits a very long key—as long as the message.
A one-time key that will not be used again.
A one-time key as long as a message cannot be cracked by cryptanalysis.
If an interceptor reads part of the key in transit, this will be immediately apparent to the sender and receiver.
Quantum Key Cracking:
Tests many keys simultaneously.
If quantum key cracking becomes capable of working on long keys, today’s strong key lengths will offer no protection.
3.9: Cryptographic Systems
Cryptographic Systems
Combine all cryptographic protections, including confidentiality, authentication, and integrity into a single system.
Protect user dialogues from attackers and eliminate the need for users to understand the specific cryptographic details.
Virtual Private Networks (VPNs)
A VPN is a cryptographic system over an untrusted network (the Internet, a wireless LAN, etc.) that provides secure communication.
Host-to-Host SSL/TLS VPN
The PC only needs a browser. Nearly all PCs already have browsers. There is no need to install software on clients. This is the main benefit of SSL/TLS.
SSL/TLS works at the transport layer. Only protects applications that are SSL/TLS-aware (WWW and sometimes e-mail).
IP Security (IPsec) versus SSL/TLS
IPSec offers gold standard cryptographic security protections and supports central management whereas SSL/TLS does not.
IPsec Operation: Tunnel and Transport Modes
Transport Mode
Security within site network and on the Internet.
Significant setup costs: digital certificate, host configuration
Tunnel Mode
Security on the Internet.
No setup costs, no security within site network
Comparing IPsec Transport and Tunnel Modes
Transport Mode
No VPN gateway.
Protection from source host to destination host, including Internet and site networks.
High setup costs (digital certificates for each client).
Bad firewall friendliness (cannot filter encrypted content).
End-to-end security at high cost.
Tunnel Mode
Uses an IPsec VPN gateway.
Protection only over the Internet between IPsec gateways.
Low setup costs (only IPsec gateways need configuration).
Good firewall friendliness (can filter decrypted packets).
Low cost, protects the most dangerous part of the journey.
IPsec security association (SA)
An IPsec security association (SA) is an agreement about what security methods and options the two hosts or two IPsec gateways will use during their communication