Information Systems Security Cryptography Domain #3.
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Transcript of Information Systems Security Cryptography Domain #3.
Cryptography Now and Before
In the past – mainly used for confidentiality Today
– Still used for confidentiality– Data integrity– Source authentication– Non-repudiation
Definitions
Cryptography– Hiding the meaning of communication
Cipher– Transforms characters or bits into an
unreadable format Cryptographic Algorithm
– Procedure that turns readable data into an unreadable format (usually through mathematical formulas
More Definitions
Cryptanalysis– Science of studying and breaking encryption
mechanisms Cryptology
– Study of cryptography and cryptanalysis Key Clustering
– When two keys generate the same ciphertext from the same plaintext
MORE Definitions
Cryptosystem– Encompasses all of the components for encryption and
decryption Plaintext
– Readable format / decrypted Ciphertext
– Unreadable format / encrypted Work Factor
– Time, effort, and resources necessary to break a cryptosystem (should be too high for compromise)
Symmetric Cryptography
Two instances of the same key– One key used for encryption and decryption– Sender/receiver use same key (public/shared)
Key distribution a problem Secure 2nd channel needed or offline Does not implement non-repudiation
Historical Symmetric Keys
Hieroglyphics– Recorded in use in 2000 B.C.– 1st known type of cryptography– Only certain people knew the symbols– Used to glorify the life of deceased
Scytale Cipher
Paper or leather was wrapped around a rod Message was written on paper and
delivered by messenger Sender and receiver had to have rod of
same diameter Was used around 400 B.C. Transposition Cipher
Caesar Cipher
Symmetric cipher Keys shifted ‘n’ number of letters Also referred to as a shift cipher If one alphabet is used – monoalphabetic
substitution If two alphabets are used – polyalphabetic
substitution
Vigenere Cipher
Polyalphabetic substitution cipher Proposed by court of King Henry III An extension of the Caesar Cipher Strength is no frequency analysis can be
performed
Enigma Machine
Rotor cipher used polyalphabetic substitution
Employed in WWII Symmetric – original setting of rotors and
how rotors moved must be the same Sender entered characters and the rotors
moved
Vernam Cipher
One-time pad– Only method that is completely secure– Still used today for backups encryption method– Senders uses one-time pad to encrypt and
receiver uses twin to decrypt
Vernam
Deemed unbreakable if:– Pad is truly random values– Used only once– Securely distributed– Securely stored– Pad is at least as long as message
Key & Algorithm Relationship
Key – long string of values Algorithm – group of mathematical
equations that can be used for the encryption process
Used together – key values are used by the algorithm to indicate which equations to use, in what order, and with what values
Breaking Cryptosystems
Brute Force– Attempts all possible combinations of a given key space
to derive the key Takes ciphertext, applies key, and sees if understandable
plaintext is derived
– How many possible keys? 40-bit key = 1 trillion 56-bit key = 72 quadrillion
– Easily breakable due to Moore’s Law– In 1998, broke DES in three days with 1536 PCs
running at 40 MHz
Frequency Analysis
Patterns in ciphertext are identified and matched back to plaintext
For example, the most common letters in the English language are “iron seat”
Allows attackers to reverse-engineer encryption process
Characteristics of Strong Algorithms
Confusion– Complexity of the process to increase the workfactor of
reverse engineering– Interceptor should not be able to predict what changing
one character will do to ciphertext
Diffusion– Component going through an encryption should have
MANY things take place– Change should affect many part of the ciphertext
Kerckhoff’s Principle
Only secrecy involved with cryptography should be the key
Should not base security on figuring out the algorithm
Algorithms should be publicly known Government does NOT agree
Type of Ciphers Used Today
Substitution Methods Transposition Methods Symmetric Ciphers
– Block– Stream
Asymmetric Ciphers
Encryption/Decryption
Substitution– Substitute one bit for another– Destination has to have the correct key to
indicate how to substitute Transposition
– Bits are moved to new place in stream– No new bits are introduced– Destination must have correct key to
unscramble
Symmetric – Block Cipher
Message is divided into blocks and put through mathematical function
Each block is encrypted separately While message is not encrypted as one
entity Best used in software implementation
S-Box
Substitution box is common component Table where a lookup using a few bits as an
index yields some other bits For example, in an 8 by 32 s-box, you enter
with 8 bits and come out with 32 DES uses 6 by 4 boxes Blowfish uses 8 by 32 boxes
Symmetric Stream Cipher
Encrypts individual bits of the message Bits are X-ORed with a bit from message Stream algorithms have keystroke
generators Best used in hardware implementations Caesar cipher is an example
Message Authentication Code (MAC)
MACs are computed and verified with same key Four types
– Unconditionally secure One time pad
– Has function-based MAC (HMAC) Uses key with hash function (MD5)
– Stream – broken into two substreams– Block – encrypt message and output final block of
ciphertext as checksum (DES)
Digital Signatures
Provide Authenticity and Non-Repudiation– After hashed, MD value is encrypted with
sender’s private key– Receiver validates the digital signature by
decrypting it with the sender’s public key– Provides data integrity, authenticity, and non-
repudiation
Digital Signature vs. MAC
Symmetric Cryptography– MAC = hash + symmetric key
Asymmetric Cryptography– DS = hash + asymmetric key
US Government Standard
Digital Signature Standard (DSS)– Secure hashing algorithm (SHA) must be used
for message digest creation.– DSA, RSA, and ECDSA asymmetric algorithms
can be used for digital signature creation ECDSA = elliptic curve digital signature algorithm
Symmetric Algorithms
Data Encryption Standard (DES) – Break work in half and XOR several times. Became standard in 1977.
Triple DES – Encrypted/decrypted with 3 separate keys. Strength of 2 112.
Advanced Encryption Standard (AES) - Adopted in 1997 by NIST. Had larger block size then DES
Advantages of Symmetric
Very fast and secure method for confidentiality
Implemented in either hardware or software Usually available at no cost to user
Disadvantages
Not able to provide non-repudiation Can not provide access control or digital
signatures Need to share key
Security in Hashing
Strength of Hashing Algorithm– The hash should be computed over the entire
message– Messages cannot be disclosed by MD value– Different messages should generate different
MD values Collision free Resistant to birthday attacks
Hashing Issue
It is easier to find 2 messages that have the same MD than looking for one particular MD value on a message
Hashing value (n) Brute force to find hash value (2n) Brute force to find any 2 matching hash values 2(n/2)
Crux – A hashing algorithm that generates a larger MD value is less vulnerable to a bday attack than one that creates a smaller one
Key Management
Responsibilities– Secure key creation and distribution– Secure key recovery– Secure key storage and destruction
Characteristics– Split knowledge and control– Length of key– Never available in clear text
Asymmetric Key AlgorithmsPublic Key Cryptography
No key exchange needed Users can generate their own public/private
key pairs and exchange them If you receive a public key from Bob, how do
you know it is really Bob? Need a trusted third party to vouch for the
identity of the owner of a public key
Asymmetric Keys
Allows non-repudiation and access control Extremely slow Output may be much larger than the
plaintext Hashing Integrity Digital Signatures
CA and RA Roles
Registration Authority– Accepts registration requests from users– Validates users identities– Passes request to CA
Certificate Authority– Creates digital signature– Binds identity to signature– Maintains certificate during lifetime
Verisign, Thawte, IP, Belsign, etc.
Example
Bob wants to participate in a PKI– Bob send request to RA– RA validates Bob’s identity– RA sends request to CA– CA generates certificate and sends to Bob– Bob’s identity is now bound to the public key
that is embedded in the digital certificate
Components of PKI
Certificate Revocation Lists (CRL)– Certificates can become revoked– CRL is list of revoked certificates signed by CA– Method to tell others not to trust (compromised)
Certificate Directory– Storage of certificates– Usually publicly accessible– Each certificate is digitally signed
Steganography
Hiding information in media– No algorithm, key or encryption required– Hides data– Digital watermarks are used to detect– Messages can be sent without knowledge– Specialized tools to carry this out
Hide and Seek Steg Detect
Email Standards
Pretty Good Privacy (PGP)– Free email client that provides security– Uses passphrases instead of passwords– Web of trust instead of hierarchy of CAs
PGP provides confidentiality via IDEA Integrity via MD5
Secure Protocols
Secure Hypertext (SHTTP)– Protects each message– Older less used
HTTPS– HTTP runs on top of SSL
Provides secure communication channel All messages are protected
Secure Socket Layer (SSL)– Netscape developed; requies PKI– Works at transport layer
Secure email Standard
Secure MIME (SMIME)– Secure Multipurpose Internet Mail Extension– Allows for encryption, hashing, and digital
signatures to take place in a uniform manner– Email client vendors follow this standard
Transparent between different email clients
Secure Electronic Transaction
SET– Developed by Visa and Mastercard for more
secure monetary transactions over the Internet– Uses PKI to protect sensitive data and
authenticates each ‘hop’ in the transaction Slow in acceptance and deployment Goal to replace SSL Slow the throughput of the transaction
Network Layer Protection
IPSec– Developed because IPv4 has no security– Sets up a secure channel between computers
instead of between applications Application secure channels provided by SSL
– Network layer security– Provides host-to-host, host-to-subnet, and
subnet-to-subnet connections
Key Issues with IPSec
Internet Key Exchange (IKE)– Used to negotiate the key exchange between two
entities– Suite of ISAKMP and Oakley protocols
Internet Security Association and Key Management Protocol (ISAKMP)– Framework for key negotiation
Oakley Protocol– Negotiates key information using Diffie-Hellman
algorithm
IPSec Modes of Operation
Transport Mode– Offers payload protection– Protects header up to the network layer
Tunnel Mode– Protects the whole packet– Includes payload and all headers– Creates a new IP header`
Attacks on Crypto
Ciphertext-only Attack– Attacker only has ciphertext– Goal to reverse the encryption process
Known plaintext attack– Attacker has ciphertext and plaintext
Chosen-plaintext attack– Attacker can choose what plaintext is encrypted
Chosen-ciphertext attack– Attacker can choose ciphertext to be decrypted
More Attacks
Replay Attack– Attacker obtains a set of credentials and sends them to
authentication source– Timestamps and sequence numbers protect against this
attack
Man in Middle Attack– Attacker injects itself between two users and reads
messages– Sequence numbers and digital signatures protect
against this attack