- Cracked via frequency analysis - Most common English letters are E, T, A, O, N, R, I, S, H. Attackers find the common substitutions and experiment until they can discern the message.
Crypto Basics
- Goals of Crypto
- Confidentiality
- Preservation of secrecy of stored information
- Symmetric crypto systems use a secret key shared by all users
- Asymmetric crypto systems use individual combos of public and private keys for each user
- Data in transit aka data on the wire - exam terms, same thing
- Message digests - aka digital signatures, created when message is transmitted, used to ensure data wasn’t altered
- Challenge-response authentication
- Prove that one can encrypt/decrypt something to validate identity
- Only offered by asymmetric systems
- Assurance that the message originated by the sender, not someone pretending to be the sender
- Plaintext - Before a message is coded
- Ciphertext - After a message is encrypted
- Keys - Used in encryption calculations.
- Key space - The range of values that are valid for use as a key in a specific algorithm, aka bit size
- Kerckhoff’s Principle - A Cryptographic system should be secure even if everything about the system, except the key, is public knowledge
- Cryptovariable - Sometimes used to refer to keys
- Cryptography - Implementing secret codes and ciphers
- Cryptanalysis - The study of methods to defeat codes and ciphers
- Cryptology - Cryptography and cryptanalysis together
Cryptographic Mathematics
- Boolean Math - Logical Operations
- AND - Represented by the ^ symbol, checks whether two input values are both true
- OR - Represented by the v symbol, checks whether at least one input value is true
- NOT - Represented by the ! symbol, reverses the value of an input variable
- XOR - Represented by the ⊕ symbol, returns true when only one of the input values is true
- M of N control - Requires a minimum (M) number of the total agents (N) work together to perform a high-security action
- Time and effort required to complete a brute-force attack
- Work function only needs to be slightly greater than the time value of the asset
Codes vs. Ciphers
- Codes - Crypto systems of symbols that represent words or phrases, sometimes secret, not always meant to provide confidentiality
- Ex: “The eagle has landed”
- Ex: The transformation of plaintext to ciphertext
Transposition Ciphers
- Transposition cipher - Use an encryption algorithm to rearrange the letters of a plaintext message to form ciphertext
- Can used a keyword to perform a columnar transposition
Substitution Ciphers
- Substitution cipher - Use an encryption algorithm to replace each character or bit of the plaintext with a different character
- Vigenère cipher - Uses a chart of the alphabet shifted once per line, then a key is used to decrypt
One-Time Pads
- One-time pads - Use a different substitution alphabet for each letter of the plaintext message
- AKA Vernam ciphers
- One-time pad must be randomly generated
- Most be physically protected against disclosure
- OTP must be used only once
- Key must be at least as long as the plaintext
- When used properly, OTP is unbreakable - no repeating patterns
Running Key Ciphers
- Running key cipher - aka a book cipher
- Key is as long as the message itself, often chosen from a common book
Block Ciphers
- Block ciphers - Operate on chunks or blocks of a message
Stream Ciphers
- Stream ciphers - Operate on one character or bit of a message (or data stream) at a time
Confusion and Diffusion
- Confusion - Occurs when the relationship between plaintext and the key is too complicated for an attacker to altering the plaintext and analyzing the ciphertext to determine the key
- Diffusion - Occurs when a change in the plaintext results in multiple changes throughout the ciphertext
- Substitution introduces confusing
- Transposition introduces diffusion
Modern Crypto
- Crypto keys
- Kerckhoff’s Principle - Opening algorithms to public scrutiny actually improves their security
- Modern cryptosystems rely on secrecy of one or more cryptographic keys
- AKA secret key and private key crypto
- Rely on a shared secret
- Weaknesses
- Need to distribute the key - See Diffie-Hellman
- Does not implement nonrepudiation
- Algo doesn’t scale well - secure private comms between individuals can only be achieved if every possible combo of users has their own shared key
- (n * (n - 1)) / 2 = number of keys needed
- AKA public key algos
- Each user has two keys
- Public key - Shared with all users
- Private key - Kept secret, known only to the user
Symmetric Cryptography
- Data Encryption Standard
- No longer considered secure
- Superseded by Advanced Encryption Standard (AES)
- Uses a long series of XOR operations, repeated 16 times (aka 16 rounds of encryption)
- 56 bit key size
- Electronic Code Book Mode - ECB
- Weakest mode
- 64 bit blocks processed
- The same block of input produces the same encrypted block
- Only for exchanging small amounts of data
- Each block of plaintext is XORed with the preceding block of ciphertext before it’s encrypted with DES
- CBC has an initialization vector and XORs it with the first block of the message - IV must be sent to recipient
- Errors propagate
- Streaming cipher version of CBC
- Operates against data produced in real time
- Uses a memory buffer instead of block size
- Uses an IV and chaining
- Almost the same as CFB, except instead of XORing an encrypted version of the previous ciphertext, it’s XORed with a seed value
- Still uses an IV to create the seed value
- Future seeds are derived by running DES on previous seed
- No chaining function, so transmission errors don’t propagate
- Stream cipher similar to CFB and OFB
- Uses a simple counter that increments for each operation
- Errors do not propagate
- Well suited for use in parallel computing
- Effective key = 64 bit
- 64 bit block size
- Four versions
- Encrypt plaintext 3 times using 3 different keys
- Effective key size of 112 bits
- Key is broken up in a series of operations into 52 16-bit subkeys
- Same 5 modes as DES
- Used in PGP - Pretty Good Privacy
- 64-bit blocks of text
- Keys between 32 bits and 448 bits
- Much faster than DES and IDEA
- Released for public use with no license
- US Government
- 64-bit blocks of text
- 80-bit keys
- Clipper and Capstone encryption chips - just know these are related to Skipjack
- Supports escrow of encryption keys
- Not widely embraced because of mistrust of escrow processes within US government
- Block sizes - 32, 64, 128 bits
- Key sizes - between 0 and 2040 bits
- 128-bit keys require 10 rounds of encryption
- 192-bit keys require 12 rounds of encryption
- 256-bit keys require 14 rounds of encryption
- Prewhitening - XORing the plaintext with a separate subkey before the first round of encryption
- Postwhitening - Uses a similar operation after the 16th round of encryption
There is a table (6.2) that I won’t reproduce here that you should familiarize yourself with before your exam
Symmetric Key Management
- Creation and distribution
- Offline - physical distribution
- Use public key encryption to setup an initial communications link, exchange a secret key over the secure public key link
- Diffie-Hellman
- Never store keys on the same system as encrypted data
- Sensitive keys can be split up and then split knowledge can be used
- Fair cryptosystems - Secret keys used in communication are divided into two or more pieces, each of which is given to an independent third party
- Escrowed encryption standard - Provides the government with a technological means to decrypt ciphertext - used in Skipjack
Cryptographic Lifecycle
- Moore’s Law - except One-Time Pad, all crypto systems have a limited life span
- A cited trend in the advancement of computing power that states the processing abilities of a microprocessor will double approximately every two years
- Not a law, just a previous trend
- Means what is hard to crack today might not be hard to crack tomorrow - especially with quantum computing
- Encrypt plaintext 3 times using 3 different keys
- Confidentiality