1. Principles of Encryption
Michael Jones

2. Introduction to Encryption
Review
Data can be held in three forms:
Plain text: e.g., txt, php, html
Binary: e.g., jpg
Encrypted
Plain text documents are encoded
Encoding: character for character substitution
Because of machine and other requirements
Examples: UTF-8 (ASCII), UTF-16, etc.
Michael Jones
Introduction to Encryption

3. Introduction to Encryption
Also known as: enciphering
Application of: a cipher (algorithm)
Output: ciphertext – (binary) data
Difference between encoding and encryption
Encoding: character-by-character substitution
Encryption: multiple characters processed at a time
Michael Jones
Introduction to Encryption

4. Introduction to Encryption
Key Terms
CIA
Confidentiality: Entitlement to view, modify
Integrity: Is data unchanged?
Authenticity: With whom is the data actually associated?
Availability: who can access the data
CAIN
Non-repudiation: Cannot deny sending or receiving a message
PANIC
Privacy: wider view of availability
Encryption can be used in all aspects
Michael Jones
Introduction to Encryption

5. Introduction to Encryption
Scope
Three scopes relating to encryption
Transit
Data moving around a network
Rest
Data stored on persistent storage
Archive
Long-term data storage
Each has a different set of requirements for encryption
Michael Jones
Introduction to Encryption

6. Introduction to Encryption
Terms in Encryption
Plaintext
The original data
Ciphertext
The encrypted data
Cipher
The algorithm used to encrypt the data
Key
Used for encryption or decryption
Michael Jones
Introduction to Encryption

7. Example – Using OpenSSL
openssl is available as a command on most *nix systems
To encrypt a file using the AES-256-CBC cipher:
openssl enc –e -aes-256-cbc -in a.txt -out b.dat -k password
To decrypt the file:
openssl enc -d -aes-256-cbc -in b.dat -out c.txt -k password
The files a.txt and c.txt should be identical
Michael Jones
Introduction to Encryption

8. Introduction to Encryption
Adding Encoding
The output from the encryption will use all 8 bits in the byte
Making the result unreadable (by an editor)
There may be issues in communication
Adding a ‘-a’ flag encodes the result into Base64
Making the result readable in a text editor
And easier to transmit
Michael Jones
Introduction to Encryption

9. Introduction to Encryption
Origins
Encryption is the product of the application of cryptography
Origins: Greek for ‘hidden or secret writing’
Techniques
Produce ciphertext
Hide in ‘plain view’
Steganography
Early example: slave and tattoo
Example: German Enigma machine
Encoder or encipher?
Michael Jones
Introduction to Encryption

10. Introduction to Encryption
Cracking Codes
A cipher manipulates plaintext into ciphertext
Cracking involves three basic techniques
Reverse engineering
Analysing lots of examples to identify patterns
Forward engineering
E.g., trying all possibilities: brute-force
Social engineering
Gain access to the cipher, keys used
Use influence to dictate the ciphers and keys
Zero day and cracking
Useful time to exploit a cracked (illegally obtained) cipher and key
Michael Jones
Introduction to Encryption

11. Introduction to Encryption
Modern Cryptography
To compensate for increased computational power for ‘crackers’
Ciphers include the use of ‘strong’ keys
Even if the algorithm is known, cracking will not be simple
Cryptanalysis: science of cracking
Objective of cryptography:
Make the effort involved in cryptanalysis greater than the value of that which is being encrypted
Michael Jones
Introduction to Encryption

12. Introduction to Encryption
Pre-computation
The power and memory of modern computers creates the possibility of pre-computing the ciphertexts (hashes) for all possible plaintexts
Example: cracking passwords
Suppose we have access to a ‘users’ table, but all passwords have been encrypted
And we know that the passwords are all 8 digit numbers
And we know the cipher (e.g., MD5)
Michael Jones
Introduction to Encryption

13. Size of Pre-computation Table
8 digits = 10 to the power 8 = 100,000,000
Each entry in the table consists of:
A number – 4 bytes
A hash – 32 bytes
Size required: 36 x 100MB = 3.6GB
What if:
Keys can be variable sized
Keys can include letters and special characters
Michael Jones
Introduction to Encryption

14. Introduction to Encryption
Rainbow Tables
A complete pre-computation table will require too much memory
What is needed is a means to link subsets of possible plaintexts
Then only one of each subset is required
A rainbow table is a means of creating subsets of plaintexts
Using what is called a ‘reduction’ function
Michael Jones
Introduction to Encryption

15. How Rainbow Tables Work
Start with a possible plaintext value:
Using MD5 as the cipher, produces:
25d55ad283aa400af464c76d713c07ad
Now select the first 8 digits –
And compute the ciphertext (hash) again
Repeat while each plaintext value is unique
We only then need to store the first value
Michael Jones
Introduction to Encryption

16. Introduction to Encryption
Notes
A number of sequences will be needed to cover all possible plaintext values
Each item in a sequence must be unique across all sequences
Processing overhead
Michael Jones
Introduction to Encryption

17. Using Salt to Combat the Rainbow
Pre-computed rainbow tables can be found
Theses represent a threat to password protection
Solution: create an additional (random) item
Called an Initialisation Vector (IV)
Use this in the creation of the hash
A rainbow table will be needed for each IV value
In OpenSSL
Add a ‘-salt’ flag to the command line
Michael Jones
Introduction to Encryption

18. Introduction to Encryption
Key Exchange
Two people can exchange a key using a ‘key and box’ metaphor:
A puts a secret message in a box, and locks it. A keeps the key, and sends the box to B.
B receives the box, puts a second lock on the box. B keeps the second key, then sends the box back to A.
A receives the box, and uses his/her key to unlock his/her lock and takes it off, then sends the box back to B.
B can now remove the second lock on the box with his/her key. As there are no longer any locks on the box, B can open the box and access the secret message inside.
Michael Jones
Introduction to Encryption

19. Introduction to Encryption
Types of Encryption
Symmetric key
Block or stream ciphers
Same key used to encrypt, decrypt
Asymmetric key
E.g., Public Key Infrastructure
One key used for encryption, another for decryption
Michael Jones
Introduction to Encryption

20. Symmetric Key Encryption
Block:
Each block is encrypted with a key into a block of the same size
Examples:
Data Encryption Standard (DES)
Deprecated
See also: Triple DES (TDES)
Advanced Encryption Standard (AES)
Stream:
Arbitrary length output
Based on manipulation of internal state
Example: RC4
Block ciphers can be used in stream mode
Michael Jones
Introduction to Encryption

21. Introduction to Encryption
AES Principles
AES is an iterative block cipher with variable length keys, based on the Rijndael algorithm
Winner of a competition organised by US government
Block cipher
128 bits
Key of variable lengths:
128, 192, 256 bits
Iteration
Number of times the algorithm is applied
Michael Jones
Introduction to Encryption

22. Introduction to Encryption
How AES Works (128 bit)
State: 4 x 4 matrix of bytes
Key: 4 x 4 matrix of bytes (if using 128 bit key)
Number of rounds
128 bit: 10, 192-bit: 12, 256-bit: 14
In each round
Generation of a round key
Subsitutions
Shifts of each row a certain number of bits to the left
Transformations on columns
Application of the round key
Michael Jones
Introduction to Encryption

23. Introduction to Encryption
Issues
Single key = single point of failure
Key usage may persist
To avoid problem of managing keys
Michael Jones
Introduction to Encryption

24. Asymmetric Encryption
Basic idea:
One key to encrypt
Different key to decrypt
i.e., a pair of keys
To be used in addition to symmetric key
For reliable transfer of keys
Origin
Diffie-Hellman – mid 1970’s
Also CESG
Michael Jones
Introduction to Encryption

25. Diffie-Hellman Protocol
A and B each have a key pair
Public and private
Each sends the other their public keys
A encrypts the symmetric key using his/her private key, and sends this to B
B decrypts the message using A’s public key
B then sends his/her symmetric key using the same process
Michael Jones
Introduction to Encryption

26. Introduction to Encryption
Issues
Asymmetric encryption is much more computationally expensive
Susceptible to man-in-the-middle attacks
Michael Jones
Introduction to Encryption

27. Introduction to Encryption
Creating PKI Keys
Based on the concept of an inverse function
A ‘trapdoor’
If a function (f1) has an inverse function (f2)
Then: x == f1(f2(x))
Problem: finding the inverse for a given function is computationally prohibitive
Michael Jones
Introduction to Encryption

28. Introduction to Encryption
Basic Principle
The RSA (Rivest, Shamir, Adleman) algorithm is a demonstration of the Diffie-Hellman (Merkle) proposal
Basic elements:
Prime numbers
Modulus arithmetic (remainders)
Michael Jones
Introduction to Encryption

29. Introduction to Encryption
Basic Principle
The 2 people share a (numeric secret)
Computed two ways:
b^c mod a and d^e mod a
The issue is:
Each person must know 2 things
A secret they keep to themselves
Something received from the other person
Michael Jones
Introduction to Encryption

30. Introduction to Encryption
The Process
The 2 people agree to share two prime numbers – e.g., 3 and 5
Each selects a secret number – e.g., A selects 4 and B selects 2
Each calculates 3^(selected number) mod 5
For A: 3^4 mod 5 = 81 mod 5 = 1
For B: 3^2 mod 5 = 9 mod 5 = 4
They tell each other these numbers
Michael Jones
Introduction to Encryption

31. Introduction to Encryption
Process…
Both know the original numbers (3 and 5)
A also knows his/her secret number (4) and the number supplied by B (4)
B also knows his/her secret number (2) and the number supplied by A (1)
Both now calculate the shared secret number:
Supplied number ^ secret number mod 5
For A: 4^4 mod 5 = 1
For B: 1^2 mod 5 = 1
Michael Jones
Introduction to Encryption

32. Introduction to Encryption
Notes
Even if all the numbers are sent in plaintext the secret number cannot be calculated unless one or other of the secret numbers is known
Much larger prime numbers are needed
There is a relationship between the 2 original numbers
For more information search for ‘Diffie-Hellman explanation’
Michael Jones
Introduction to Encryption

33. Introduction to Encryption
Summary
Encryption is the process of producing ciphertext from plaintext
Decryption is the opposite
Cryptanalysis attempts to understand the algorithm (to break it)
Symmetric encryption uses one key
Asymmetric encryption uses 2 keys
Key terms: CAAIN
Michael Jones
Introduction to Encryption