Two data security techniques, a model of a cryptosystem, and symmetric-key encryption with block and stream ciphers.
Ali sends Sara a door code in a group chat that others read. Plan 1 mixes up the code with a secret rule. Plan 2 hides it in a normal sentence. Which plan can hide that a secret exists?
Plan 1, the mixed-up code.
Plan 2, the normal sentence.
No plan. People always notice a secret.
This deck has two parts. Techniques and model: data security techniques, steganography and the security model. Symmetric-key: the types of cryptography and symmetric-key encryption.
Your slides name Crypt analysis, Symmetric-key algorithms, RSA and Hash function here, but do not teach them in this lecture.
You can transform a message to make it secure. Or you can hide the message inside something else.
“Cryptography means secrete writing.”
“It refers to the art of transforming messages to make them secure and immune to attacks.”
“It involves three distinct mechanisms: symmetric-key encipherment, asymmetric-key encipherment, and hashing.”
Secrete: secret. Immune to: safe from. Distinct mechanisms: separate tools. Encipherment: turning a message into secret code.
“Steganography means covered writing.”
“It refers to concealing the message itself by covering it with something else.”
“It is possible to insert secrete binary information into data during digitization process.”
Concealing: hiding. Binary: made of 0s and 1s. Digitization: turning something into digital data.
Objective: “It hides information either for secrecy or to protect copyright, prevent tampering or add extra information.”
Secrecy: keeping something secret. Copyright: the owner’s right over a work. Tampering: changing something without permission.
Text cover: “We can use single space between words to represent bit 0 and double space to represent bit 1.”
Represent: stand for.
Example: “Consider the ASCII code of letter A: 01000001.”
Your slides’ sentence. Each ▽ is one space.
Step 1: read each gap. One space gives 0. Two spaces give 1.
Step 2: the 8 gaps give 01000001: the code of A.
Try: which bits does “Meet▽▽me▽at▽▽noon” hide?
Answer: two, one, two spaces: 101.
“Secrete data can be covered under a color image.”
“Images are made of pixels, each of 3 bytes.”
“In LSB method, LSBs are set to 0s.”
“The ASCII code of the character is inserted in LSBs.”
LSB: least significant bit, the last bit on the right of a byte. Inserted: put in.
Example: “Consider the ASCII code of letter M: 01001101.” “This can be hide in 3 pixels.”
Be hide: be hidden.
010100111011110001010101011001011011110001011110000101010100101001111110Your slides’ 9 bytes, in the order the slide lists them. The marked last bit of each byte is its LSB. our grouping Three bytes per pixel.
Step 1: 3 pixels × 3 bytes = 9 LSBs. The 8 bits of M need 8 of them.
Step 2: the first 8 LSBs read 10110010: M’s code 01001101 from its last bit to its first.
Step 3: the last of the 9 LSBs stays 0.
Note: your slides do not say this order. Ask your instructor which order your exam uses.
Security system
“The two parties (principals) must cooperate for the exchange of the message.”
“A channel is established by the two principals.”
“It is desirable to protect the information transmission from an opponent who may present a threat.”
“All the techniques for providing security have three components.”
Parties: sides. Cooperate: work together. Established: set up. Desirable: wanted. Transmission: sending. Opponent: the enemy side.
Notice: in your slides’ picture, the trusted third party is linked to both principals and to the channel. The opponent is linked to the channel.
Your slides ask: “List the three components of the Security System?” Their answer:
1 “A security-related transformation of information to be sent. Examples include:
2 “Some secret information shared by the two principals and unknown to the opponent. An example is an encryption key.”
3 “A trusted third party for distributing secret information to the two principals.”
Unreadable: cannot be read. Verify the identity: check who it really is. Trusted: relied on. Distributing: giving out.
Generate: make. Distribution: giving out. Specify: state exactly. Protocol: a set of rules the parties follow. Achieve: reach.
our links The arrows to the three components are our links, not your slides’.
Your slides’ picture of a message on its way.
The sender starts with plaintext, the message you can read.
The encryption algorithm uses the key Ke. It gives ciphertext, the mixed-up message.
The ciphertext crosses the network.
The decryption algorithm uses the key Kd. The receiver gets the plaintext again.
Your slides ask: “Differentiate between categories of cryptography Techniques?” Their categories:
Conventional: traditional. Symmetric: the same on both sides. Asymmetric: not the same on both sides. Character: a letter, number or sign.
our link Cryptography’s three mechanisms on your slides name both kinds too: symmetric-key encipherment and asymmetric-key encipherment, with hashing.
For public-key methods, your slides give only the name (Extra)Extra: not in your slidesIn public-key methods, the encryption key and the decryption key are different..
“Ingredients of Symmetric-key Cryptography
Fed into: given to. Scrambled: mixed up so it cannot be read. In reverse: the other way round. Recipient: receiver.
Notice: one secret key feeds both algorithms.
“Properties
Till: until. Referred to as: called. Inverse: the opposite. Deduce: work out.
How to read it: m times (m minus 1), then divided by 2.
Worked example: 4 users, A to D.
Why it works (our reading): each pair of users shares one key. Each user pairs with m − 1 others. Each pair is counted twice, so we divide by 2.
Try: 5 users?
Answer: 5 × 4 = 20. 20 ÷ 2 = 10 keys.
“A block of plaintext symbols of size m (m > 1) are encrypted at a time creating a block of ciphertext of the same size.”
“The block size is usually a multiple of 64 bits.”
“A single key is used to encrypt the whole block even if the key is made of multiple values.”
Symbols: characters or bits. m > 1: m is more than 1. A multiple of 64: 64, 128, 192 and so on. Multiple values: several parts.
Notice: m bits in, m bits out, one key.
“It processes the input elements continuously, producing one symbol (a character or a bit) at a time, as it goes along.”
“There are plaintext stream, cipher stream, and key stream.”
“Stream ciphers are more immune to the corruption of bits during transmission.”
“They are faster than block ciphers.”
“Their hardware implementation is easier.”
Input elements: the pieces that go in. Continuously: without stopping. Corruption of bits: bits damaged. Hardware implementation: building it in hardware.
Notice: a block cipher takes m symbols at a time. A stream cipher gives one symbol at a time.
Ali sends Sara a door code in a group chat that others read. Plan 1 mixes up the code with a secret rule. Plan 2 hides it in a normal sentence. Which plan can hide that a secret exists?
Plan 1 is cryptography: “the art of transforming messages to make them secure and immune to attacks.” The others still see the mixed-up code.
Plan 2 is steganography: “concealing the message itself by covering it with something else”, like single and double spaces.
Steganography hides the message itself, so others can miss it.
If Plan 1 uses symmetric-key encryption, Ali and Sara share the same secret key.
Next: Lecture 6.
Everything you need is in this deck. Each question has one check. Your score waits on the final slide.
Choose one
Pick the best answer.
True or false
Judge one claim.
Fill the gap
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Match
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Sort
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Put in order
Tap the steps in order.
Find the mistake
Tap the wrong part.
Work it out
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Label the figure
Put each label on its number.
1 · Ali hides one character in a color image with the LSB method. What happens to the LSBs before the character’s ASCII code goes in?
2 · An opponent learns the encryption algorithm and the key of a symmetric-key system. What can he deduce?
3 · “In the security system, a trusted third party distributes secret information to the two principals.”
4 · “In symmetric-key encryption, the encryption key Ke and the decryption key Kd are different.”
5 · Component 2 of the security system: some secret information shared by the two principals and to the opponent.
6 · Symmetric-key encryption is also called conventional, secret-key, or -key encryption.
7 · Ingredients of symmetric-key cryptography
8 · The two data security techniques, and steganography’s two covers
9 · Block cipher or stream cipher?
10 · Cryptography or steganography?
11 · Your slides’ four basic security tasks, first to last:
12 · The first component of the security system
13 · Categories of cryptography
14 · In a text cover, a single space stands for bit 0 and a double space for bit 1. Letter B has the ASCII code 01000010. How many double spaces hide it?
15 · For m users, symmetric-key encryption needs [m×(m-1)]/2 keys. How many keys do 6 users need?
16 · Your slides’ model of a cryptosystem. Which part is at each number?
Answer the questions, then come back here.