SHA256 and Symmetric Encryption Examples

Summary

This video tutorial covers two main questions. The first involves using Python's hash library to find an English word with a specific SHA-256 hash value. It guides viewers to use a provided dictionary file, strip newline characters, and loop through words to match the hash. The second question tackles decrypting cipher text generated by a symmetric encryption algorithm. The video explains the decryption process, which involves XORing each block of the cipher text with the key, and emphasizes the importance of key length for security. Additionally, it suggests an exhaustive search method to find both the key and the encrypted word from a given ciphertext.

Takeaways

• 🔐 **Hashlib Usage**: The video discusses using Python's hashlib library to find an English word with a specific SHA-256 hash digest.
• 📄 **Dictionary Utilization**: A 'dictionary.txt' file is provided, containing a list of meaningful English words to be used for hash matching.
• 🔍 **Word Processing**: The script explains how to process each word in the dictionary by removing newline characters.
• 🔢 **Hash Calculation**: It details the process of calculating the SHA-256 hash of each word and converting it to a hexadecimal digest.
• 🔎 **Target Matching**: The goal is to find a word whose hash matches a given target hash value.
• 🔄 **Decryption Function**: The video explains how to write a Python function to decrypt a cipher text generated by a symmetric encryption algorithm.
• 🔑 **Key Manipulation**: It describes how the encryption algorithm uses an 8-bit key to create a 32-bit extended key by repeating the key four times.
• 🔡 **Block Conversion**: The script mentions converting blocks of text into 32-bit integers for encryption and decryption processes.
• 🔐 **XOR Operation**: The exclusive OR (XOR) operation is central to both the encryption and decryption processes.
• 🔍 **Exhaustive Search**: For the security attack example, the script suggests an exhaustive search through all possible keys (0-255) to find the correct decryption.

Q & A

What is the task described in the first part of the video?

The task is to use Python's hash library to find an English word whose SHA-256 hash digest matches a given decimal value.

What hint is provided for finding the word in the first task?

The hint is to use the 'hashlib.sha256()' function in Python to calculate the hash digest of words from a provided dictionary.

How is the dictionary of words accessed according to the video?

The dictionary is accessed by reading a file named 'dictionary.txt' which contains a list of meaningful English words.

What preprocessing is done to each word read from the dictionary?

Each word is stripped of newline characters, particularly by removing the last character which is a newline.

What is the formula used to calculate the hash digest of a word?

The formula used is 'hashlib.sha256(word.encode('ascii')).hexdigest()' which calculates the SHA-256 hash digest of the word in ASCII encoding.

How is the matching word identified in the first task?

A loop is used to iterate over each word, calculate its hash digest, and compare it to the target hash digest to find a match.

What is the second task discussed in the video?

The second task is to write a Python function for decrypting a cipher text generated by a specified symmetric encryption algorithm.

How is the encryption algorithm described in the video?

The algorithm breaks the message into four-character blocks, converts each block to a 32-bit integer, and then XORs it with an extended key to produce the cipher text.

What is the decryption process as described in the video?

Decryption involves XORing the cipher text with the same key used for encryption to retrieve the original message.

What is the significance of the exclusive OR (XOR) operation in the encryption and decryption process?

The XOR operation is used because it is reversible; applying it twice with the same key returns the original value, which is essential for both encryption and decryption.

What is the approach to find the key and word if only the cipher text is known?

An exhaustive search is performed over all possible keys (0 to 255) to decrypt the cipher text and match the result with a list of English words to find the original word and key.

Outlines

00:00

🔐 Hashing Words with Python

The speaker introduces a Python programming exercise involving the use of the hashlib library to find an English word with a specific SHA-256 hash digest. They provide a dictionary file 'dictionary.txt' containing a list of meaningful English words and instruct the audience to read this file to create a list of words. Each word must be stripped of newline characters, particularly the last character which is a newline. The target is to find a word whose SHA-256 hash digest matches a given long decimal number. The formula for hashing involves converting the word to its ASCII representation, hashing it with hashlib.sha256(), and then ignoring the 'ignore' part of the hash to get the hexadecimal digest. The process involves looping through each word, hashing it, and comparing it to the target hash value. Upon finding a match, the word is returned as the result.

05:04

🔄 Decrypting Symmetric Encryption

The second paragraph discusses a Python function for decrypting cipher text generated by a symmetric encryption algorithm. The encryption process involves taking a message, breaking it into four-character blocks, and converting each block into a 32-bit integer. An extended key is created by repeating an 8-bit key four times, shifting it by 8, 16, and 24 bits respectively, and then XORing each block with this extended key. The result is converted to hexadecimal, and any '0x' prefix is removed. To decrypt, the process is reversed by XORing the encrypted block with the key again, which yields the original block due to the properties of XOR. The function takes the cipher text and the key as inputs and returns the decrypted plain text. The speaker also touches on the security implications of short keys and the potential for exhaustive key searches.

10:04

🔍 Finding the Key and Decrypting

The final paragraph addresses a more complex challenge where the goal is to find both the encryption key and the original word from a given cipher text. The speaker suggests an exhaustive search method, iterating through all possible keys (0 to 255) and using the previously mentioned decryption function to test each key. If the decrypted text matches any word in the 'stripped Words' list (a list of English words without newlines), the correct key and word are found. The speaker reassures the audience that if the decryption function is implemented correctly, this process will yield the correct results. They also recap the steps for the first question, emphasizing the need to read the dictionary, loop through each word, and compare their hash values to the target.

Keywords

💡hashlib

hashlib is a Python library used for generating hash values. In the video, it is used to find an English word that matches a given SHA-256 hash digest. The script mentions using hashlib to compute the hash of each word in a dictionary to find a match for the target hash value.

💡SHA-256

SHA-256 is a cryptographic hash function from the SHA-2 family. The video discusses using SHA-256 to generate a hash digest of a word in hexadecimal format. The task is to find a word whose SHA-256 hash matches a given target hash.

💡hexadecimal digest

A hexadecimal digest is a hash value represented in hexadecimal format. The video script instructs to ignore the hexadecimal digest and instead focus on the decimal digest when comparing hash values.

💡dictionary.txt

dictionary.txt is a file mentioned in the video that contains a list of meaningful English words. It is used to search for a word whose hash matches a given target hash value.

💡strip

In the context of the video, 'strip' refers to the action of removing newline characters from each word read from the dictionary.txt file. This is necessary to clean the data before processing.

💡loop

A loop is a programming construct used to iterate over a collection of items. The video describes using a loop to go through each word in the list to find a match for the target hash value.

💡hash value

A hash value is the result produced by a hash function. In the video, the hash value is used to identify a word that matches a specific SHA-256 hash digest.

💡encryption algorithm

The encryption algorithm discussed in the video is a symmetric encryption function that encrypts messages using a key. The script describes how the algorithm breaks messages into blocks and applies the encryption process.

💡decrypt function

The decrypt function is a part of the video's discussion on reversing the encryption process. It is used to convert encrypted text back into its original form using the same key that was used for encryption.

💡XOR

XOR, or exclusive or, is a bitwise operation used in both the encryption and decryption processes described in the video. It is used to combine the message block with the extended key to produce the encrypted text, and then again in reverse to decrypt it.

💡ciphertext

Ciphertext is the encrypted form of plaintext. The video script includes a task to implement a function that takes ciphertext and a key to return the original plaintext.

💡exhaustive search

Exhaustive search is a method of trying all possible keys to decrypt a ciphertext when the key size is small. The video mentions using exhaustive search to find the key and the original word from a given ciphertext.

Highlights

Introduction to the video's content about questions and homework.

Explanation of using Python's hashlib library to find a meaningful English word with a given hash.

Use of hint 'hashlib.sha256' to find the hash of a word.

Requirement to use a dictionary of meaningful English words for the task.

Instructions on how to read and process the 'dictionary.txt' file.

Details on creating a list of words and removing newline characters.

Description of the target hash value and the process of matching it with word hashes.

Formula for calculating the hash value of a word using hashlib.sha256.

Process of looping over words to find a match for the target hash value.

Method to return the word that matches the target hash as the result.

Introduction to the second question about decrypting cipher text.

Explanation of the symmetric encryption function and its process.

Details on breaking the message into four-character blocks for encryption.

Process of creating an extended key by repeating the original key four times.

Description of the XOR operation used for both encryption and decryption.

Instructions on implementing the decrypt function for the given encryption algorithm.

Explanation of the decryption process by XORing the key with each piece of cipher text.

Description of how to handle the leading zeros in the encrypted blocks.

Final step of returning the decrypted plain text.

Introduction to the third question about finding the key and word from a ciphertext.

Explanation of the exhaustive search method to find the key and word.

Details on the creation of the decrypt function and its usage in the exhaustive search.

Final advice on the importance of secure encryption and the weakness of short keys.

Transcripts

hello in this video I'm going to briefly

talk about

uh the questions and the homework

the first question I want to talk about

is

uh this one it says using the Python's

hash lib Library find a meaningful

English word who's asking coding has the

following shot 256 has a decimal digest

and it says basically as a hint use

hashlip.shaw uh word that in kodaski and

ignore that hex digest to get the

hexadecimal

Digestive and asking encoding of a given

word

so in order to

find a meaningful word you need to have

a list of all words I have posted a

dictionary.txt which is the list of all

the meaning rewards in English assuming

you have that one and you have it in

your

python project then you have it

available you can open the

actual file read that

read all the lines

and then in each line you're going to

see one word so basically you can create

and list of a strict words and for every

word in words you can strip them new

line

um out of the world and you can drop the

last letter by doing word called a

negative one that's going to drop the

last character of the award which is the

new line because in the dictionary.txt

as you saw every

line

belongs to one

word and there's a new line at the end

this operation is going to drop the last

character

and after that what you're gonna do is

you're going to

use the Target that is given which is

this pretty long

uh 256 uh

number in his decimal and uh

try to find out which meaningful word

has this hash digest or this hash value

in order to do that the best way is to

Loop over every single word in the strip

words array this one is gonna made of

is stewards array is made of

90 some thousand meaningful words

without a new line at their end that's

what I call the surfboard and uh

when you look over every word in this

list

try to find out if the target matches

any of the awards hash value calculated

by this formula the formula should be

SDR which means the string

representation a hash leap.shot 256 in

parentheses word that encode ASCII and

ignore dot hash dot hex Digest

and uh

basically in this

formula hex digest Returns the hex value

so you first calculate the shot 56 hash

value and then you convert it to hex and

see if that's equals the target when you

find the word whose hash is equal to

Target you return that as the result and

you submit your assignment this way

another question is that is tricky is

question three which is asking you to

write down a function a python function

for the decrypting

a cipher text

that is generated by the following

encryption algorithm the symmetric

encryption function we have

you can I mean the the encrypted

function we have here basically gets a

key which is a 8-bit integer it's going

to be a integer from two to zero to 255.

and uh as you see I have it here it

basically

gets a message breaks it into four

character pieces or four character

blocks then it's gonna call the make

block on every four characters this

Matlock is going to concatenate all the

four characters into one 32-bit integer

because every character is eight bits

long when you concatenate eight four

eight bits it gives you a 32-bit integer

and uh you don't need to be worried

about this part this is already done you

need to take care of the decryption

decryption is the reverse operation and

basically uh after it breaks the message

into four bits four character pieces or

four character blocks

and it applies the make lock on it to

convert every four character into one

32-bit integer it's going to basically

create an extended key using the same

Key by repeating that key four times

because the key is a bit long when you

repeat that

G four times you know

line you're going to get a 32-bit number

there is the way it's

repeats the same key four times is to

First add the key by the key after if

after it shifts at 8 bit to the left and

then adds it by the same key after it

shifts at 16 bit and added by the same

key after it shifts at 24 bits this way

is going to create the 32-bit integer

which is made of four identical quarters

and each quarter would be equal to the

key and after extended key is created

and the number is created all it does it

um

converts

the block which is uh going to be the

um

one of the blocks that we created before

it was an integer uh by it's gonna

cover the existing more the converts the

following to his decimal the block

exclusive or this carrot sign is

exclusive or with the extended key so in

other words it xores every block with X

and the key

um so basically we xor 2 hexadecimal two

32-bit numbers and then it's going to

convert it to hexadecimal and then

converts it to a string when you convert

his decimal to a string there would be a

zero X this two the colon is going to

drop that 0x because we don't like the

zero X we know what it's already X so we

don't want to keep the zero X which

emphasizes this is a hexadecimal so we

already know it so leave that

out of the encrypted parts and this is

going to be the the encrypted version of

that block

then we make sure that the length of the

encrypted block is not less than eight

uh hexadecimal digits because

uh the encrypted block is the encryption

of 32 bits which means eight hexadecimal

digits if the result of encryption is

less than eight digits less than here's

the small digit we're going to pair it

with leading zeros to a Shore

that the the overall length uh would be

the same the overall length of the

decrypted

ciphers to text is the same as the

actual

um

message blocks uh

uh that we created in this for Loop so

um anyways after it's

checks whether or not the length is less

than eight or not it's going to add

leading zeros if the length is less than

eight

um and may call the

um

encrypted P says eight digit longer it

takes this decimal digit launch and at

the end every encrypted block is going

to be added with the return value and at

the end we're going to return the return

value

the first part of this question says

implement the decrypt function for this

uh encryption so that it gets a

ciphertext and it returns to playing

text as output so basically we need to

create a function like this the cipher

is the first parameter the key is the

second parameter this is an 8-bit number

this can be any string what we're gonna

do is we're gonna break the cipher into

multiple eight bits pieces or epic

blocks you can also break it into 32-bit

pieces but because key is eight bits you

don't need to break it into 32-bit

pieces you can just break it in the same

size as the key and uh basically

exclusive or the key with each piece

and this is going to give you the

original value you may say how is it

possible well I'm just going to briefly

mention you know so you understand if a

exclusive or B is equal to C

imagine this is the plain text

this is the key and this is uh the the

cipher text

then I can say

be exclusive C is equal to a

and you can prove that because because

of the properties of exclusive

so because of this

um implication

I can say when I want to do the

decryption I do the same xor that I did

before so for encryption we do xor for

decryption we do X1 as well so that's

how the that's the nature of exclusive

and that's pretty much it after you do

the exclusive or for each piece

uh you exclusive the key with each piece

you get the result of exclusive or and

append the result of exclusive or with

the return value return value is an

empty string at the beginning but as you

go forward you're going to build that

return value gradually at the end you're

going to return the whole return value

return value basically begins with an

empty string and a

we gradually

decrypt every uh block and appended to

the return value at the end you get the

whole decrypted plain text and that's

the question that's the answer to the

question 3.8 for 2. B uh the question

says if we know that the following

ciphertext is the result of encrypting a

single meaningful English word with some

key find the key and find the word in

this case we have two unknown things one

is the key one is the

uh the word itself but there's nothing

to be worried about and then the

the ciphertext is given right so this is

kind of an a security attack that you

should always be aware and you know

avoid uh the security attack basically

does an exhaustive search on all the

possible keys because the key is pretty

short it's a very very you know weak key

and therefore it's a very weak

encryption algorithm

um

if you want to make it stronger you get

a elongate the length of key and make it

longer and longer but eight bit is

pretty simple I'll show you how to break

it uh basically you go every key

possible so you go for key from 0 all

the way to 255 for key in range 256 you

get the following you say if the Crypt

of ciphertext and key

the decrypt is the same function we call

there we created their uh I I didn't

create it but I'm going to show you the

creation of it but I just told you that

what you need to do is these three steps

should you follow it so if you've called

this function if you created correctly

create that function and then you call

it on ciphertext and key for every key

from 0 to 255 and see if the result is

in their stripped Words which is the

same list that I showed you this is a

list of all English

words

you get a print the key followed by the

decrypted

ciphertext with the key and that's gonna

generate the output if you make the

decrypt function correctly this uh for

Loop is going to answer the question

trippy hope you understand

um

how to uh do the rest basically you need

to implement the decrypt function and

um yeah that's pretty much all you need

to do

for this question for the first question

again as I said

uh first part you need to read the

dictionary and then you need to go over

every word in the dictionary to see if

the target matches any of the words

hash value calculated by the following

formula the formula was given as well

all right

have a good day