I have an assignment in which I have to decrypt a text file encrypted with the Vigenere cipher. However, the alphabet used is the entire ASCII table (256 different characters), and I haven't found any material on how to deduce the key for this alphabet (as all material I've found assumes only a limited alphabet is used). Does anybody know how I should approach this problem? An explanation of good algorithms would be appreciated.
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There's a program that encrypts any values. The problem is, I can't understand what the algorithm is.
So, e.g.
input is 1, output is cwjtCNNxuYsB+fns/5h66g==
input is 2, output is UR/EJ8GNC/eG5zFXBwbXDw== and so on.
When the input becomes bigger, the output becomes bigger as well:
input London is the capital of Great Britain, output mnmxU29GVF+e+zn6Y8k246TdbF3wafzl7/ohdgA9KEvZNoLG02JW5HdcwZJNiZmA.
The strange things here are these "+", "/" and "=".
I can't understand how to classify such cipher.
That output (with the "+", "/" and "=") is base64 encoding.
Base64 is an encoding standard that uses a number of ASCII characters to represent binary data, by translating it into a radix-64 representation.
It's widely used to make messages that are encrypted easy to transport over email, WhatsApp, iMessage, etc.
Looking at your examples, they also seem to be encrypted, however, all ciphers will produce larger and larger ciphertext as plaintext input increases.
I expect this is some type of symmetric block cipher.
AES - the Advanced Encryption Standard, has a block size of 128-bits. Ergo, no matter how small the input (even 1 bit) it will be padded to at least 128-bits. Given that your inputs of '1' and '-2' are in fact encrypted to a 128-bit output, I expect this is likely AES.
Indeed "London is the capital of Great Britain" is also a multiple of 128-bits, at 384-bit's of ciphertext.
mnmxU29GVF+e+zn6Y8k246TdbF3wafzl7/ohdgA9KEvZNoLG02JW5HdcwZJNiZmA
->
100110100111100110110001010100110110111101000110010101000101111110011110111110110011100111111010011000111100100100110110111000111010010011011101011011000101110111110000011010011111110011100101111011111111101000100001011101100000000000111101001010000100101111011001001101101000001011000110110100110110001001010110111001000111011101011100110000011001001001001101100010011001100110000000 == 384-bits/128-bit block size = 3 Blocks of data.
I have an API to call where I have to encrypt my data using RSA/ECB/PKCS1 Padding & AES/CBC/PKCS5PADDING.
Sample Data: {"KEY":"VALUE"}
Step.1:
I have to generate a random number of 16 digit. eg: '1234567890123456'
Step.2:
Do RSA/ECB/PKCS1Padding to random number and base64Encode the result. we get "encrypted_key"
Step.3:
Concatenate random number & data:
DATA = 1234567890123456{"KEY":"VALUE"}
Step.4:
Do AES/CBC/PKCS5Padding on DATA (from Step 3) using random number(1234567890123456) as KEY & Base64Encoded random number as IV. we get "ENCRYPTED_DATA"
So, for Step 1 I am using JSEncrypt javascript library.
for Step 4 I am using CrytoJS.AES.encrypt() function. I am pretty sure that my JSEncrypt function is running fine as the client is able to decrypt it but client is not able to decrypt my data. I feel that I am making a mistake while using CryptoJS.
Can someone guide me properly on how to use the library.
What I am doing is:
KEY = '1234567890123456'
IV = MTIzNDU2Nzg5MDEyMzQ1Ng== (result of btoa('1234567890123456') )
DATA = "1234567890123456{"KEY":"VAL"}"
cryptedData = Crypto.AES.encrypt(DATA, KEY, {iv: IV, mode: CryptoJS.mode.CBC,padding:CryptoJS.pad.Pkcs7})
I am told to use PKCS5Padding in AES/CBC Encryption ( Step 4 ) but it seems that AES does not support PKCS5Padding but PKCS7Padding.
I think I am making a mistake in the way I am passing KEY & IV to CryptoJS.
Any help will be greatly appreciated.
For the start lets see why are you doing the exercise. RSA is intended to encode only limited amout of data. So we use "hybrid encryption", where the data are encrypted using a symmetric cipher with a random key and the key itself is encrypted using RSA
Encryption works on binary data, to safely transmit binary data, the data are encoded to printable form (hex or base64)
Step.1: I have to generate a random number of 16 digit
What we see is 16 digits 0-9. That's not really safe. Generating 16 digits you will get a key of 10^16, which is equals of approx 2^53 (if I did the math wrong, please comment).
You need to generate 16 random bytes (digits 0-256 resulting in 2^128 key). That is your DEK (data encryption key).
You may encode the DEK to be in printable form, in hexadecimal encoding it will have 32 characters.
Step.2:
ok, you now get encrypted encoded_encryption_key
Step 3, Step 4
And here you should understand what are you doing.
encrypt DATA using DEK ( not encoded random number in binary form), you will get encrypted_data. You can encode the result to encoded_encrypted_data
concatenate the encrypted key and encrypted data. It. is up to you to choose if you encode it before or after encoding. I suggest you make concatenation of encoded_encryption_key and encoded_encrypted_data with some separator, because if RSA key length changes, the length of encoded_encryption_key changes too
Make sure to discuss with the client what format is expected exactly.
Notes:
IV needs to be 16 bytes long for AES and for CryptoJS I believe it needs to be Hex encoded, so using btoa may not be the best idea. I believe the CryptoJS just trims the value to 16 bytes, but formally it is not correct.
CBC cipher needs some sort of integrity check, I suggest to add some HMAC or signature to the result (otherwise someone could change the ciphertext without you being able to detect the tamper)
but it seems that AES does not support PKCS5Padding but PKCS7Padding.
Indeed AES supports Pkcs7. Pkcs5 is functionally the same, but defined on 64 blocks. The designation is still used in Java as heritage from DES encryption.
I am working on encryption & decryption of data using AES-CCM.
While studying AES, I came across a word called S-Box.
What is S-Box, and the relationship with AES? How can it be calculated? Is it depends on symmetric key or not?
How will cypher text be generated in AES-CCM 128 bit?
The S-Boxes are a system that is used in symmetric cryptographic algorithms to substitute and obscure the relationship between the key and the text that you want to cypher.
You can see more in this article. Here, you have a part:
There are different types of cyphers according to their design [68]. One of these is the Substitution–PermutationNetwork (SPN) that generates the ciphered text by applying substitution and permutation rounds to the original text and the symmetric key to create confusion. To do this, it must be used the Substitution boxes (S-boxes) and Permutation boxes (P-boxes). The S-boxes substitute one-to-one the bits of a block of the input text in the round with bits of the output text. This output is taken as an input in the P-boxes and then it permutes all the bits that will be used as S-box input in the next round.
As #CGG said, S-boxes are a component of a Substitution-Permutation Network. The Wikipedia entry has good diagrams which will help explain how they work.
Think of an S-box as a simple substitution cipher -- A=1, B=2, etc. In an SPN, you run input through an S-box to substitute new values, then you run that result through a P-box (permutation) to distribute the modified bits out to as many S-boxes as possible. This loop repeats to spread the changes throughout the entire cipher text.
In general, an S-box replaces the input bits with an identical number of output bits. This exchange should be 1:1 to provide invertibility (i.e. you must be able to reverse the operation in order to decrypt), should employ the avalanche effect (so changing 1 bit of input changes about half the output bits), and should depend on every bit of input.
I tried to find the list of possible characters that are contained in the encrypted output after AES 256 bit encryption. But, it seems like they are not on the internet? Mind to help? thanks.
The output of an AES cipher is not character data, it is simply bytes. The output should be indistinguishable from random data.
You can represent the output as a string by encoding it as Base64 or Hex if you like.
Well I have been working on an assigment and it states:
A program has to be developed, and coded in C language, to decipher a document written
in Italian that is encoded using a secret key. The secret key is obtained as random
permutation of all the uppercase letters, lowercase letters, numbers and blank space. As
an example, let us consider the following two strings:
Plain: “ABCDEFGHIJKLMNOPQRSTUVXWYZabcdefghijklmnopqrstuvwxyz0123456789 ”
Code: “BZJ9y0KePWopxYkQlRjhzsaNTFAtM7H6S24fC5mcIgXbnLOq8Uid 3EDv1ruVGw”
The secret key modifies only letters, numbers, and spaces of the original document, while
the remaining characters are left unchanged. The document is stored in a text file whose
length is unknown.
The program has to read the document, find the secret key (which by definition is
unknown; the above table is just an example and it is not the key used for preparing the
sample files available on the web course) using a suitable decoding algorithm, and write
the decoded document to a new text file.
And I know that I have to upload an English dictionary into the program but I don't why it has been asked (may be not in that statement but I have to do THAT). My question is, while I can do that program using simple encryption/decryption algorithm then what's the use of uploading the English dictionary in our program? So is there any decryption algorithm that uses a dictionary to decrypt an encrypted algorithm? Or can somebody tell me what approach or algorithm should I use to solve that problem???
An early reply (and also authentic one) will be highly appreciated from you.
Thank you guys.
This is a simple substitution cipher. It can be broken using frequency analysis. The Wikipedia articles explain both concepts thoroughly. What you need to do is:
Find the statistical frequency of characters in Italian texts. If you can't find this published anywhere, you can build it yourself by analyzing a large corpus of Italian texts.
Analyze the frequency of characters in the cipher text, and match it to the statistical data.
The first Wikipedia article links to a set of tools that implement all of the above. You just need to use and possibly adapt it to your use case.
Your cipher is a substitution cipher. That is it substitutes one letter for another.
consider the cipher text
"yjr,1drv2ry1od1q1..."
We can use a dictionary to find the plaintext.
Find punctuation, since a space always follows a comma, you can find the substitution rule for spaces.
which gives you.
"yjr, drv2ry od q..."
Notice the word lengths. Since there only two 1 letter words in the english language the q is probably i or a. "yjr" is probably "why", "the", "how" etc.
We try why with the result
"why, dyv2yw od q..."
There are no english words with two y's, and end in w.
So we try "the" and get
"the, dev2et od q..."
We conclude that the is a likely answer.
Now we search our dictionary for words that start look like ?e??et.
rinse repeat.
That is, find some set of words which fit into the lengths available and do not break each others substitution rules.
Personally I just do the frequency analysis suggested above.
Frequency analysis, as both other respondents said, is the way to go, and you can use digrams and trigrams to make it much stronger. Just grab tons of Italian text from the web and churn ahead! It's really pretty simple programming.