One time pad used twise

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In summary, the speaker is asking for help with a challenge involving two ciphertexts encrypted with the same one-time pad. The ciphertexts start with the same message header and are split into the same number of blocks. The speaker has tried XOR-ing the ciphertexts but is unsure of what to do next. They mention a hint from someone who has solved the cipher, which involves the time at which the ciphertexts were sent being prime numbers. The speaker speculates that the RSA algorithm may be involved.
  • #1
everQuest
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Hi everyone

I've been given a challenge and i was wondering if you guys could shed some light on it...

i have been given two ciphertexts that have been encrypted with the same one time pad... the ciphertexts start the same (message header), but I am not given any more information about it...

the ciphertexts are split up into the same number of blocks so i think padding may have been used...

im assuming xor, they haven't given us any information (im also assuming one time pad used twice, because of the message headers)...

heres the cipher texts so you see what i mean

3:17am :
UWCHR IFLFK AFFTG WVOOM SZYUJ
QVGBR LZFSC CWAXZ KCIUE KDZEA
RIAAY HHJTN DLCWV CIIJE WARON
JEDZO IKFNM AFJJL FVSZE MEFOM

4:29am :
UWCHR IFLFK AFFTG WVOHI LDOQX
NYQUE PHRSC BDIBB FHMTI VKVOA
RHMTQ EZACG INLDH ALTHI ISGHV
AGSPI WPJML BSICH BKZNX TAFGE

so they start the same, each cipher text has exactly 100 characters in it, I've xor'd the ciphertexts but I am not quite sure what to do next.

Also there is a hint from one guy who has solved this cipher:

just look at the time at which the ciphertext was sent: 3:17( = 3x60+17 = 197) and 4:29 ( = 4x60 + 29 = 269), both of these are primes.

any help would be great!
 
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  • #2
Two primes are used in the RSA algorithm, so that's my guess.
 
  • #3


Hi there,

It seems like you have been given a challenging task! It is definitely possible to decrypt these ciphertexts, but it may require some trial and error. Here are a few things you can try to help you decrypt the messages:

1. Use the XOR method: As you mentioned, it is likely that the ciphertexts were encrypted using the XOR method. This means that you can XOR the two ciphertexts together to get the original message. However, since they were encrypted using the same one time pad, you will need to find the key used to encrypt them. This can be done by guessing the key and then XORing it with one of the ciphertexts. If the result looks like a readable message, then you have found the correct key. If not, try another key until you find the correct one.

2. Look for patterns: Since the ciphertexts start the same, it is possible that the original message also starts with the same words or phrases. Try to look for patterns in the ciphertexts that may help you guess the key or the original message.

3. Use the hint: The hint given by the person who solved this cipher may be the key to decrypting the messages. The fact that the times are prime numbers may be a clue to the key used to encrypt the messages. Try using the prime numbers 197 and 269 as keys and see if you can decrypt the messages.

4. Use frequency analysis: If all else fails, you can try using frequency analysis to decrypt the messages. This involves looking at the frequency of letters and words in the ciphertexts and comparing them to the frequency of letters and words in the English language. This can give you a hint as to what the original message may be.

I hope these tips help you in decrypting the messages. Remember to keep trying different methods and don't give up! Good luck!
 

1. What is a one time pad?

A one time pad is a type of encryption method that uses a random key of the same length as the message to create a completely random and unbreakable ciphertext. It is considered to be one of the most secure methods of encryption.

2. How does a one time pad work?

A one time pad works by using a random key, which is only used once and is the same length as the message. The key is used to encrypt the message by combining it with the plaintext using a mathematical operation such as XOR. The recipient then uses the same key to decrypt the message and reveal the original plaintext.

3. What is the advantage of using a one time pad?

The main advantage of using a one time pad is that it is considered to be completely secure as long as the key is kept secret and is only used once. This is because the ciphertext is completely random and does not reveal any information about the plaintext, making it impossible for anyone to decipher it without the key.

4. Can a one time pad be used more than once?

No, a one time pad should only be used once. If it is used more than once, it becomes vulnerable to attacks and the security of the ciphertext is compromised. This is because patterns can emerge when the same key is used to encrypt multiple messages, making it easier for an attacker to break the encryption.

5. What are the limitations of using a one time pad?

One of the main limitations of using a one time pad is the need for a completely random and secret key that is the same length as the message. This can make it difficult to use for large messages and can also be challenging to generate truly random keys. Additionally, the key must be exchanged securely between the sender and recipient, which can be a logistical challenge.

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