Decrypting an AES-Encrypted Message Without the Key: A Cryptographic Challenge


I am investigating the possibility of decrypting an AES-encrypted message when the encryption key is unknown and unguessable. My query is aimed at understanding the limitations and potential of current cryptographic methods in such scenarios.


The message in question has been encrypted using the AES algorithm in CBC mode with a symmetric 128-bit key. The ciphertext is as follows: b'gAAAAABltItzoD0IHmJdqSKdZfchXMr151w1eMj-6pTwf3mP3q8K9ZnYC9t91yCVeXcC0W-fdDS-aU5gmAzT0s9ebock5LDkdhu3badVAaikXET15kQxuus='. I am exploring the theoretical and practical aspects of decrypting the message without access to the encryption key, aiming to contribute to academic discourse on cryptographic security.

  • Encryption Algorithm Details: AES in CBC mode
  • Key Details: Symmetric 128-bit key
  • Encryption Algorithm Details: AES in CBC mode
  • Key Details: Symmetric 128-bit key

Research Conducted: I have looked into various cryptographic texts and discussions around AES and its security, focusing on the resilience of AES-128 against different forms of cryptanalytic attacks.

Decryption Attempts: Theoretical considerations have been made for brute force approaches, including exhaustive key search attacks like the bicycle attack. However, given the vastness of the key space with a 128-bit key, this seems impractical.


Is there a known method or theory that could feasibly decrypt an AES-encrypted message under the conditions mentioned above, without the original key or guessing it? What would be the computational requirements for such a decryption process? Are there any documented instances or studies that have managed to achieve this, and what implications would such a capability have on the perceived security of AES encryption?

  • $\begingroup$ If such a thing were even remotely possible, AES would be completely useless. $\endgroup$
    – Mikero
    Jan 27 at 7:38
  • $\begingroup$ Is there no way of guessing/brute forcing the matching key to the encrypted message and printing the decrypted message(es) into a text file and then see if some message/string spits out the right result. Then we could derive the key from that couldn't we? $\endgroup$
    – NeoX
    Jan 27 at 12:34
  • $\begingroup$ Brute force is possible, if (depending on the key size) you get finished before the universe ends. $\endgroup$ Jan 27 at 15:18
  • $\begingroup$ How about quantum algorithms, is there any algorithm that would be able to brute force it for any t < t(end_of_universe) $\endgroup$
    – NeoX
    Jan 28 at 17:45

1 Answer 1


In general no, properly applied AES+CBC with proper key and IV is secure. Known attacks are far from practical.

However, if the message is guessable, there are many practical scenarios where one could verify such a guess without discovering the key.

There may be directly an operation you can make the key holder do which will help.

If the IV isn't chosen properly, you may be able to validate a guess, or even compare encrypted message fragments. A Fixed or controlable IV and an encryption oracle will allow you to easily check guesses for a message. You won't find the key but you can find the plain text.

Another source of information leakage is message size. AES+CBC doesn't hide message size, and this can have a lot of information. Especially if the message is expected to be one of a few options with known sizes or possibly one of a few groups of messages. If I no the secret message is either a "Stand Down" message or an "Attack" order, even if the details might remain hidden we could detect the type just by message length. A more sophisticated attack was shown on voice calls which utilized compression differences and timing to identify common phrases and words. (Though here it was on stream ciphers, but the point was message recovery without touching the encryption)

AES+CBC is secure. But full or partial message recovery can be done practically in many scenarios without breaking the encryption, full setup is needed to actually assess.


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