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algorithmencryptioncryptographyverificationverify

Verify validity of a key without weakening of the chipher strength

I have a very large block of code (few seconds to crypt).

I use KeyA to encrypt it.

later in the process, i receive a key (not necessarily KeyA)...
but i don't need to open the block yet,
what i really need, is to validate that this is really the Key that will open the code correctly.

I Assumed i can keep a known block, and encrypt it, and in order to validate the key, only open it, but it feels like weakenning the power of the cryptography (brute-force is easier, one can learn few things about the key properties).

  1. Does my assumption really weakening the chipher? why yes/why no?
  2. Is there a different way to ensure the match of a key without opening the whole block.
Solution

  • I am assuming you are using Symmetric-Key Cryptography (the kind where the key used to decrypt the file is the same as the one used to encrypt it).

    If the cipher is vulnerable to a Known-Plaintext Attack, then the known block of plaintext may reveal information about the key. The stream cipher used for ZIP files suffered from this problem. Because ZIPs are compressed, it was difficult to guess enough plain-text, but the checksum used to verify passwords (among other factors) helped provide sufficient plain-text for a practical attack.

    In principle you could publicize the hash of KeyA (assuming that the hash algorithm is strong enough that it cannot be reversed, and that the hash algorithm isn't also used internally by the cipher). This would allow you to quickly reject invalid keys without changing the way the message is encrypted.

    Taking this idea further, you could use a Message authentication code such as HMAC. A message authentication code will validate that the message (in this case your very large block of code, or perhaps just its file path) has not been tampered with, as well as validating that the key is correct.

    If you are concerned that this will make brute force easier or expose properties of the key, you could split the key into two parts. The first part of the key could be purely for validation, and the second part purely for decryption. e.g. MyKey = AuthenticationPart,DecryptionPart

    (Disclaimer: This is based on my very incomplete understanding of crypto. You might get better responses from the experts on security.stackexchange.com and/or crypto.stackexchange.com)