can half of an encrypted photo be decrypted with the key?

if we have like some part of the encrypted picture,is it there any way that the picture can be decrypted. is there any specific algorithm that allows this. like can from 60 per cent of the encrypted picture, can we build it up to determone the actual picture we also has access to the key. we know the algorithm. our purpose is to convert yhe 60 percent of cipher to image form

• This question is a bit unclear to me. Do you have 60% of the plaintext? 60% of the cipher text? Do you have the key? Do you have the full cipher text? Do you know exactly which algorithm is used? Do you want a system where given x,y and z one can recover the full image? Or do you rather want to know whether leaking 60% of the image is OK? – SEJPM Mar 22 '17 at 23:05
• ok ive edited the qs – user45273 Mar 22 '17 at 23:38

OK, for clarity I'll quickly summarize your scenario:

• You have the key and know the encryption algorithm.
• You have 60% of the cipher text of an image encrypted under above key and algorithm.
• You seek to decrypt the cipher text and reconstruct the full image.

There are two possible answers to this:

• It's pretty much impossible. If the 60% are highly fragmented and you only got a bit of ciphertext every now and then, you may either suffer from desynchronization (if you don't know how large the gaps are) or you'll lose potentially even more information because you can't decrypt all the bits (highly dependent on the mode of operation in use)
• You're lucky and have about 60% of the picture as a continuous block. This means that chances likely are that you can fully decrypt those 60% of the cipher text and start reconstruction of the image. How much of the image you can recover from 60%, I don't know. I suspect it depends on the details of the format and how interrelated the pixels for it are.

Depends on the image and the goal of the adversary. While the theoretical answer is no for partial decryption, there may be enough correlation between the encrypted pixels and the unencrypted pixels for one to retrieve (but not decrypt) the full image. Consider a pathological example of an all white/black image.

• so you mean that like if a hacker has access to 60 per cent of the encrypted picture and the key can he make out the whole picture?and by the picture i mean casual daily life picture – user45273 Mar 22 '17 at 21:00
• If he has the key, why does he need anything else? – Ayende Rahien Mar 22 '17 at 21:05
• I didn't get it. The attacker has access to partially decrypted picture or only part of the encrypted data? – Sauron Mar 22 '17 at 21:14
• just only part of the encrypted data and the key – user45273 Mar 22 '17 at 21:16
• Then he cannot decrypt the rest of the picture because he does not have the remaining ciphertext. Again, depending on the image, he can approximate remaining image but probably not easy for a regular picture. – Sauron Mar 22 '17 at 21:18

Not an expert, but that seems to be basically the known plain text issue.

Depending on the actual encryption mode you use, that can be problematic, but most modern crypto algorithms shouldn't allow that (AES doesn't, for example)