When Decrypting Image, gives javax.crypto.BadPaddingException: pad block corrupted Android

Question

Hi i\'m new to android and Image encryption. My scenario is like this,

• first I\'m encrypting the image file.
• Then I\'m uploading it to the Server

Answer 1

There are a lot of things that might cause a Bad Padding exception. Obvious things to check are that for both encryption and decryption you are using:

1. the same key, that is byte-for-byte the same.

2. the same encryption mode (CBC, CTR or GCM usually).

3. the same IV/Nonce, again byte-for-byte the same.

4. the same padding (PKCS5 or PKCS7 are common).

Do not rely on system defaults, especially when encrypting on one system and decrypting on another, as you seem to be doing. If the system defaults are different, then your decryption will fail. Always explicitly set key, mode, IV and padding. There will be documented ways to do so in any reasonable crypto library.

If that doesn't solve it then you will need to do a bit more digging. Set the decryption method temporarily to NoPadding or whatever equivalent your library uses. That will let the decryption method ignore padding errors, and give you some output. Have a look at the output and compare it to the original input; you may have to look at hex dumps here to be sure what is happening.

Among the possibilities are:

1. the output is complete garbage: your key is wrong, or the IV/Nonce is wrong for a stream cypher or GCM mode or CTR mode.

2. the first block is garbage with the rest matching the plaintext: you have the wrong IV in CBC mode.

3. the output matches with some extra stuff at the end: the extra stuff is padding. Set your decryption method to expect that type of padding.

If none of these happen, then ask again here, describing the symptoms.

When you have got a solution, you must set your decryption method back to expect the correct padding. Leaving it set to NoPadding is not secure since any old garbage can be added to the decrypted plaintext.