Android crypto badpadding

android crypto badpadding

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The simplest of which just fills the missing bits with zeros. There is practically no security implication in the choice of padding in AES. This way, each ciphertext block depends on all plaintext blocks processed up to that point. Using the same image as before the result would be noise not distinguishable from random data: Image encrypted with CBC block mode looks random So what about the first block? The easiest way is to just use a block full of e.

Also if you reuse the same key for different plaintexts it would make it easier to recover the key. A better way is to use a random initialization vector IV. This is just a fancy word for random data that is about the size of one block bit. Think about it like the salt of the encryption , that is, an IV can be public, should be random and only used one time.

Mind though, that not knowing the IV will only hinder the decryption of the first block since the CBC XORs the ciphertext not the plaintext of the previous one. When transmitting or persisting the data it is common to just prepend the IV to the actual cipher message. This block mode is interesting because it turns a block cipher into a stream cipher which means no padding is required.

In its basic form all blocks are numbered from 0 to n. Every block will now be encrypted with the key, the IV also called nonce here and the counter value. Image from Wikpedia The advantage is, unlike CBC, encryption can be done in parallel and all blocks are depended on the IV not only the first one. A big caveat is, that an IV must never be reused with the same key because an attacker can trivially calculate the used key from that.

Can I be sure that nobody altered my message? The hard truth: encryption does not automatically protect against data modification. It is actually a pretty common attack. Read here on a more thorough discussion about this issue. So what can we do? A MAC is similar to a digital signature, with the difference that the verifying and authenticating key are practically the same. There are different variations of this method, the mode that is recommend by most researchers is called Encrypt-then-Mac.

That is, after encryption a MAC is calculated on the cipher text and appended. So now it starts getting complicated. This is also slow since the whole message must be processed twice. The opposite side has to to the same but for decrypting and verifying.

Fortunately there is a thing called authenticated encryption which simultaneously provides confidentiality, integrity, and authenticity assurances on the data. This authentication tag is then usually appended to the cipher text. Its size is an important security property, so it should be at least bit long. It is also possible to authenticate additional information not included in the plaintext. This data is called associated data. Why is this useful? For example the encrypted data has a meta property, the creation date, which is used to check if the content must be re-encrypted.

An attacker could now trivially change the creation date, but if it is added as associated data, GCM will also verify this piece of information and recognize the change. A heated discussion: What Key Size to use? So intuition says: the bigger the better — it is obvious that it is harder to brute force a bit random value than a bit. With our current understanding brute forcing through all values of a bit long word would require astronomically amount of energy , not realistic for anyone in sensible time looking at you, NSA.

AES actually has three distinct key sizes because it has been chosen as a US Federal Algorithm Apt at being used in various areas under the control of the US federal government [including the military]. So basically bit key is enough security for most of every use case with the exception of quantum computer protection.

Also using bit encrypts faster than bit and the key-schedule for bit keys seems to be better protected against related-key attacks however this is irrelevant to most real-world uses. As a Side Note: Side Channel Attacks Side channel attacks are attacks that aim to exploit issues specific to certain implementations.

To use the kony. NOTE: This value in the table is mandatory only if the scheme is passphrase. If the subalgo is aes, it contains a single string, whereas if the subalgo is tripledes, it contains three strings. NOTE: tripledes - in Android, if the passphrase length is less than 24 bytes or greater than 24 bytes an exception will be thrown with error message Invalid Keystrength and error code This is a mandatory parameter irrespective of the scheme.

Possible values are: aes and tripledes. NOTE: This value in the table is applicable only if the scheme is passphrase. Various error conditions related to CryptoError will be covered through the following error codes. Return Values The following are the return values for this API: key [userdata] - object The key that is created using the specified algorithm.

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