Do I have provide AES key + IV for someone to be able to decrypt encrypted data?
Does that increase key length from 128 bit to 256 bits?
Yes, both the key and the IV are needed to decrypt something. Generally, the key should be exchanged using a secure channel or key exchange mechanism. The IV can be transmitted along with the encrypted data in plain text. An IV should ideally be used only once. The main motivation behind using a changing IV is that encrypting the same thing twice should not result in the same ciphertext both times, because this can allow an attacker to draw conclusions about the data encrypted.
Related
I am using node and the crypto module to encrypt and decrypt a large binary file. I encrypt the file using crypto.createCipheriv and decrypt it using crypto.createDecipheriv.
For the encryption I use a random IV as follows:
const iv = crypto.randomBytes(16);
const encrypt = crypto.createCipheriv('aes-128-cbc', key, iv)
What I don't understand, do I need to pass a random IV for createDecipheriv as well? The SO here says:
The IV needs to be identical for encryption and decryption.
Can the IV be static? And if it can't, is it considered to be a secret? Where would I store the IV? In the payload?
If I use different random IVs for the encryption and decryption, my payload gets decrypted but the first 16 bytes are corrupt. This means, it looks like the IV needs to be the same but from a security perspective there is also not much value as the payload is decrypted except 16 bytes.
Can anyone elaborate what the go-to approach is? Thanks for your help!
The Key+IV pair must never be duplicated on two encryptions using CBC. Doing so leaks information about the first block (in all cases), and is creates duplicate cipher texts (which is a problem if you ever encrypt the same message prefix twice).
So, if your key changes for every encryption, then your IV could be static. But no one does that. They have a key they reuse. So the IV must change.
There is no requirement that it be random. It just shouldn't repeat and it must not be predictable (in cases where the attacker can control the messages). Random is the easiest way to do that. Anything other than random requires a lot of specialized knowledge to get right, so use random.
Reusing a Key+IV pair in CBC weakens the security of the cipher, but does not destroy it, as in CTR. IV reused with CTR can lead to trivial decryptions. In CBC, it generally just leaks information. It's a serious problem, but it is not catastrophic. (Not all insecure configurations are created equal.)
The IV is not a secret. Everyone can know it. So it is typically prepended to the ciphertext.
For security reasons, the IV needs to be chosen to meet cryptographic randomness security requirements (i.e. use crypto.randomBytes( ) in node). This was shown in Phil Rogaway's research paper. The summary is in Figure 1.2 of the paper, which I transcribe here:
CBC (SP 800-38A): An IV-based encryption scheme, the mode is secure as a probabilistic encryption scheme, achieving indistinguishability from random bits, assuming a random IV. Confidentiality is not achieved if the IV is merely a nonce, nor if it is a nonce enciphered under the same key used by the scheme, as the standard incorrectly suggests to do.
The normal way to implement this is to include the IV prepended to the ciphertext. The receiving party extracts the IV and then decrypts the ciphertext. The IV is not a secret, instead it is just used to bring necessary security properties into the mode of operation.
However, be aware that encryption with CBC does not prevent people from tampering with the data. If an attacker fiddles with ciphertext bits within a block, it affects exactly two plaintext blocks, one of which is in a very controlled way.
To make a very long story short, GCM is a better mode to use to prevent such abuses. In that case, you do not need a random IV, but instead you must never let the IV repeat (in cryptography, we call this property a "nonce"). Luke Park gives an example of how to implement it, here. He uses randomness for the nonce, which achieves the nonce property for all practical purposes (unless you are encrypting 2^48 texts, which is crazy large).
But whatever mode you do, you must never repeat an IV for a given key, which is a very common mistake.
I am new in cryptography. I need to encrypt a text using AES with some configuration
Encryption mode: GCM
Key size: 256 bits
Nonce size: 96 bits
MAC size: 128 bits
As AES is a symmetric algo. so i have a secrete key. I googled and found
nonce is a random number used to make sure a message is unique
But i have a doubt, how i can perform decryption, if nonce is a random number. do i need to store nonce along with each encryption. or any other way i need to split nonce, cipher and mac using key.
how can i use the provided configuration for encryption.
But i have a doubt, how i can perform decryption, if nonce is a random number. do i need to store nonce along with each encryption.
Yes, result of the encryption stored/sent is nonce, ciphertext, mac.
how i can perform decryption, if nonce is a random number
Nonce is randomly generated when encrypting the input and the nonce is then passed along the ciphertext (very often the nonce is prepended as the first block). Indeed you need THE SAME nonce value when decrypting, then the nonce is part of the input for decrpytion, not random
or any other way i need to split nonce, cipher and mac using key. how can i use the provided configuration for encryption.
There is a standardized message format for encrypted document or encrypted XML messages, but these are pretty pretty complex.
In reality - for simpler applications very often we see the encryption output composed as IV || ciphertext || MAC (as concatenation). IV and MAC are having fixed length, so you can cut them out and use the parameters for decryption.
decryption is happening on different machine and language. they have shared only a key for encryption. same key they are using for decryption. if i will add any random value as nonce then how hey will know what logic i have used for nonc
It doesn't matter until you provide the same input. As already mentioned - you may pass the IV (nonce) as part of the message and before decryption separate the nonce and original ciphertext.
BTW: I have a few encryption examples linked
Precisely, the nonce must be stored with the cipher text.
Remember, the nonce being part of the cipher text doesn't give the attacker any advantage.
From Wikipedia:
An initialization vector has different security requirements than a key, so the IV usually does not need to be secret. However, in most cases, it is important that an initialization vector is never reused under the same key. For CBC and CFB, reusing an IV leaks some information about the first block of plaintext, and about any common prefix shared by the two messages.
The purpose of initialization vector is to insert some randomness the encryption process so that an attacker cannot know when two identical plaintext messages have been encrypted with the same key.
IV is required for decryption, you can simply send it concatenated with the ciphertext.
IV || ciphertext
The most common way to transmit an initialization vector is, indeed, to prepend it immediately before the ciphertext.
If I am using Rijndael CBC mode, I have no idea why we would need salt.
My understanding is even if people know the password, but he cannot get the data without IV.
So from my perspective, password + IV seem to be sufficent secure.
Do I get anything wrong?
Yes, you need all of these things.
Salt (and an "iteration count") is used to derive a key from the password. Refer to PKCS #5 for more information. The salt and iteration count used for key derivation do not have to be secret. The salt should be unpredictable, however, and is best chosen randomly.
CBC mode requires an initialization vector. This is a block of random data produced for each message by a cryptographic random number generator. It serves as the dummy initial block of ciphertext. Like the key-derivation salt, it doesn't have to be kept secret, and is usually transmitted along with the cipher text.
The password, and keys derived from it, must be kept secret. Even if an attacker has the parameters for key derivation and encryption, and the ciphertext, he can do nothing without the key.
Update:
Passwords aren't selected randomly; some passwords are much more likely than others. Therefore, rather than generating all possible passwords of a given length (exhaustive brute-force search), attackers maintain a list of passwords, ordered by decreasing probability.
Deriving an encryption key from a password is relatively slow (due to the iteration of the key derivation algorithm). Deriving keys for a few million passwords could take months. This would motivate an attacker to derive the keys from his most-likely-password list once, and store the results. With such a list, he can quickly try to decrypt with each key in his list, rather than spending months of compute time to derive keys again.
However, each bit of salt doubles the space required to store the derived key, and the time it takes to derive keys for each of his likely passwords. A few bytes of salt, and it quickly becomes infeasible to create and store such a list.
Salt is necessary to prevent pre-computation attacks.
An IV (or nonce with counter modes) makes the same plain text produce different cipher texts. The prevents an attacker from exploiting patterns in the plain text to garner information from a set of encrypted messages.
An initialization vector is necessary to hide patterns in messages.
One serves to enhance the security of the key, the other enhances the security of each message encrypted with that key. Both are necessary together.
First things first: Rijndael does not have a "password" in CBC mode. Rijndael in CBC mode takes a buffer to encrypt or decrypt, a key, and an IV.
A "salt" is typically used for encrypting passwords. The salt is added to the password that is encrypted and stored with the encrypted value. This prevents someone from building a dictionary of how all passwords encrypt---you need to build a dictionary of how all passwords encrypt for all salts. That was actually possible with the old Unix password encryption algorithm, which only used a 12-bit salt. (It increased the work factor by 4096). With a 128-bit salt it is not possible.
Someone can still do a brute-force attack on a specific password, of course, provided that they can retrieve the encrypted password.
However, you have an IV, which does pretty much the same thing that a Salt does. You don't need both. Or, rather, the IV is your salt.
BTW, these days we call "Rijndael" AES.
A salt is generally used when using a hash algorithm. Rijndael is not a hash, but a two-way encryption algorithm. Ergo, a salt is not necessarily needed for encrypting the data. That being said, a salted hash of a password may be used as the Key for encrypting data. For what you're looking for, you might wish to look at hybrid cryptosystems.
The Key should be considered private and not transmitted with your encrypted data while the IV may be transmitted with the encrypted data.
Firstly I'm new to cryptography.
Suppose I encrypt a plain test using AES 128 bit Encryption and passed to the receiver. The key is also passed to the receiver. At the receiver side how can I check the KEY is correct or not without fully decrypting the message.
What I mean is can we check like this
IF(KEY==something)
DECRYPT
ELSE
NOT a correct key.
Is there anything to do with IV?
I really Don't understand what the IV is.
Firstly, passing the ciphertext and key through an insecure channel is, as you can probably determine, insecure. Keeping the ciphertext and key together in the same place is the same as keeping the plaintext, so don't do that.
Rather than determine if a given key is correct for the ciphertext, cryptographic systems instead determine if the ciphertext is legitimate before they even decrypt. The most common way to do this is using a MAC, or Message Authentication Code. HMACs are a common way to do this, as are Authenticated block modes like GCM.
Lastly, an IV is used to ensure that duplicated blocks of plaintext don't result in repeated blocks of ciphertext. E.g. in ECB mode, which doesn't use an IV, each identical block of plaintext will encrypt to the same ciphertext under a given key. Applying an IV (in modes like CBC), will ensure that identical plaintext blocks look different due to a chained XOR operation that starts with the IV.
To solve your problem, either use GCM mode or use a KDF to derive a symmetric key and a key for an HMAC.
If a random IV is used in encrypting plain text, how does the receiver of the cipher text know what the IV is in order to decrypt it?
This is a follow-up question to a response to the previous stackoverflow question on IVs here.
The IV allows for plaintext to be encrypted such that the encrypted text is harder to decrypt for an attacker. Each bit of IV you use will double the possibilities of encrypted text from a given plain text.
The point is that the attacker doesn't know what the IV is and therefore must compute every possible IV for a given plain text to find the matching cipher text. In this way, the IV acts like a password salt. Most commonly, an IV is used with a chaining cipher (either a stream or block cipher). ...
So, if you have a random IV used to encrypt the plain text, how do you decrypt it? Simple. Pass the IV (in plain text) along with your encrypted text.
Wait. You just said the IV is randomly generated. Then why pass it as plain text along with the encrypted text?
The answer that you quote is wrong. So don't worry if it does not make sense to you.
IVs don't make breaking a ciphertext harder. IVs are ususlly just prepended to the ciphertext and hence known to a potential attacker.
The main reason to use IVs is to randomize the ciphertext. If you encrypt the same plaintext twice with the same key, but different IVs then the ciphertext should be different. Ideally an attacker should not be able to tell if two ciphertexts correspond to the same plaintext or different plaintexts of the same length. More formally, IVs are used so that the encryption has ciphertext indistinguishability.
If a random IV is used in encrypting
plain text, how does the receiver of
the cipher text know what the IV is in
order to decrypt it?
The Wikipedia article on Initialization vectors provides several examples of ways to tell the receiver what the IV is.
Wait. You just said the IV is randomly
generated. Then why pass it as plain
text along with the encrypted text?
If the IV is randomly generated (at encrypt time), then only the sender knows what it is. In order to decrypt the message, the receiver needs to know the IV too.