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194

what is the default AES config in crypto-js?

rated 0 times [ 200] [ 6] / answers: 1 / hits: 6873 / 4 Years ago, thu, november 12, 2020, 12:00:00

https://cryptojs.gitbook.io/docs/ just said "CryptoJS supports AES-128, AES-192, and AES-256. It will pick the variant by the size of the key you pass in. If you use a passphrase, then it will generate a 256-bit key."

But what is default config like mode, padding, iv ? I notice there is a CipherOption for mode & padding

interface CipherHelper {

    encrypt(message: WordArray | string, key: WordArray | string, cfg?: CipherOption): CipherParams;

    decrypt(ciphertext: CipherParams | string, key: WordArray | string, cfg?: CipherOption): WordArray;

}

But I can't figure what the default value, e.g. it does not look like ECB mode by default.

My second question is how does it decide to use ace-128, aes-192, ase-256 based on the key I input, e.g. if I use a short string key like "my password" will it decide to use ace-128 then ? How ?

--- update ---

Except for the answer, I find cryptojs: How to generate AES passphrase is also helpful to understand the passphrase used in CryptoJS.

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answered 4 Years ago brendan · Accepted Answer

Your link to the CryptoJs-docs (https://cryptojs.gitbook.io/docs/) reveals the answers to your questions but we do have two scenarios.

you are feeding the encrypt function with a passphrase or 2) you are giving an explicit key as input.

Scenario 1: you feed a password/passphrase like "myPassword" to the function:

CryptoJS.AES.encrypt("Message", "Secret Passphrase");

Now CryptoJs derives a 32 byte long encryption key for AES-256 and a 16 byte long initialization vector (iv) from the password, encrypts the "Message" using this key, iv in AES mode CBC and (default) padding Pkcs7.

Scenario 2: you feed a key to the function:

var key = CryptoJS.enc.Hex.parse("000102030405060708090a0b0c0d0e0f");

var iv = CryptoJS.enc.Hex.parse("101112131415161718191a1b1c1d1e1f");

var encrypted = CryptoJS.AES.encrypt("Message", key, { iv: iv });

Now CryptoJs takes the key for AES-128/192/256 depending on the key length (16/24/32 bytes) and the 16 byte long initialization vector (iv) from the password, encrypts the "Message" using this key, iv in AES mode CBC and (default) padding Pkcs7.

more options: You can use other modes or padding when using the options like this (key + iv derivation from passphrase, AES mode CFB and padding AnsiX923):

var encrypted = CryptoJS.AES.encrypt("Message", "Secret Passphrase", {

  mode: CryptoJS.mode.CFB,

  padding: CryptoJS.pad.AnsiX923

});