Decrypting file from crypto module using openssl - node.js

Question

I have a use case where we're streaming log data to a file using a simple NodeJS app. We'd like to be able to encrypt this data as we're streaming it and then decrypt it later as required using OpenSSL or similar.
What we're doing a basically as below:
var crypto = require('crypto'),
algorithm = 'aes256',
password = 'password';
var fs = require('fs');
var Readable = require('stream').Readable
var r = new Readable
r.push('This is a test')
r.push(null)
var encrypt = crypto.createCipher(algorithm, password);
var decrypt = crypto.createDecipher(algorithm, password)
var w = fs.createWriteStream('file.out');
//Write encrypted stream to file. Decrypt with openssl fails with 'bad magic number'
r.pipe(encrypt).pipe(w)
//Decrypt using cipher library. Decrypted text displays as expected
//r.pipe(encrypt).pipe(decrypt).pipe(w)
Assuming we're just encrypting the data as we're reading it in I'd assumed we could decrypt it using open OpenSSL e.g.
openssl enc -d -aes256 -in file.out -out file.out.decrypted
But this is just giving me the error
Bad magic number
Any help would be really appreciated.

Answer 1

Looking at the documentation of the crypto.createCipher() function, it mentions that the OpenSSL EVP_BytesToKey() function is used for deriving the key from the passphrase, just like the openssl enc app does. So that seems compatible.
However, that same documentation does not mention that any salt is applied and the function does not seem to have the possibility to pass a salt in as a parameter either. Therefore, you will have to pass the -nosalt option to openssl enc to make it work, as in:
openssl enc -d -aes256 -nosalt -in file.out -out file.out.decrypted
You can simulate what is happening by just using the openssl enc tool, with and without expecting a salt on the decrypt end:
Your current situation:
$ echo -n 'This is a test' | openssl enc -aes256 -nosalt -pass pass:password | openssl enc -d -aes256 -pass pass:password
bad magic number
Adding -nosalt when decrypting:
$ echo -n 'This is a test' | openssl enc -aes256 -nosalt -pass pass:password | openssl enc -d -aes256 -nosalt -pass pass:password
This is a test

Answer 2

By default, the OpenSSL encrypted file format starts with an 8-byte "magic number", the US-ASCII encoding of "Salted__". That's followed by another 8-byte value that's hashed with the password to derive the encryption key and IV for the message. Your NodeJS code isn't deriving the key the same way or providing the necessary header, so it won't work.
OpenSSL's key derivation algorithm is insecure and non-standard. If you derive the key from a password yourself using a good key derivation algorithm like PBKDF2 (or, better yet, randomly choose a key), you can provide it (encoded in hexadecimal) to the enc command with the -K option, along with the IV using the -iv option. I haven't checked to see if you also need the -nosalt option to avoid complaints about the magic number in this case.