AES/CBC/PKCS5Padding implementation in Ruby (for r

Question

I need to decrypt text encrypted using AES/CBC/PKCS5Padding scheme. The encrypted text I got was generated using some Java software.

All values below are changed by me to something fictional.

What I get is a Key aHjgYFutF672eGIUGGVlgSETyM9VJj0K (256-bit = 32-chars * 8-bit) and IV: rxYoks3c8hRRsL2P (16-bit)

and (I supposed) Base64 encoded encrypted result ETlAHS5ZcshKxQUaHVB8==

What I need is to decrypt in Ruby this ETlAHS5ZcshKxQUaHVB8== to get in the and a simple string, like 'blablablabla'

I tried to decrypt what I got using both Ruby and just common linux console openssl command. NOTE: Key and IV below are not the ones used in real code:

# require 'openssl'
# require 'base64'
# decryption
aes = OpenSSL::Cipher::AES256.new(:CBC)
aes.decrypt
aes.padding = 1 # actually it's on by default
aes.key = "aHjgYFutF672eGIUGGVlgSETyM9VJj0K"
aes.iv="rxYoks3c8hRRsL2P"
aes.update(Base64::decode64("ETlAHS5ZcshKxQUaHVB8=="))+aes.final
=> OpenSSL::Cipher::CipherError: bad decrypt

Same as above but in console, key and iv converted to hex with:

$ echo -n $key256 | hexdump -e '16/1 "%02x"'
$ echo -n $iv     | hexdump -e '16/1 "%02x"'

$ echo "ETlAHS5ZcshKxQUaHVB8==" | openssl enc -d -aes-256-cbc -a  -K 61486a675946757446363732654749554747566c67534554794d39564a6a304b -iv 7278596f6b73336338685252734c3250
bad decrypt
140378046432928:error:06065064:digital envelope routines:EVP_DecryptFinal_ex:bad decrypt:evp_enc.c:539:

BTW. to get back original key and iv in the console you an use:

$ echo 61486a6... | xxd -r -p
#or , but then need to add \x before every character pair
$ eval `printf "\x61\x48......"

Please give me some clues as I hoped in the beginning that I will be able to use https://github.com/chicks/aes gem. The gem seems fine, it's just a nice wrapper for OpenSSL::Cipher::Cipher.

Is it possible that ruby/openssl use different PKCS, let's say PKCS#7, Java uses PKCS#5 and I need to preprocess my data ? Or there is a vesion mismatch between ruby/openssl and that Java's PKCS #7 and #5? #5 is meant for 8byte data blocks and #7 is for 16byte? Just a wild guess ...

Answer 1

The Ruby code in my first post is correct, the problem was this AES/CBC/PKCS5Padding used by Java part.

Java program should not use this scheme for AES-CBC-256. PKCS5 pads to a 64 bit (8 byte) block size, but AES-256-CBC uses 16 byte blocks. Therefore, PKCS7 must be used.