xyz.rb

require "openssl"

p plain = ("a".."z").to_a.shuffle.join

cipher = OpenSSL::Cipher.new("aes-128-ecb").encrypt # key_len == 16
cipher.key = "0123456789abcdef" # 16 bytes
p out1 = cipher.update(plain) << cipher.final

cipher = OpenSSL::Cipher.new("aes-128-ecb").encrypt
cipher.key = "0123456789abcdefghi" # 19 bytes
p out2 = cipher.update(plain) << cipher.final

p out1 == out2 # => true

__END__

"kxzghrawqtbylmicovusjdnpef"
"SX\x18z\xB3\x8C\xC3e|\x9B\xD0\xF8=\xC9\xD2Z\x13\x01\xA2w\xD5\xD5\xAE\xCA5q/\xDF\x15Y\xDB\xF5"
"SX\x18z\xB3\x8C\xC3e|\x9B\xD0\xF8=\xC9\xD2Z\x13\x01\xA2w\xD5\xD5\xAE\xCA5q/\xDF\x15Y\xDB\xF5"
true

Note: This is the behavior on Ruby <= 2.3. Ruby 2.4 (Ruby/OpenSSL >= 2.0) will raise ArgumentError on line 10.

This helped me out a ton, thank you!

Is the same truncation behavior occurring on the IV assignment operator?

I'm getting an Argument Error when assigning an IV of length > 16, in Ruby 2.4.5. Ruby 2.1.x used to not complain. If I truncate the IV to length 16, the decryption I'm trying to invoke works as I would expect in Ruby 2.4.5.

I think my project's IV is longer than 16 bytes because it was base64 encoded (if I decode I get bytes == 16 in size). But it looks like the bytes beyond the 16th have been simply being dropped from truncation all this time.