I try to get the full reasoning behind the above statement. First, after reading articles here and at wikipedia i understand that using an IV only once is good practice.
For stream ciphers not doing so will result in compromising security because of
Cipher1 xor Cipher2 = Plain1 xor Plain2
For block ciphers using the same IV will degrade to ECB mode.
But, is there a real attack like with the stream cipher (or does the stream cipher reasoning apply, too)? E.g. if i have two (short) messages, create a single IV, encrypt both using AES and save the three values for later decryption - what is a concrete attack?
A concrete example from a fictious "preferences vault" implementation. Having an xml like
<vault> <items> <item iv="" name="f2e342435" value="2034b324"> </item> ... </items> </vault>
where iv is the initialization vector for encrypting both name and value properties, would this add any vulnerability in comparison to having two iv's (always assuming AES).