[Cryptography] AES related issue

[Philipp Gühring]

Hi,

I am currently playing around with an encryption/decryption module, which
claims to be using AES-256. Under normal circumstances, Encryption and
Decryption seem to work fine. 
Now I tried to play a bit with the key during the decryption operation,
and when I seem to insert the wrong key, the module returns high entropy
blocks.
Surprisingly, the high entropy blocks contains only on average 7.58 bits
of entropy
per byte (it was in the range of 7.53-7.604). 
The keys I inserted were similar to the original key, so possibly just
single bit-flips difference to the original key.
>From AES-256 I would have expected something like 7.99 or 7.999 bits of
entropy per byte. 7.58 looks like a problem to me, thats a bit more than
5% too deterministic, I would say.
I took a look at bit biases, but I couldn't find any yet.
I took a look at ECB encryption artifacts, but I couldn't find that either.
Are there any encryption modes where you would expect such problems?

If anyone is interested in further analyzing this issue, I can produce any
amount of cleartext+wrongly-decoded samples.

Best regards,
Philipp