I try to understand a oracle Padding Oracle Attack example. I try to solve this question but I didn't figure out the answer.
A 7-byte message is padded per PKCS#5 and encrypted using the CBC mode of operation. The resulting ciphertext is
0x07 06 05 04 03 02 01 01, 0x08 09 0A 0B 0C 0D 0E 0F
Assume an Attacker modified that to
0x07 06 05 04 03 02 00 02, 0x08 09 0A 0B 0C 0D 0E 0F
and observed that the ciphertext was decrypted without any problems.
What is byte 7 of the original message?
What I did so far is separate the blocks as
0x07 06 05 04 03 02 00 02
0x08 09 0A 0B 0C 0D 0E 0F
The modified block is changed at the 7th and 8th byte so the key length should be 7. The Xor operation could be 0x02 xor 0x07 xor cipher text.