Decrypt AES 256 bit ciphertext with key

Solution 1:

OpenSSL defaults (mostly) to CBC, so -aes256 is really -aes-256-cbc, which needs an IV. Specify -aes-256-ecb instead, which doesn't. (If you haven't learned about block cipher modes yet, see https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation .) In addition the data here appears not to be padded, although it happens to sort-of work; better to specify -nopad. With those I get:

$ od -tx1 su1670756
0000000 53 9b 33 3b 39 70 6d 14 90 28 cf e1 d9 d4 a4 07
0000020
$ echo $K
8000000000000000000000000000000000000000000000000000000000000001
$ openssl <su1670756 enc -aes-256-ecb -d -K $K -nopad | od -tx1
0000000 80 70 60 50 40 30 20 10 08 07 06 05 04 03 02 01
0000020