[aeon10]

Actually the first few characters remained the same with page loads.

[nwh]

CBC mode AES?

[drdaeman]

More likely a header. It's 3081 byte blob after base64 decoding. If we consider it's encrypted with some block cipher with 128 or 256-bit blocks (IV and/or MAC would be likely to be 128-bit, too), there are 9 bytes for some header and/or padding.

[nwh]

Good catch.