I came across the following enterprise encryption scheme. I laughed when I first saw it, but I'm not a specialist and I'd like to know how bad it really is.
- k1=k2=k3 for 3DES
- IV for CBC is repeated every 256 messages. Every communication party has a different set of 256 IVs but they are all predictable and similar in many places. IV has a fixed part unique for each communication party and an 8-bit counter.
- 16bit checksum of plaintext is known (sum of all bytes modulo 2^16)
- plaintext of some messages or part of it is known but it is not obvious, which message contains it
I wonder if this information can be used to further weaken an already weak algorithm and if it opens doors for feasible brute-force attacks or any other more clever methods.