$2\mathrm{.}4$ Perhaps the simplest $$serious$$ symmetric block encryption algorithm is the Tiny

Encryption Algorithm $($TEA$).$ TEA operates on $64-$bit blocks of plaintext using a $128-$bit key. The plaintext is divided into two $32-$bit blocks, and the key is divided into four $32-$bit blocks. Encryption involves repeated application of a pair of rounds, defined as follows for rounds I $+1$: