Simplest possible consensus model: Proof of Work. Grin uses Nakamoto consensus just like Bitcoin.
Simplest possible emission: 1 grin per second forever. Grin uses Tail Emission from launch, the complete opposite of Bitcoin's Capped Supply. The latter is known to suffer from insecurity and mining instability once the block subsidy becomes insignificant, unless a constant backlog of high fee paying transactions can be created (which Bitcoin seems to manage with its constrained block size). Emission properties are further explored in [1a] and [1b].
Simplest possible blockchain protocol: Pure Mimblewimble. In Mimblewimble, outputs are Pedersen commitments r*G+v*H which combine value and blinding factor into a single curve point. The blinding factor serves both to hide the value and to control ownership. Correspondingly, a single (multi-)signature serves both to prove value balance (non-inflation) and to authorize transfer of ownership. The magic doesn't stop there, as transaction cut-through results in the collapse of the entire transaction history into a single transaction with no inputs and the current UTXO set as outputs [2].
Simplest possible blockchain sync: verify UTXO set. Grin still verifies transaction history by means of a ~100 byte kernel that remains for every transaction, but doesn't need to know anything about spent outputs.
Simplest possible (memory hard) Proof of Work Algorithm: Cuckatoo Cycle. Its mathematical specification is only 13 lines [3a] based on the very simple siphash-2-4 hash function. Which translates to just 42 lines of C code [3b]. Like Bitcoin, solutions can be instantly verified, but unlike Bitcoin, a single solution attempt (searching a graph) takes on the order of a second.
Simplest possible Difficulty Adjustment Algorithm: wtema. Just one line of code [4] that outperforms many other DAAs [5].
Simplest possible scripting functionality: scriptless scripts. Grin does away with Bitcoin's script and all its complexity, but retains a lot of its functionality, including multi-signatures, and both absolute and relative timelocks. It easily supports atomic swaps, discreet log contracts, and bidirectional payment channels. It lacks hash locks, but finds a superior alternative in adaptor signatures
The simplicity is reflected in the relatively small Rust codebase of the reference implementation [6] and the alternative C++ implementation [7].
[1a]
https://john-tromp.medium.com/a-case-for-using-soft-total-supply-1169a188d153[1b]
https://medium.com/@CryptoProfG/grin-money-explained-4-exploring-grins-monetary-model-e48b1761653[2]
https://phyro.github.io/what-is-grin/mimblewimble.html[3a]
https://github.com/tromp/cuckoo/blob/master/doc/mathspec[3b]
https://github.com/tromp/cuckoo/blob/master/doc/spec[4]
https://github.com/mimblewimble/grin/blob/master/core/src/consensus.rs#L371-L372[5]
https://read.cash/@jtoomim/bch-upgrade-proposal-use-asert-as-the-new-daa-1d875696[6]
https://github.com/mimblewimble/grin[7]
https://github.com/GrinPlusPlus/GrinPlusPlus
