add commitment.md

src/mpc/commitments.md

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+# Commitments
+
+Here we illustrate the commitment scheme used to create authenticated commitments to the plaintext in scenarios where a general-purpose [`Notary`](/intro.html#tls-verification-with-a-general-purpose-notary) is used. (Note that this scheme is not used when the `Prover` proves directly to the `Verifier`)
+
+A naive approach of extending the [`Encryption and Decryption`](/protocol/mpc-tls/encryption.html) steps to also compute a commitment (e.g. BLAKE3 hash) using MPC is too resource-intensive, prompting us to provide a more lightweight commitment scheme.
+
+The high-level idea is that the `Prover` creates a commitment to the active plaintext encoding from the MPC protocol used for [`Encryption and Decryption`](/protocol/mpc-tls/encryption.html). 
+
+We also hide the amount of commitments (to preserve `Prover` privacy) by having the `Prover` commit to the Merkle tree of commitments.
+
+
+![Commitment](/diagrams/encoding_commitment.svg)