Jeremy Rubin suggested using OP_SHA256STREAM instead of OP_CAT, but ZmnSCPxj expressed concern about its potential obsolescence if SHA256 is ever broken. Ethan Heilman agreed with ZmnSCPxj and proposed increasing the size of inputs for OP_CAT to enhance performance and utility. The group also discussed the limitations of OP_CAT for building Merkle trees in Bitcoin blocks and considered implementing tagged SHA256 as a new opcode to address this issue.Furthermore, ZmnSCPxj proposed removing SIGHASH from signatures and putting them on public keys instead, which would improve security by ensuring that public keys determine the sighash algorithm rather than the signature itself. To achieve this, ZmnSCPxj suggested the addition of a new opcode called OP_SETPUBKEYSIGHASH. This opcode allows public keys to be indirectly committed to every output scriptPubKey, with an automatically output tagging to allow for a particular SIGHASH.The proposal includes guidelines for how to utilize different ways to hide public keys until they are needed, such as MAST-inside-Taproot. The OP_SETPUBKEYSIGHASH opcode would have several possible byte inputs, including 0x1 to signify "just use the Taproot internal pubkey," a 33-byte public key with the sighash byte prepended to it, or a 34-byte public key with the first byte replaced by the sighash byte. If the sighash byte is set to 0x00, the result is a 33-byte public key, implying SIGHASH_ALL. By default, public keys will not have an attached SIGHASH byte, disallowing non-SIGHASH_ALL options unless specifically allowed in the output script.The proposal aims to retain the old feature of selecting the sighash at time-of-spending while allowing the SCRIPT to form the witness stack after the signature.