https://en.bitcoin.it/wiki/OP_CHECKSIG (note that the diagram has a couple of errors and is only for bare scripts, not P2SH)
https://github.com/bitcoin/bips/blob/master/bip-0143.mediawiki
SIGHASH_NOINPUT : https://github.com/bitcoin/bips/blob/master/bip-0118.mediawiki eltoo: https://blockstream.com/eltoo.pdf jl's sighash2 proposals: https://github.com/jl2012/bips/blob/sighash2/bip-sighash2.mediawiki bitcoin-dev mailing list post on SIGHASH proposals: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-November/016488.html and https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-December/016534.html