This is supposed to supersede https://github.com/sipa/bips/pull/158.
I tried to say this carefully. I don't think that multiparty signing is in general broken with short hashes. For example the attack in #158 could be avoided by letting everybody not only commit to the nonce but also to the message. It's just that using a collision-resistant hash just eliminates the problem entirely...
In the test case "Case: PSBT With invalid output witnessScript typed key", after PSBT_OUT_WITNESS_SCRIPT key with garbage data (which ends with `...478ef51309d`, follows value `2b` which would denote the length of the data value of the key. But the length of actual remaining data is only 7 bytes. Thus, an implementation that reads key-value pairs and checks for validity of the key data after it has read the current key-value pair, will not be able to hit the exact condition intended for this test case: extra data within the key itself. This is because such implementation will hit serialization error when it will try to read the data of the value and will get the short read.
Reading full key-value pair and then checking key format afterwards is fairly normal thing to do, as the format of the keys with all their meaning is an abstraction of higher level than just the simple key-value serialization format.
The proposed change is to replace byte `2b` after the key data to `06` and thus make the value length in the key-value pair valid (not going beyond the end of the data).
base64 encoding has been changed accordingly.
As the key type is now defined as compact size integer, `At the beginning of each key is a compact size unsigned integer representing the type`, the comment in the first table in the document, about first byte of the key being the key type is no longer accurate.
As the structure of the key data is described further in the text after the table, and the comment that it starts with the compact size integer seems a bit long to be in that table, I think it is better to just remove the comment about the key data structure from the table, and leave the explanation to the text after the table.
