Is there a security flaw in this that I'm not seeing? Is spending coins to/from these addresses safe?
Probably only the fact that you don't gain security benefit from multisig address & your Bitcoin will be stolen even if one private key is revealed to thief.
But aside from that, AFAIK it should be safe to send/spend to/from that address as long as the script is valid. If the one who make such multisig script can make this mistake which increase tx size/fee, i doubt he can make proper script.
Yes I know that this completely ignores the concept of multisig because I'm using a single private key, but that's not the security risk I meant. I'm talking about the cryptographic signature weakness and such, I have very little knowledge on the field so it would be great if someone can confirm if using the same private key to sign txes from all these addresses could weaken it.
The cases I linked above are also a 1-of-n multisigs, so technically the output script isn't going to get bigger every time because only one signature is required.. It's really cool because you just need the input and you can build a transaction for all these addresses, exactly the same way you would for a normal address.
I'm almost tempted to start doing this myself but I really need more insight on the mathematical part..
