user83352: Various block explorers exist, which offer REST APIs. So far been using blockchain.info and btc.com
Here is what we have:
- List of bitcoin addresses
Here is what can easily be got with that info:
 - Historic list of transactions (hashes, or IDs) for the given addresses
 - I can also discover which input or output was OUR address, and therefore:
 - value, and whether the transaction was being sent or received
Now, here's what we need to obtain:
 - the OTHER (to/from) address!
So:
How can you use a transaction ID to obtain BOTH the sender's and receiver's wallet address - with some kind of REST API preferably?
(...knowing that sometimes transaction IDs are associated with multiple inputs and multiple outputs)
Endpoint references:
https://blockchain.info/q
https://btc.com/api-doc

user52021: The Bitcoin system doesn't have a "sender's address".  You can walk the transaction history to find out what destination the coins you were paid was last sent to but that is not a "sender's address".
Imagine if you had a magic wand you could wave over any package you received in the mail that would tell you where it was last mailed before you. You wave the wand over a package and find it was last mailed to a large distribution warehouse in china.  Is this the sender's address?  No. You bought the package from a shop in New York that purchased it for drop shipment from a distributor in Texas that fulfilled it out of a third party operated warehouse in Seattle, who recently pulled it out of a container from transoceanic shipment from the warehouse in china that your wand returned.
The previous to address might be useful information, just like the wand example-- but it is not the sender-- or at least not necessarily so--, it is not a return or refund address.  And this failure to map to the sender is not a fringe situation, just like the wand returning someone other than the party who sold you something would not be fringe: people frequently pay bitcoin's out of third parties wallets.

83352: <p>Various block explorers exist, which offer REST APIs. So far been using blockchain.info and btc.com</p>

<p><strong>Here is what we have:</strong><br>
- List of bitcoin addresses</p>

<p><strong>Here is what can easily be got with that info:</strong><br>
 - Historic list of transactions (hashes, or IDs) for the given addresses<br>
 - I can also discover which input or output was OUR address, and therefore:<br>
 - value, and whether the transaction was being sent or received<br></p>

<p>Now, <strong>here's what we need to obtain</strong>:<br>
 - the <strong>OTHER (to/from)</strong> address!<br></p>

<p><strong>So:</strong><br>
How can you use a transaction ID to obtain BOTH the sender's and receiver's wallet address - with some kind of REST API preferably?<br></p>

<p>(...knowing that sometimes transaction IDs are associated with multiple inputs and multiple outputs)<br></p>

<p><em>Endpoint references</em>:<br>
<a href="https://blockchain.info/q" rel="nofollow noreferrer">https://blockchain.info/q</a><br>
<a href="https://btc.com/api-doc" rel="nofollow noreferrer">https://btc.com/api-doc</a></p>
 52021: <p>The Bitcoin system doesn't have a "sender's address".  You can walk the transaction history to find out what destination the coins you were paid was last sent to but that is not a "sender's address".</p>

<p>Imagine if you had a magic wand you could wave over any package you received in the mail that would tell you where it was last mailed before you. You wave the wand over a package and find it was last mailed to a large distribution warehouse in china.  Is this the sender's address?  No. You bought the package from a shop in New York that purchased it for drop shipment from a distributor in Texas that fulfilled it out of a third party operated warehouse in Seattle, who recently pulled it out of a container from transoceanic shipment from the warehouse in china that your wand returned.</p>

<p>The previous to address might be useful information, just like the wand example-- but it is not the sender-- or at least not necessarily so--, it is not a return or refund address.  And this failure to map to the sender is not a fringe situation, just like the wand returning someone other than the party who sold you something would not be fringe: people frequently pay bitcoin's out of third parties wallets.</p>
