https://www.jbs.cam.ac.uk/faculty-research/centres/alternative-finance/publications/cryptoasset-regulation/#.XLWdcYrTWf0
https://archive.is/BGKOo
...a valid signature does not automatically provide proof that the owner of the corresponding private key has produced the signature. Instead, it provides a guarantee that a holder of the private key has initiated the transaction. The use of a private key underpins the presumption that a transaction was authorised. In the context of cryptoassets, the notion of custody thus no longer refers to the direct holding of assets but to the secure storage of cryptographic keys. Despite this functional equivalence, it is far from straightforward that (exclusive) knowledge of a private key is equivalent for all purposes to legal possession.23
There is no title in bitcoin.