overt /əʊˈvəːt,ˈəʊvət/ adjective, done or shown openly; plainly apparent.

Karbo is built on CryptoNote protocol that ensures privacy of the transactions by default. Payments in Karbo are untraceable and unlinkable thanks to cryptographic technology ring signature. All transactions are signed on behalf of a group so that it is impossible to determine who exactly from the the group signed transaction and, accordingly, one can not say with certainty who carried out the payment. Moreover, the transactions can not be linked, because by using a variation of the Diffie-Hellman exchange protocol, it is sent to a receiver’s unique one-time address, derived from his single public key. After funds are sent to these one-time addresses they can only be redeemed by the receiver and it would be impossible to cross-link payments.

But there are cases where users would actually want to send a transparent transaction, for example, for compliance with local regulations. For this case, we introduce the “Overt transactions” which adds the ability to send a public transaction or a private transaction on a per transaction basis. This approach is backward compatible and does not require a hardfork. It leverages the same techniques that are used in a so-called “Proof of payment”, introduced to Karbo previously.

The “overt transaction” is a normal transaction that includes a special attachment in its “extra” field that contains disclosures of sender and recipient(s) public addresses and corresponding cryptographic signatures proving payment to destination by only revealing key derivation, not the actual transaction secret key, and allowing to determine which transaction outputs belongs to which address, as well as received amounts. These transactions, however, do not reveal involved parties’s wallet balances, nor previous or subsequent operations.

The disclosure attachment in transaction's extra consists of:

• a vector of 'destination public address and payment proof signature' pairs
• a signature by sender's private key to distinguish who is the sender

To find out who is the sender iterate through addresses from vector and try to check the signature until it's valid.