# description
A signed receipt transmitted in the synchronous `Ack` response body, proving that the receiver authenticated, received, and processed the inbound request.  `CounterSignature` shares the same wire format as `Signature` but differs: - **Signer**: the response receiver (not the request sender) - **Location**: transmitted in the `Ack` response body (not in the `Authorization` header) - **`digest`**: covers the Ack response body (not the inbound request body) - **`(request-digest)`** and **`(message-id)`** MUST be included in the signing string  Signing string format: ``` (created): {unixTimestamp} (expires): {unixTimestamp} digest: BLAKE-512={base64DigestOfAckBody} (request-digest): BLAKE-512={base64DigestOfInboundRequestBody} (message-id): {messageId} ```