Secure FIFO Data Transfer Without DB | Dataford Interview Questions

How would you send data from one place to another place without leaking information, using FIFO ordering, and without using any database?

Implement an in-memory sender and receiver. Encrypt each message, authenticate its ciphertext, preserve FIFO order, and reject reordered or replayed packets.

Use secure_fifo_transfer(messages, key, arrival_order, nonce). Return a result object containing received messages and an error value, without persisting data externally.

Constraints

0 <= len(messages) <= 100
Each message is a UTF-8 string
The key and nonce are non-empty strings
arrival_order contains packet indices and may contain reordering, duplication, or omission
The function must not use files, databases, or external services
A nonce must be unique for each transfer when the same key is reused

Function Signature

def secure_fifo_transfer(messages, key, arrival_order, nonce):

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