SwiftSync - smarter synchronization with hints

In a recent exchange between Ruben Somsen and Weikeng Chen on the Bitcoin Development Mailing List, an interesting cryptographic method was proposed for proving relationships between different unspent transaction outputs (UTXOs). The conversation delved into the feasibility of using encryption, specifically AES in place of hashing, to enforce that certain conditions between UTXOs are met without revealing their values.

Chen suggested employing a symmetric key k to encrypt UTXOs and then perform modular arithmetic operations (+ and -) on them. The goal is to prove either one of two conditions: (A==C && B==D) or (A==D && B==C), without directly revealing the UTXOs' values. This proposal contrasts with the traditional hashing approach, aiming for enhanced security or efficiency.

However, concerns were raised regarding the suitability of AES in Counter (CTR) mode for this purpose due to its operational characteristics. It was noted that while modular addition and subtraction, as well as XOR operations, share similarities, they are not entirely interchangeable, especially in cryptographic applications. The discussion highlighted potential collision resistance issues tied to AES's block size and the necessity for careful consideration of the encryption mode used. The Cipher Block Chaining Message Authentication Code (CMC) mode was mentioned as a possible solution to mitigate these concerns, emphasizing the need for a mode that avoids problems such as those imposed by the birthday bound.

This conversation underscores ongoing efforts to innovate within the field of cryptocurrency development, exploring new cryptographic techniques to secure transactions and enhance privacy. The discussion's technical depth reflects the complexity of developing cryptographic solutions that are both secure and practical for implementation in blockchain technologies.