Off-Key: Inconsistent MAC Keys Compromise Privacy in SPDZ-Family MPC

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SPDZ-family protocols for maliciously secure multi-party computation protect secret-shared values with message authentication codes (MACs) under a secret-shared global key. Their security requires that all values and preprocessing used together be authenticated under the same key. We present Off-Key, a privacy attack that exploits violations of this requirement. Depending on which specific MAC check protocol is used, a corrupted party can either recover the value of a shared secret from a single