feat: merge-train/fairies-v5 - #24673
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…24665) ## Problem When encrypting a message, the ephemeral secret key comes from an unconstrained routine, so a malicious sender can substitute any value while proving. Substituting `eph_sk = 0` yields the point at infinity as the ephemeral public key, which passed the y-sign check: its y-coordinate is 0, which counts as positive. Its x-coordinate (0) is then broadcast, but 0 is not a valid x-coordinate on the curve, so the recipient can never reconstruct the key and the message is permanently undecryptable. This breaks the constrained-delivery guarantee that a note delivered by an untrusted sender remains decryptable by the recipient. ## Fix `generate_positive_ephemeral_key_pair` now asserts the ephemeral public key is not the point at infinity, alongside the existing sign check. A test emulates the substitution by mocking the randomness oracle to return 0. Fixes F-799
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fix(aztec-nr): reject infinity ephemeral key in message encryption (#24665)
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