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is_low_order compared the raw bytes against the canonical encodings of 0, 1 and p - 1, so the non-canonical encodings p and p + 1 were not detected and x448() returned an all-zero shared secret for them.
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MontgomeryPoint::is_low_ordercompares the raw 56 bytes againstLOW_A/LOW_B/LOW_C, the canonical encodings of 0, 1 and p - 1. RFC 7748 has implementations accept non-canonical u values and reduce them, which the ladder does, sopandp + 1decode to 0 and 1 but get past the check.x448::PublicKey::from_bytesaccepts them andx448(sk, p)returnsSome([0; 56]), although its docs say it never returns a low order result.The check now reduces u before comparing. Only 0 and 1 have non-canonical encodings in 56 bytes, so this covers all of them.
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test_non_canonical_low_orderin x448, which fails on master for both values. #1306 moves this function toMontgomeryXpointunchanged, so it would need the same change if that lands first.