Postgres implementation of a 63-bit (unbalanced) feistel cipher

FeistelCipher63.sql

-- AZURE doesn't support this extension yet
-- ;CREATE EXTENSION IF NOT EXISTS permuteseq;

CREATE OR REPLACE FUNCTION feistel_cipher_63(
    value     bigint,
    crypt_key bigint,
    decrypt   boolean DEFAULT false
)
RETURNS bigint LANGUAGE plpgsql STRICT IMMUTABLE PARALLEL SAFE
AS $$
DECLARE
    num_rounds constant int := 12;
    result bigint;
    L31 bigint; R32 bigint;
    Rhi bigint; R31 bigint;
    s int; i int; Fi bigint;
    kh bigint; kl bigint; Ks bigint;
BEGIN
    IF value < 0 THEN RAISE EXCEPTION 'Value must be non-negative'; END IF;

    -- Initial split
    L31 := (value >> 32) & x'7FFFFFFF'::bigint;
    R32 := value & x'FFFFFFFF'::bigint;

    kh := (crypt_key >> 32) & x'FFFFFFFF'::bigint;
    kl := crypt_key & x'FFFFFFFF'::bigint;

    FOR i IN 0..num_rounds - 1 LOOP
        s := CASE WHEN decrypt THEN num_rounds - 1 - i ELSE i END;
        Ks := (CASE WHEN (s & 1) = 0 THEN kl ELSE kh END) * s;

        IF NOT decrypt THEN
            -- ENCRYPT
            Rhi := (R32 >> 31) & 1;
            R31 := R32 & x'7FFFFFFF'::bigint;

            Fi  := (R31 # Ks);
            -- Stir ...
            Fi := (Fi # (Fi << 13)) & x'FFFFFFFF'::bigint;
            Fi := Fi # (Fi >> 17);
            Fi := (Fi # (Fi << 5)) & x'7FFFFFFF'::bigint;

            R32 := ((L31 # Fi) << 1) | Rhi;
            L31 := R31;

        ELSE
            -- DECRYPT (explicit inverse)
            Rhi := R32 & 1;
            R31 := L31;

            Fi  := (R31 # Ks);
            -- Stir ...
            Fi := (Fi # (Fi << 13)) & x'FFFFFFFF'::bigint;
            Fi := Fi # (Fi >> 17);
            Fi := (Fi # (Fi << 5)) & x'7FFFFFFF'::bigint;

            L31 := (R32 >> 1) # Fi;
            R32 := (Rhi << 31) | R31;
        END IF;
    END LOOP;

    RETURN (L31 << 32) | R32;
END;
$$;

/*
;WITH const AS (SELECT 6287830383155378919::bigint as key)
SELECT 
     values.seq as original
    ,LPAD(to_hex(feistel_cipher_63(values.seq, const.key)), 16, '0') as encoded
    ,feistel_cipher_63(feistel_cipher_63(values.seq, const.key), const.key, true) as roundtrip
    ,values.seq - feistel_cipher_63(feistel_cipher_63(values.seq, const.key), const.key, true) as "error"
FROM const CROSS JOIN generate_series(0::bigint,1000) AS values(seq)
UNION ALL
SELECT 
     values.seq as original
    ,LPAD(to_hex(feistel_cipher_63(values.seq, const.key)), 16, '0') as encoded
    ,feistel_cipher_63(feistel_cipher_63(values.seq, const.key), const.key, true) as roundtrip
    ,values.seq - feistel_cipher_63(feistel_cipher_63(values.seq, const.key), const.key, true) as "error"
FROM const CROSS JOIN generate_series((~(1::bigint<<63)) - 1000, (~(1::bigint<<63)) - 0) AS values(seq)

;WITH const AS (SELECT 6287830383155378919::bigint as key)
SELECT 
    feistel_cipher_63(0, const.key, true) AS zero, 
    feistel_cipher_63(1, const.key, true) AS one
FROM const
*/