Cheapo Enigma machine (Cops)

Question

For robbers' post, Cheapo Enigma machine (Robbers)

A cop's submission will consist of a program/function that accepts a single byte of data and return a single byte of data. Every possible input must produce a unique output. (In the other words, your function must be bijective)

Robbers will attempt to create the inverse function of yours using as short a code as possible. So your objective is to make your function difficult to invert.

You cannot use built-ins that have the sole purpose of hashing or encryption.

Your byte count cannot exceed 64 bytes. 0-byte solutions are not eligible for winning.

Input/Output format

8 bits (0 or 1), or a base-10 integer in range 1-256, 0-255 or -128 to 127. Can use standard I/O or file I/O. Function can also return a value as output. Input and output should belong to the same range (binary, 1-256, 0-255 or -128 to 127). The robber will also be required to use this range for input and output.

Scoring

Ratio of your byte count to that of the best robber's attempt against you. Lowest score wins.

You are eligible to win (as a cop) only if a robber has attempted to defeat you. (This robber may be you)

Example

C++, uses 0-255 range, 31 bytes

int x;
cin>>x;
cout<<(x+1)%256;

Possible robber submission in C++, 32 bytes

int f(int x)
{return x?x-1:255;}

Using the same language or a similar algorithm isn't a requirement

This gives a score of 31/32 = 0.97 to both the cop and the robber.

Answer 1

Javascript, 11 8 bytes, Score: 8 / 5

x=>x^x/2

Simple implementation of gray code. Decoding usally needs a whole loop. Let's see who comes up with the smallest or even without a loop !

Answer 2

C, 64 bytes, Score 64/71 = 0.901

T[256];f(x){srand(x&&f(x-1));for(;T[x=rand()%256]++;);return x;}

Takes input in the range [0 255].

Try it online! — on TIO (using GCC), this produces:

103,198,105,115,081,255,074,236,041,205,186,171,242,251,227,070,
124,194,084,248,027,232,231,141,118,090,046,099,051,159,201,154,
102,050,013,183,049,088,163,037,093,005,023,233,094,212,178,155,
180,017,014,130,116,065,033,061,220,135,112,062,161,225,252,001,
126,151,234,107,150,143,056,092,042,176,059,175,060,024,219,002,
026,254,067,250,170,058,209,230,148,117,216,190,097,137,249,187,
168,153,015,149,177,235,241,179,239,247,000,229,202,011,203,208,
072,071,100,189,031,035,030,028,123,197,020,075,121,036,158,009,
172,016,080,021,111,034,025,125,245,127,164,019,181,078,152,224,
077,052,188,095,119,108,134,043,085,162,004,211,054,226,240,228,
079,073,253,169,008,138,010,213,068,091,243,142,076,215,045,066,
006,196,132,173,222,048,246,032,133,007,244,185,195,217,160,120,
218,106,083,144,087,238,207,096,210,053,101,063,098,128,165,089,
140,012,192,131,047,039,022,147,184,109,182,193,199,237,018,069,
057,157,174,104,122,166,055,110,003,040,139,086,145,114,129,113,
206,167,191,214,146,221,136,038,156,082,200,029,044,204,223,064

Note that on other systems, it may produce different (but still valid) output, since C does not mandate a specific rand implementation. My submission is specifically the version running on TIO (as linked).

---

I'm quite disappointed that I wasn't able to get a version like my original (f(x){return rand(srand(x*229))/229%256;}) to work on TIO, since I think that's much more elegant. Since that only works on Clang running on OS X, it isn't fair for the competition. This one's still pretty awkward to reverse, so that's enough I guess.

Answer 3

Jelly, 2/5

^H

Try it online to see the whole table.

Answer 4

JavaScript, 44 bytes 22/3

x=>a.sort()[x]
for(a=[],i=0;i<256;)a[i]=i++;

Uses lexicographic sort (Javascript Default) to rearrange all the numbers from 0-255

Try it online!

Answer 5

C (gcc), 32 27/30 bytes

Thanks to christoph for golfing 5 bytes.

f(x){x=x?f(x*5+1&255)+1:0;}

Try it online!

Answer 6

JavaScript, 13 bytes 13/12

x=>(x<<9)%257

Input and output are both in the range 1->256

Try it online!

Answer 7

Octave, 16/6

@(x)mod(x*3,256)

Try it online!

Answer 8

Javascript, 11/8 bytes

x=>x**5%257

Domain/range is 1 through 256.

Answer 9

Javascript, 27/29 bytes

x=>x-6?x*95%127+x*98%131:65

Edit: Range/Domain is 1..256. Generated via brute force, more or less.

Answer 10

Java, 35 bytes

int a(int v){return (v*v+v)%512/2;}

Domain/Range are 0-255

Answer 11

Ruby, 23 bytes

->x{('228'*x).to_i%257}

Range and domain is 0..255. Concatenate 228 to itself x times, then take the result modulo 257 (0 maps to 0). 228 is the first magic number after 9 that works for this range (gives distinct values that don't include 256).

Answer 12

Python 3, 32 bytes 32/23

V=lambda x:((3+x)%16)*16+x//16

Domain/Range is 0-255.

Flips the first four bytes with the last four, and adds a three to the first bytes.

Answer 13

Python 3, 55 bytes

y=lambda x,i=3:x if i==0 else pow(3,y(5*x,i-1),257)-1

Domain/Range is 0-255.

Answer 14

brainfuck, 37/11

,+++++[>+++++++<-]>++[<+++++++++>-]<.

Try it online!

Not very good but range of 0-255

Answer 15

Mathematica, 13 bytes

Mod[#^7,257]&

Uses the range [1..256], although it's equally valid on the range [0..255]. To see the entire table, copy/paste one of the following lines of code into the Wolfram sandbox:

Array[ Mod[#^7,257]&, 256]   (for a list of values in order)
Array[ Rule[#,Mod[#^7,257]]&, 256]   (for a list of input-output rules)