#!/bin/bash
 
awk -v from="foo" -v to="bar" '
    BEGIN {
        if (length(from)!=length(to)) {
            print "length mismatch" >"/dev/stderr"
            exit(42)
        }
        n = length(from)
        for(i=1; i<=n; i++) {
            f[i] = tolower(substr(from, i, 1))
            g[i] = toupper(substr(from,i, 1))
            t[i] = tolower(substr(to, i, 1))
            u[i] = toupper(substr(to, i, 1))
        }
        for(i=0; i<2**n; ++i) {
            a = b = ""
            q = i
            k = 2**(n-1)
            for(j = 1; j<=n;  j++) {
                c = (q > 0) && (q - k >= 0)
                a = a (c ? f[j] : g[j])
                b = b (c ? t[j] : u[j])
                if (c) q -= k
                k = int(k/2)
            }
            s[a] = b
        }
    }
    { for(r in s) gsub(r, s[r]) } 1'

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

SSd2ZSBnb3QgbXkgZm9vZCBpbiB5b3VyIGJhcm4KZm9vIGZvTyBGb28gRk9PIGZPb2QK

I've got my food in your barn
foo foO Foo FOO fOod