My first Go program: Caesar Cipher
It looks like it was written in another language and then translated to Go. It looks complicated. Go is designed to be efficient. Maps are not as efficient as slices and arrays for simple indexing. decode(secret, i) executes 26 times instead of once. Etc.

Here's an alternate version,

package main

import "fmt"

const alphabet = "abcdefghijklmnopqrstuvwxyz"

func rotate(s string, rot int) string {
    rot %= 26
    b := []byte(s)
    for i, c := range b {
        c |= 0x20
        if 'a' &lt;= c &amp;&amp; c &lt;= 'z' {
            b[i] = alphabet[(int(('z'-'a'+1)+(c-'a'))+rot)%26]
        }
    }
    return string(b)
}

func decode(cipher string, rot int) (text string) {
    return rotate(cipher, -rot)
}

func encode(text string, rot int) (cipher string) {
    return rotate(text, rot)
}

var frequencies = []float32{
    .0817, .0149, .0278, .0425, .1270, .0223, .0202, .0609, .0697, .0015, .0077, .0402, .0241,
    .0675, .0751, .0193, .0009, .0599, .0633, .0906, .0276, .0098, .0236, .0015, .0197, .0007,
}

func crack(cipher string) (text string) {
    var sums [26]float32
    for _, c := range cipher {
        c |= 0x20
        for j := range sums {
            if 'a' &lt;= c &amp;&amp; c &lt;= 'z' {
                sums[j] += frequencies[(int(c)-int('a')+j)%26]
            }
        }
    }
    var maxi, maxsum = 0, float32(0)
    for i, sum := range sums {
        if sum &gt; maxsum {
            maxsum = sum
            maxi = i
        }
    }
    return rotate(cipher, maxi)
}

func main() {
    fmt.Printf("%s\n", encode("hello", 10))
    fmt.Printf("%s\n", decode("rovvy", 10))
    fmt.Printf("%s\n", crack("pybrscpexnkwoxdkvmyxdbsledsyxcdydronopsxsdsyxkxnnocsqxypzbyqbkwwsxqvkxqekqoc"))
}


Output:

rovvy
hello
forhisfundamentalcontributionstothedefinitionanddesignofprogramminglanguages