Goで書くはじめてのデジタル署名 ref: http://qiita.com/keitaj/items/00aede60e64e8eebbb8a

main.go

package main

import (
    "bufio"
    "bytes"
    "crypto"
    "crypto/rand"
    "crypto/rsa"
    "crypto/x509"
    "encoding/base64"
    "fmt"
    "log"
    "net/http"
    "os"
    "strconv"
    "time"
)

func main() {
    // 秘密鍵の読込み
    privateKeyStr, err := readPrivateKey("private.key")
    if err != nil {
        log.Fatal(err)
    }

    // 署名対象の送信データをJson形式にしてみる　例）-> {"message":"Hello World","timestamp":1481610623}
    message := `{"message":"Hello World","timestamp":` + strconv.FormatInt(time.Now().Unix(), 10) + `}`

    // 送信データ（メッセージ）と秘密鍵を入力とし、署名を生成する
    signature, err := createSignature(message, privateKeyStr)
    if err != nil {
        log.Fatal(err)
    }

    // リクエストを送る際に、署名はヘッダに設定する
    req, err := http.NewRequest("POST", "http://localhost:8080/foo", bytes.NewBuffer(([]byte)(message)))
    defer req.Body.Close()
    req.Header.Set("Content-Type", "application/json")
    req.Header.Set("Signature", signature)

    client := http.Client{}
    res, err := client.Do(req)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(res.StatusCode)
}

func readPrivateKey(filepath string) (string, error) {
    s := ""
    fp, err := os.Open(filepath)
    if err != nil {
        return "", err
    }
    defer fp.Close()
    scanner := bufio.NewScanner(fp)
    for scanner.Scan() {
        text := scanner.Text()
        if text == "-----BEGIN RSA PRIVATE KEY-----" || text == "-----END RSA PRIVATE KEY-----" {
            continue
        }
        s = s + scanner.Text()
    }
    if err := scanner.Err(); err != nil {
        return "", err
    }

    return s, nil
}

func createSignature(message, keystr string) (string, error) {
    keyBytes, err := base64.StdEncoding.DecodeString(keystr)
    if err != nil {
        return "", err
    }

    private, err := x509.ParsePKCS1PrivateKey(keyBytes)
    if err != nil {
        return "", err
    }

    h := crypto.Hash.New(crypto.SHA256)
    h.Write(([]byte)(message))
    hashed := h.Sum(nil)

    signedData, err := rsa.SignPKCS1v15(rand.Reader, private, crypto.SHA256, hashed)
    if err != nil {
        return "", err
    }

    signature := base64.StdEncoding.EncodeToString(signedData)
    return signature, nil
}