RTB

RTB 广告是一种实时竞价广告，针对每个广告在有广告位置的时候,会实时多方竞争，价格最有优势的广告主会竞得这次展示机会，在媒体测在拿到素材的时候，需将本次成交的价格，上报给指定的监控服务器，这时就需要将实时价格按照指定的加密方案加密后，替换GET链接中的请求参数中的价格宏来上报。

官网

1. 官方给出的源代码有Java和C++版本，地址如下：https://code.google.com/archive/p/privatedatacommunicationprotocol/source/default/source
2. 官方解释说明中， WINNING_PRICE 主要基于SHA-1 HMAC实现，官方文档给的解释都是相关伪代码，下面将如何用Go语言实现
3. i_key和 e_key为字符串，用于价格加密

Go Source Code

package main

/*
golang 实现 google 的rtb 价格加密方案
https://developers.google.com/authorized-buyers/rtb/response-guide/decrypt-price#encryption-scheme
*/

import (
    "crypto/hmac"
    "crypto/md5"
    "crypto/sha1"
    "encoding/base64"
    "encoding/binary"
    "encoding/hex"
    "fmt"
    "hash"
    "math"
    "strings"
)

const (
    PayloadSize    = 8
    InitVectorSize = 16
    SignatureSize  = 4
    EKey           = ""
    IKey           = ""
    UTF8           = "utf-8"
)

func AddBase64Padding(paramBase64 string) string {
    if i := len(paramBase64) % 4; i != 0 {
        paramBase64 += strings.Repeat("=", 4-i)
    }
    return paramBase64
}

func CreateHmac(key, mode string, isBase64 bool) (hash.Hash, error) {
    var b64DecodedKey, k []byte
    var err error
    if isBase64 {
        b64DecodedKey, err = base64.URLEncoding.DecodeString(AddBase64Padding(key))
        if err == nil {
            key = string(b64DecodedKey[:])
        }
    }
    if mode == UTF8 {
        k = []byte(key)
    } else {
        k, err = hex.DecodeString(key)
    }
    if err != nil {
        return nil, err
    }
    return hmac.New(sha1.New, k), nil
}

func HmacSum(hmacParam hash.Hash, buf []byte) []byte {
    hmacParam.Reset()
    hmacParam.Write(buf)
    return hmacParam.Sum(nil)
}

func GenerateHmac(key string, buf []byte) ([]byte, error) {
    hmacParam, err := CreateHmac(key, UTF8, true)
    if err != nil {
        err = fmt.Errorf("jzt/encrypt: create hmac error, %s", err.Error())
        return nil, err
    }
    return HmacSum(hmacParam, buf), nil
}

func EncryptPrice(price float64) (string, error) {
    var (
        iv         = make([]byte, InitVectorSize)
        encoded    = make([]byte, PayloadSize)
        signature  = make([]byte, SignatureSize)
        priceBytes = make([]byte, PayloadSize)
    )

    sum := md5.Sum([]byte(""))
    copy(iv[:], sum[:])

    // pad = hmac(e_key, iv)  // first 8 bytes
    pad, err := GenerateHmac(EKey, iv[:])
    if err != nil {
        return "", err
    }
    pad = pad[:PayloadSize]

    bits := math.Float64bits(price)
    binary.BigEndian.PutUint64(priceBytes, bits)
    // enc_price = pad <xor> price
    for i := range priceBytes {
        encoded[i] = pad[i] ^ priceBytes[i]
    }

    // signature = hmac(i_key, data || iv), first 4 bytes
    sig, err := GenerateHmac(IKey, append(priceBytes[:], iv[:]...))
    if err != nil {
        return "", err
    }
    signature = sig[:SignatureSize]

    // final_message = WebSafeBase64Encode( iv || enc_price || signature )
    finalMessage := strings.TrimRight(
        base64.URLEncoding.EncodeToString(append(append(iv[:], encoded[:]...), signature[:]...)),
        "=")
    return finalMessage, nil
}

func DecryptPrice(finalMessage string) (float64, error) {
    var err error
    var errPrice float64
    encryptedPrice := AddBase64Padding(finalMessage)
    decoded, err := base64.URLEncoding.DecodeString(encryptedPrice)
    if err != nil {
        return errPrice, err
    }
    var (
        iv         = make([]byte, InitVectorSize)
        p          = make([]byte, PayloadSize)
        signature  = make([]byte, SignatureSize)
        priceBytes = make([]byte, PayloadSize)
    )

    copy(iv[:], decoded[0:16])
    copy(p[:], decoded[16:24])
    copy(signature[:], decoded[24:28])

    pad, err := GenerateHmac(EKey, iv[:])
    if err != nil {
        return errPrice, err
    }
    pad = pad[:PayloadSize]
    for i := range p {
        priceBytes[i] = pad[i] ^ p[i]
    }

    sig, err := GenerateHmac(IKey, append(priceBytes[:], iv[:]...))
    if err != nil {
        return errPrice, err
    }
    sig = sig[:SignatureSize]
    for i := range sig {
        if sig[i] != signature[i] {
            return errPrice, fmt.Errorf("jzt/decrypt: Failed to decrypt, got:%s ,expect:%s", string(sig), string(signature))
        }
    }
    price := math.Float64frombits(binary.BigEndian.Uint64(priceBytes))
    return price, nil
}

func main() {

    result, err := EncryptPrice(0.11)
    if err != nil {
        err = fmt.Errorf("Encryption failed. Error : %s", err)
        return
    }
    fmt.Println(result)

    price, err := DecryptPrice("1B2M2Y8AsgTpgAmY7PhCfumh5TxN_8-DUn4uHg")
    if err != nil {
        err = fmt.Errorf("Encryption failed. Error : %s", err)
        return
    }
    fmt.Println(price)
}