base64篇
base64嚴格意義說並不是一種加密方式,而是一種編碼方式,標準的base64有很多的特徵例如字符串中常常會看到"+“和”",在末尾也時常看到兩個連續的"="
1、原理:
base64將每三個字節分成一組(24bit),再將這一組數據數據按照每6個bit進行切割,這樣就切割出了4個小組,在將每個小組高位補0,湊齊8bit,在對應base64轉化表,將其轉化爲新的編碼。
這裏需要注意的是當字符串不是3的倍數時,需要用0對字符串進行補齊,將其轉化爲3的倍數,然而最後必然會產生全0值,而base64編碼表的0對應的通常是’A’,這樣在解碼時會造成極大的麻煩,因此引入了’='字符串,對末尾的0進行編碼,由於base64的特性可能會導致其末尾出現一個或者最多兩個0的情況
示例(1):當字符串爲三的倍數時:
字符 | 6 | 6 | 6 | 6 | 6 | 6 | * | * |
---|---|---|---|---|---|---|---|---|
16進製表示 | 0x36 | 0x36 | 0x36 | 0x36 | ||||
2進製表示 | 00110110 | 00110110 | 00110110 | 00110110 | 00110110 | 00110110 | ||
每6位切割處理後 | 001101 | 100011 | 011000 | 110110 | 001101 | 100011 | 011000 | 110110 |
對比base64編碼表後 | 0xD | 0x23 | 0x18 | 0x36 | 0xD | 0x23 | 0x18 | 0x36 |
對應的base64字符 | N | j | Y | 2 | N | j | Y | 2 |
示例(2):當字符串不爲三的倍數時:
這個時候就需要用0將不足三個補齊三個
字符 | 6 | 6 | * | * |
---|---|---|---|---|
16進製表示 | 0x36 | 0x36 | 0x0(補齊) | |
2進製表示 | 00110110 | 00110110 | 00000000 | |
每6位切割處理後 | 001101 | 100011 | 011000 | 00000 |
對比base64編碼表後 | 0xD | 0x23 | 0x18 | 0x0(末尾0) |
對應的base64字符 | N | j | Y | = |
2.1、C語言實現(加密):
1.對輸入的字符串進行判斷並補成3的倍數
unsigned int SixBitGroupSize; //明文6Bit組大小
unsigned char* BitPlainText; //明文Bit組
if (strlen(PlainText) % 3) //不是3的倍數
{
BitPlainSize = (strlen(PlainText) / 3 + 1) * 3;
SixBitGroupSize = (strlen(PlainText) / 3 + 1) * 4;
}
else //是3的倍數
{
BitPlainSize = strlen(PlainText);
SixBitGroupSize = strlen(PlainText) / 3 * 4;
}
這很簡單根據實際情況對明文bit組的大小和6bit組的大小進行修正就可以了
2.將已經爲3的倍數的字符串(3x8bit)轉化成(4x6bit)
int TransitionSixBitGroup(unsigned char *BitPlainText, unsigned char* SixBitGroup, unsigned int SixBitGroupSize)
{
int ret = 0;
//1、每4個6Bit組一個循環
for (int i = 0, j = 0; i < SixBitGroupSize; i += 4, j += 3)
{
SixBitGroup[i] = ((BitPlainText[j] & 0xFC) >> 2);
SixBitGroup[i + 1] = ((BitPlainText[j] & 0x03) << 4) + ((BitPlainText[j + 1] & 0xF0) >> 4);
SixBitGroup[i + 2] = ((BitPlainText[j + 1] & 0x0F) << 2) + ((BitPlainText[j + 2] & 0xC0) >> 6);
SixBitGroup[i + 3] = (BitPlainText[j + 2] & 0x3F);
}
return ret;
}
for循環代碼分析:
(1)SixBitGroup[i] = ((BitPlainText[j] & 0xFC) >> 2);
根據&操作符的特性,將8位二進制數據保留前6位,在將前6位數據右移2位實現補零操作
(2)SixBitGroup[i + 1] = ((BitPlainText[j] & 0x03) << 4) + ((BitPlainText[j + 1] & 0xF0) >> 4);
將第一個8位的後兩位提出來,左移4位作爲第二個6bit的高位和第二個8位的前4位進行相加
(3)SixBitGroup[i + 2] = ((BitPlainText[j + 1] & 0x0F) << 2) + ((BitPlainText[j + 2] & 0xC0) >> 6);
將第二個8位的後四位提取出來,向左移2位作爲高位和第三個8位的前兩位右移6位到最低位,後相加
(3)SixBitGroup[i + 3] = (BitPlainText[j + 2] & 0x3F)
將第3個8位的後6位直接提取出來
3.獲取6bit轉換成功後的字符串
unsigned char Base64Table[64] =
{
'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
'I', 'G', 'K', 'L', 'M', 'N', 'O', 'P',
'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
'w', 'x', 'y', 'z', '0', '1', '2', '3',
'4', '5', '6', '7', '8', '9', '+', '/'
};
int GetBase64String(unsigned char *CipherGroup, unsigned char *SixBitGroup, unsigned int SixBitGroupSize)
{
int ret = 0;
for (int i = 0; i < SixBitGroupSize; i++)
{
CipherGroup[i] = Base64Table[SixBitGroup[i]];
}
return ret;
}
4.獲取6bit轉換成功後的字符串最後兩位,若爲’A’則將其轉化爲’=’
for (int i = SixBitGroupSize - 1; i > SixBitGroupSize - 3; i--)
{
if (CipherGroup[i] == 'A')
{
CipherGroup[i] = '=';
}
}
2。2、C語言實現(解碼):
1.首先將編碼的字符串轉化爲對應的值
int GetBase64Index(unsigned char *CipherText, unsigned char *Base64Index, unsigned int Base64IndexSize)
{
int ret = 0;
for (int i = 0; i < Base64IndexSize; i++)
{
//計算下標
if (CipherText[i] >= 'A' && CipherText[i] <= 'Z') //'A'-'Z'
{
Base64Index[i] = CipherText[i] - 'A';
}
else if (CipherText[i] >= 'a' && CipherText[i] <= 'z') //'a'-'z'
{
Base64Index[i] = CipherText[i] - 'a' + 26;
}
else if (CipherText[i] >= '0' && CipherText[i] <= '9') //'0'-'9'
{
Base64Index[i] = CipherText[i] - '0' + 52;
}
else if (CipherText[i] == '+')
{
Base64Index[i] = 62;
}
else if (CipherText[i] == '/')
{
Base64Index[i] = 63;
}
else //處理字符串末尾是'='的情況
{
Base64Index[i] = 0;
}
}
return ret;
}
其實也可以通過索引Base64Table[64]
數組,但是這樣做會極大增加時間複雜度,因此在C語言中沒有這麼做
2.將4X6Bit組轉換爲3X8Bit組形式
int TransitionEightBitGroup(unsigned char *BitPlainText, unsigned char *Base64Index, unsigned int Base64IndexSize)
{
int ret = 0;
for (int i = 0, j = 0; j < Base64IndexSize; i += 3, j += 4)
{
BitPlainText[i] = (Base64Index[j] << 2) + ((Base64Index[j + 1] & 0xF0) >> 4);
BitPlainText[i + 1] = ((Base64Index[j + 1] & 0x0F) << 4) + ((Base64Index[j + 2] & 0xFC) >> 2);
BitPlainText[i + 2] = ((Base64Index[j + 2] & 0x03) << 6) + Base64Index[j + 3];
}
F
return ret;
}
這個跟前面一樣也是相關的位移、取出指定位的操作
3、python:
import base64
#加密
s = '66'
a = base64.b64encode(s.encode('utf-8'))
print(a)
#解密
print (base64.b64decode(a))
補充:
ctf題可能出現base64變碼的情況這個時候就需要進行一些轉化:
import base64
table_tmp = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'N', 'M','O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b','c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p','q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '9', '1', '2', '3','4', '5', '6', '7', '8', '0', '+', '/']
table_original = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
table_tmp_str = ''
str1 = ''
for i in table_tmp:
table_tmp_str += i
print(table_tmp_str)
print(base64.b64decode(str1.translate(str.maketrans(table_tmp_str,table_original))))
4.java
思路和C語言基本相同
public class Base64Realize {
//進行base64映射的字符數組
private final static char[] str = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z',
'0','1','2','3','4','5','6','7','8','9','+','/'};
/**
* 獲取解碼器
* @return
*/
public static Base64Realize getDecoder(){
return new Base64Realize();
}
/**
* 解碼
* @param code
* @return
*/
public String decode(String code){
//對字符串的長度進行計算
int length = code.length();
//判斷長度的合法性
if(length == 0 || length % 4 != 0)
return null;
//獲取字符串末尾的'='號數目
int endEqualNum = 0;
if(code.endsWith("=="))
endEqualNum = 2;
else if(code.endsWith("="))
endEqualNum = 1;
//對末尾的=號進行替換
code.replace('=','0');
StringBuilder sb = new StringBuilder(length);
//解碼
int blockNum = length / 4;
String afterDecode = "";
for(int i = 0;i < blockNum;i++){
afterDecode = decodeDetail(code.substring(i * 4,i * 4 + 4));
sb.append(afterDecode);
}
//返回字符串
String result = sb.toString();
return result.substring(0,result.length() - endEqualNum);
}
/**
* 編碼
* @param code
* @return
*/
public String encode(String code){
//初始化判斷
if (code == null || code.equals(""))
return null;
//獲取需編碼字符串的長度
int length = code.length();
StringBuilder sb = new StringBuilder(length * 2);
//轉化爲char型數組
char[] code1 = code.toCharArray();
//獲取長度對3的取餘
int mod = length % 3;
//獲取長度對3的倍數的
int div = length / 3;
//編碼
for(int i = 0;i < div;i++){
int temp = i * 3;
sb.append(encodeDetail(code1[temp],code1[temp + 1],code1[temp + 2]));
}
//對超出的進行額外的編碼
if (mod == 1) {
String str = encodeDetail(code1[length - 1], '\0', '\0');
sb.append(str.substring(0,str.length() - 2) + "==");
}
if(mod == 2) {
String str = encodeDetail(code1[length - 2], code1[length - 1], '\0');
sb.append(str.substring(0,str.length() - 1) + "=");
}
return sb.toString();
}
/**
* 編碼的詳細步驟
* @param a1
* @param a2
* @param a3
* @return
*/
private String encodeDetail(char a1,char a2,char a3){
char[] b = new char[4];
b[0] = str[((a1 & 0xFC) >> 2)];
b[1] = str[(a1 & 0x03) << 4 | (a2 & 0xF0) >> 4];
b[2] = str[(a2 & 0x0F) << 2 | (a3 & 0xC0) >> 6];
b[3] = str[(a3 & 0x3F)];
return String.copyValueOf(b);
}
/**
* 解碼的詳細步驟
* @param str
* @return
*/
private String decodeDetail(String str){
int len = str.length();
if(len != 4)
return null;
char[] b = new char[3];
int a1 = getIndex(str.charAt(0));
int a2 = getIndex(str.charAt(1));
int a3 = getIndex(str.charAt(2));
int a4 = getIndex(str.charAt(3));
b[0] = (char) (a1 << 2 | (a2 & 0x30) >> 4);
b[1] = (char) ((a2 & 0x0F) << 4 | (a3 & 0x3C) >> 2);
b[2] = (char) ((a3 & 0x03) << 6 | a4);
return String.copyValueOf(b);
}
/**
* 獲取字節的映射位置
* @param c
* @return
*/
private int getIndex(char c){
for(int i = 0;i < str.length;i++){
if(str[i] == c)
return i;
}
return -1;
}
/**
* 獲取編碼器
* @return
*/
public static Base64Realize getEncoder(){
return new Base64Realize();
}
}
參考鏈接:https://bbs.pediy.com/thread-253433.htm