Base64 encoding implementation principle Raiders

Principle of Base64 encoding

This article will achieve:

Description of the ASCII character encoding Base64 encoding implementation principle
Description of non-ASCII character encoding Base64 encoding implementation principle

Cryptanalysis series:

Base64 encoding implementation principle (that is, the article)
Url Base64 encoding algorithm implementation principle (being issued in)
Series MD5 algorithm implementation principle (being issued in)
Series SHA algorithm implementation principle (being issued in)
Series MAC algorithm implementation principle (being issued in)
Symmetric encryption DES (is issued) is
Symmetric encryption AES (is issued) is
Password-based encryption PBE (being issued in)
Asymmetric encryption DH (is issued) is
The RSA asymmetric encryption (is issued) is

1 Brief

Base64 encoding and decoding operations algorithms, may serve as the encryption and decryption operations, as well as a character map acts as a key.
Base64 algorithm of the public, which is not contrary to the principle of Kirk Hoff, but acts as a key public character map, in direct violation of the principle of Kirk Hof

密码学上的柯克霍夫原则（Kerckhoffs's principle，也称为柯克霍夫假说、公理、或定律）系由奥古斯特·柯克霍夫在19世纪提出：即使密码系统的任何细节已为人悉知，只要密匙（key，又称密钥或秘钥）未泄漏，它也应是安全的。
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Base64 encryption algorithm can not be called, but the conversion law to comply with the single-table replacement algorithm.

2 Base64 index table

Base64 index table, we can understand the configuration encrypted ciphertext, the ciphertext characters selected "AZ, az, 0-9, +, /" 64 printable characters, which is the standard Base64 agreement. In everyday use, we will see the "=" or "==" number appears in Base64 encoded result, the "=" here appears as a fill character

['A', 'B', 'C', ... 'a', 'b', 'c', ... '0', '1', ... '+', '/']
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3 coding table

Simply, the plaintext string after a series of transformations, to generate the corresponding decimal value, with reference to the code table and then, the ciphertext is represented by a single character encoded by permutation table.

4 Base64 encoding process implemented

Typically a string using Base64 encoded (encrypted), the character string is constituted, in the encoding process, as the basic unit to the characters shown in the following figure:

Encoding process to achieve

Step: "M", "a", "n" corresponding to the ASCII value are 77,97,110, corresponding to the binary value 01001101,01100001,01101110 acquired. As shown in FIG second three rows, thereby to form a 24-bit binary string.
Step two: the binary code obtained do packet switching operation, each of the three 8-bit binary code to a group, every four to convert 6-bit binary code is a set (less than 6 when low complement 0) in FIG red box , a 24-bit bit group is divided into four groups of six each, which is a process of changing the packet, three 8-bit binary code and a length of four 6-bit binary code is 24,
The third step: the four 6-bit binary complement code obtained was added to two 6-bit binary code 0 high to form four 8-bit binary code, 0 is added 2 high, the composition of four 8-bit binary code, extended to 32 bits, four bytes at this time becomes: 00010011,00010110,00000101,00101110.
The fourth step The obtained four 8-bit binary code is converted into decimal code, in the above data, 00010011,00010110,00000101,00101110 correspond decimal value: 19,22,5,46. Fifth step: to obtain with the above decimal value (Base64 encoded index) in the lookup table Base64 encoding, respectively: T, W, F, u. So "Man" after Base64 encoding becomes: TWFu.

Less than the median case

5 non-ASCII character encoding

ASCII code may be expressed in the range of 0-127 decimal character, corresponding to the binary range 0000 0000-01111111

ASCII code comprises digits, some control case letters and symbols, but not comprising a double-byte encoded characters, such as Chinese characters, with GB2312, GBK, UTF-8 encoding, etc., GB2312, GBK with two words section represents a character, UTF-8 represented by a three-byte characters.

Non-ASCII character encoding encryption implementation process

We have the string "secret" as an example

The first step obtains the encoded string "dense" utf-8 corresponding to the -27, -81, -122
Acquires the second step are utf-8 encoded (-27, -81, -122) corresponding to the binary code: 10000110 1,110,010,110,101,111
The third step of the above-described binary code into 4 groups 6 111 001 011 010 111 110 000 110
Moving towards the fourth 6-bit binary code 0 was added two high, the composition of four 8-bit binary code, adding two high 0,00111001 00011010 0,011,111,000,000,110
A fifth step of acquiring said binary code corresponding decimal code 5726626
A sixth step in the lookup table Base64 encoding, respectively 5 a + G

String "dense" obtained after Base64 encoded character string "5a + G"

6 Analysis

Here Base64 (6 th power of 2 is 64) characters transformed into 8-bit binary code to 6 units of the packet conversion represents a character, become Base64. Similarly, Base32 is a packet 5, Base16 is to use four packets.

So Base32, Base16 encoding process will clear out.