Shulou(Shulou.com)06/01 Report--

On the Ubuntu system, there are three files related to user login access control in the / etc directory: passwd, shadow, and group. In the past, the older Unix system put the hashed password directly in the passwd file, but now * nix puts the processed password in the shadow file. The purpose of these three files can be referred to: http://blog.sina.com.cn/s/blog_5edae1a101017gfn.html.

Open the / etc/shadow file as shown below:

You can see from "$6 $" to "GJr.." At the end of this paragraph, the character is a hashed user password, so now we want to know how the string is generated, we have looked at the official Ubuntu documentation:

Shadow file description: http://manpages.ubuntu.com/manpages/karmic/man5/shadow.5.html

Crypt encryption algorithm description: http://manpages.ubuntu.com/manpages/karmic/man3/crypt.3.html

We can see from the official documents that the user's password has been processed by the crypt algorithm in glibc. "$6 $3rhg9.la $" is the salt value used in the hashing process, so what is the use of the salt value? We know that for a known hash algorithm and a fixed string, the hash result is the same, so suppose there are many users in a system, then some of them may use the same password. For example, if both people use "123456" as the login password, then the processed password string is the same without using salt. Although the "illegal person" cannot obtain the plaintext password directly from the processed string, it is still known that two people have used the same password. In order to avoid this situation, we can spell a salt value (m//salt) after the original password when hashing the user's password. This has two advantages: on the one hand, because the salt value is randomly generated, the agreed hash value of the same password is avoided; on the other hand, it increases the computational complexity of brute force cracking by * * users (increased by 2 ^ | salt | times). " $6 $3rhg9.la $"is divided into two parts:" 6 "and" 3rhg9.la ". The first parameter is the hash algorithm selection parameter, which has been mentioned in the official documentation, and the second is a randomly generated string.

The crypt algorithm included in python can call the crypt algorithm in glibc. When we open the crypt.py file in the lib directory in the python source file, we can see the process of python calling the crypt algorithm in glibc:

Def crypt (word, salt=None): "" Return a string representing the one-way hash of a password, with a salt prepended. If ``salt``is not specified or is ``None``, the strongest available method will be selected and a salt generated. Otherwise, ``salt``may be one of the ``crypt.METHOD_* ``values, or a string as returned by ``crypt.mksalt () ``. " If salt is None or isinstance (salt, _ Method): salt = mksalt (salt) return _ crypt.crypt (word, salt)

Open the crypt-entry.c file in the crypt directory in the glibc source file, and we can see the code that selects the hashing algorithm based on the first three characters of the salt value:

/ * Define our magic string to mark salt for MD5 encryption replacement. This is meant to be the same as for other MD5 based encryption implementations. * / static const char md5_salt_prefix [] = "$1 $"; / * Magic string for SHA256 encryption. * / static const char sha256_salt_prefix [] = "$5 $"; / * Magic string for SHA512 encryption. * / static const char sha512_salt_prefix [] = "$6 $"; / * For use by the old, non-reentrant routines (crypt/encrypt/setkey) * / extern struct crypt_data _ ufc_foobar

In the test process, the first download is glibc2.6, this version does not have $5$ and $6$ corresponding sha256/sha512 algorithm, only MD5, and then downloaded the latest version of glibc2.18 to see these two algorithms, you can guess the use of glibc2.6 version and previous version of the linux system, the shadow file is not like this. The gblic version of this Ubuntu system is 2.11 (ldd-version view), and this version of glibc also supports sha256/sha512. The first three characters in our example are "$6 $", and we can see that the hash algorithm used by the system is sha512. Because python calls the crypt algorithm in glibc, it is natural to know that this algorithm of python cannot be called on the windows platform. Now use python to write some scripts to test it:

Enter two parameters in the crypt function, one is our login password and the other is the salt value, and we can see that the output is the same as the processed password string in the shadow file.

Shadow files can only be accessed by root users by default, and ordinary users do not have access rights. Illegal users who obtain the file through certain channels may obtain the original plaintext password through violent attempts. Therefore, users try to include multiple characters (uppercase and lowercase, numbers, special symbols) and exceed a certain length when setting the password, so as to improve the security of the system.

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