【PostgreSQL采用MD5密码认证时密码和pg_authid里rolpassword的关系】

一、PostgreSQL用户密码以MD5方式加密

PostgreSQL里用户创建密码后，是把密码按照SCRAM-SHA-256或MD5等算法处理后的值写入数据库，等到用户登陆的时候，再把输入的密码进行相应的算法处理，把两个值进行比较来判断是否密码输入正确，具体如下：
客户端收到输入的密码后，会使用密码和用户做拼接，把用户作为salt，然后对拼接好的字符串做md5。即md5(密码+用户(salt))。然后把这个值加上md5前缀与存储在pg_authid里的rolpassword做比较，如果一致，则通过认证，如果不一致，则报密码错误。

MD5算法属于单向散列算法，无法通过反推获得原始输入数据，但是MD5不算严格意义上的加密算法，可用暴力穷举法破解。SCRAM-SHA-256生成的密文长度为256位，MD5生成的密文长度为128位。SCRAM-SHA-256算法的碰撞概率比MD5更小，因为SCRAM-SHA-256使用了更复杂的哈希算法和更长的输出长度。

二、测试验证使用md5认证时密码和pg_authid里rolpassword的关系

//session1

postgres=# show password_encryption ;

 password_encryption
---------------------

 md5
(1 row)

一个session用gdb来Attach这个进程，然后在encrypt_password函数这里打上断点。

//session2

(gdb) b encrypt_password
Breakpoint 1 at 0xaaaabc4f7a40: file crypt.c, line 118.
(gdb) info b
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x0000aaaabc4f7a40 in encrypt_password at crypt.c:118

然后原本的这个数据库连接里执行创建用户的操作

//session1
执行后处于一直卡着的状态
postgres=# create user ysla with password '1qaz!QAZ';

然后去gdb端查看堆栈

(gdb) c
Continuing.

Breakpoint 1, encrypt_password (target_type=PASSWORD_TYPE_MD5, role=0xaaaacccef658 "ysla", password=password@entry=0xaaaacccef670 "1qaz!QAZ") at crypt.c:118
118     {
(gdb) bt
#0  encrypt_password (target_type=PASSWORD_TYPE_MD5, role=0xaaaacccef658 "ysla", password=password@entry=0xaaaacccef670 "1qaz!QAZ") at crypt.c:118
#1  0x0000aaaabc48d160 in CreateRole (pstate=pstate@entry=0xaaaaccdd3038, stmt=stmt@entry=0xaaaacccef750) at user.c:446
#2  0x0000aaaabc68c420 in standard_ProcessUtility (pstmt=0xaaaacccef800, queryString=0xaaaaccceec28 "create user ysla with password '1qaz!QAZ';",
    readOnlyTree=, context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0, dest=0xaaaacccf0030, qc=0xffffc7625318) at utility.c:911
#3  0x0000ffffab246270 in pgss_ProcessUtility (pstmt=0xaaaacccef800, queryString=0xaaaaccceec28 "create user ysla with password '1qaz!QAZ';",
    readOnlyTree=false, context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0, dest=0xaaaacccf0030, qc=0xffffc7625318) at pg_stat_statements.c:1145
#4  0x0000aaaabc68a6cc in PortalRunUtility (portal=portal@entry=0xaaaaccd71f18, pstmt=pstmt@entry=0xaaaacccef800, isTopLevel=isTopLevel@entry=true,
    setHoldSnapshot=setHoldSnapshot@entry=false, dest=dest@entry=0xaaaacccf0030, qc=qc@entry=0xffffc7625318) at pquery.c:1158
#5  0x0000aaaabc68a874 in PortalRunMulti (portal=portal@entry=0xaaaaccd71f18, isTopLevel=isTopLevel@entry=true,
    setHoldSnapshot=setHoldSnapshot@entry=false, dest=dest@entry=0xaaaacccf0030, altdest=altdest@entry=0xaaaacccf0030, qc=qc@entry=0xffffc7625318)
    at pquery.c:1315
#6  0x0000aaaabc68ae00 in PortalRun (portal=portal@entry=0xaaaaccd71f18, count=count@entry=9223372036854775807, isTopLevel=isTopLevel@entry=true,
    run_once=run_once@entry=true, dest=dest@entry=0xaaaacccf0030, altdest=altdest@entry=0xaaaacccf0030, qc=qc@entry=0xffffc7625318) at pquery.c:791
#7  0x0000aaaabc686768 in exec_simple_query (query_string=query_string@entry=0xaaaaccceec28 "create user ysla with password '1qaz!QAZ';")
    at postgres.c:1274
#8  0x0000aaaabc687648 in PostgresMain (dbname=, username=) at postgres.c:4637
#9  0x0000aaaabc5e0514 in BackendRun (port=0xaaaaccd23cd0, port=0xaaaaccd23cd0) at postmaster.c:4464
#10 BackendStartup (port=0xaaaaccd23cd0) at postmaster.c:4192
#11 ServerLoop () at postmaster.c:1782
#12 0x0000aaaabc5e165c in PostmasterMain (argc=argc@entry=1, argv=argv@entry=0xaaaaccc56d80) at postmaster.c:1466
#13 0x0000aaaabc298464 in main (argc=1, argv=0xaaaaccc56d80) at main.c:198

可以再加一个断点

(gdb) b pg_md5_encrypt
Breakpoint 2 at 0xaaaabc835a90: file md5_common.c, line 147.
(gdb) c
Continuing.

Breakpoint 2, pg_md5_encrypt (passwd=passwd@entry=0xaaaacccef670 "1qaz!QAZ", salt=salt@entry=0xaaaacccef658 "ysla", salt_len=4, buf=buf@entry=0xaaaaccdd32a8 "", errstr=errstr@entry=0xffffc7624ad0) at md5_common.c:147
147             size_t          passwd_len = strlen(passwd);
(gdb) bt
#0  pg_md5_encrypt (passwd=passwd@entry=0xaaaacccef670 "1qaz!QAZ", salt=salt@entry=0xaaaacccef658 "ysla", salt_len=4, buf=buf@entry=0xaaaaccdd32a8 "",
    errstr=errstr@entry=0xffffc7624ad0) at md5_common.c:147
#1  0x0000aaaabc4f7abc in encrypt_password (target_type=, role=0xaaaacccef658 "ysla", password=password@entry=0xaaaacccef670 "1qaz!QAZ")
    at crypt.c:137
#2  0x0000aaaabc48d160 in CreateRole (pstate=pstate@entry=0xaaaaccdd3038, stmt=stmt@entry=0xaaaacccef750) at user.c:446
#3  0x0000aaaabc68c420 in standard_ProcessUtility (pstmt=0xaaaacccef800, queryString=0xaaaaccceec28 "create user ysla with password '1qaz!QAZ';",
    readOnlyTree=, context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0, dest=0xaaaacccf0030, qc=0xffffc7625318) at utility.c:911
#4  0x0000ffffab246270 in pgss_ProcessUtility (pstmt=0xaaaacccef800, queryString=0xaaaaccceec28 "create user ysla with password '1qaz!QAZ';",
    readOnlyTree=false, context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0, dest=0xaaaacccf0030, qc=0xffffc7625318) at pg_stat_statements.c:1145
#5  0x0000aaaabc68a6cc in PortalRunUtility (portal=portal@entry=0xaaaaccd71f18, pstmt=pstmt@entry=0xaaaacccef800, isTopLevel=isTopLevel@entry=true,
    setHoldSnapshot=setHoldSnapshot@entry=false, dest=dest@entry=0xaaaacccf0030, qc=qc@entry=0xffffc7625318) at pquery.c:1158
#6  0x0000aaaabc68a874 in PortalRunMulti (portal=portal@entry=0xaaaaccd71f18, isTopLevel=isTopLevel@entry=true,
    setHoldSnapshot=setHoldSnapshot@entry=false, dest=dest@entry=0xaaaacccf0030, altdest=altdest@entry=0xaaaacccf0030, qc=qc@entry=0xffffc7625318)
    at pquery.c:1315
#7  0x0000aaaabc68ae00 in PortalRun (portal=portal@entry=0xaaaaccd71f18, count=count@entry=9223372036854775807, isTopLevel=isTopLevel@entry=true,
    run_once=run_once@entry=true, dest=dest@entry=0xaaaacccf0030, altdest=altdest@entry=0xaaaacccf0030, qc=qc@entry=0xffffc7625318) at pquery.c:791
#8  0x0000aaaabc686768 in exec_simple_query (query_string=query_string@entry=0xaaaaccceec28 "create user ysla with password '1qaz!QAZ';")
    at postgres.c:1274
#9  0x0000aaaabc687648 in PostgresMain (dbname=, username=) at postgres.c:4637
#10 0x0000aaaabc5e0514 in BackendRun (port=0xaaaaccd23cd0, port=0xaaaaccd23cd0) at postmaster.c:4464
#11 BackendStartup (port=0xaaaaccd23cd0) at postmaster.c:4192
#12 ServerLoop () at postmaster.c:1782
#13 0x0000aaaabc5e165c in PostmasterMain (argc=argc@entry=1, argv=argv@entry=0xaaaaccc56d80) at postmaster.c:1466
#14 0x0000aaaabc298464 in main (argc=1, argv=0xaaaaccc56d80) at main.c:198

可以往下走，可以看到这里把用户名当作salt和实际密码拼接在一起，然后根据这个做MD5的加密。

(gdb) n
150             char       *crypt_buf = malloc(passwd_len + salt_len + 1);
(gdb) n
153             if (!crypt_buf)
(gdb) n
163             memcpy(crypt_buf, passwd, passwd_len);
(gdb) n
164             memcpy(crypt_buf + passwd_len, salt, salt_len);
(gdb) p crypt_buf
$2 = 0xaaaaccdf21b0 "1qaz!QAZ"
(gdb) p passwd_len
$3 = 8
(gdb) p salt
$4 = 0xaaaacccef658 "ysla"
(gdb) p salt_len
$5 = 4
(gdb) n
166             strcpy(buf, "md5");
(gdb) p crypt_buf
$6 = 0xaaaaccdf21b0 "1qaz!QAZysla"
(gdb) n
167             ret =  	(crypt_buf, passwd_len + salt_len, buf + 3, errstr);
(gdb) p crypt_buf
$7 = 0xaaaaccdf21b0 "1qaz!QAZysla"

(gdb) s pg_md5_hash
pg_md5_encrypt (passwd=passwd@entry=0xaaaad20f35e0 "1qaz!QAZ", salt=salt@entry=0xaaaad20f35c8 "ysla", salt_len=4,
    buf=buf@entry=0xaaaad21f9448 "md5f2689c3cb387c12f0183882c6c080a05", errstr=errstr@entry=0xffffd33a3b80) at md5_common.c:169
169             free(crypt_buf);

(gdb) s pg_md5_hash
pg_md5_encrypt (passwd=passwd@entry=0xaaaad20f35e0 "1qaz!QAZ", salt=salt@entry=0xaaaad20f35c8 "ysla", salt_len=4,
    buf=buf@entry=0xaaaad21f9448 "md5f2689c3cb387c12f0183882c6c080a05", errstr=errstr@entry=0xffffd33a3b80) at md5_common.c:169
169             free(crypt_buf);

... 

(gdb) n
CreateRole (pstate=pstate@entry=0xaaaad21f91d8, stmt=stmt@entry=0xaaaad20f36c0) at user.c:448
448                             new_record[Anum_pg_authid_rolpassword - 1] =
(gdb) s
cstring_to_text (s=0xaaaad21f9448 "md5f2689c3cb387c12f0183882c6c080a05") at varlena.c:184

从“buf”输出缓冲区里查看的md5值。然后把这个值和pg_authid系统表里的记录做对比。两个值是一致的。这个pg_authid里的rolpassword字段记录的md5值就是把密码和用户名加在一起，把用户名作为salt。然后把这个值前边加上md5开头并存储在pg_authid里的rolpassword字段。

Expanded display is on.
postgres=# select * from pg_authid where rolname='ysla';
-[ RECORD 1 ]--+------------------------------------
oid            | 57735
rolname        | ysla
rolsuper       | f
rolinherit     | t
rolcreaterole  | f
rolcreatedb    | f
rolcanlogin    | t
rolreplication | f
rolbypassrls   | f
rolconnlimit   | -1
rolpassword    | md5f2689c3cb387c12f0183882c6c080a05
rolvaliduntil  |

如果使用md5函数处理下密码+用户的字符串，并加上md5前缀，可以发现和rolpassword字段的值是一样的。

postgres=# select 'md5'||md5('1qaz!QAZysla');
              ?column?
-------------------------------------
 md5f2689c3cb387c12f0183882c6c080a05
(1 row)

postgres=# select rolpassword from pg_authid where rolname='ysla';
             rolpassword
-------------------------------------
 md5f2689c3cb387c12f0183882c6c080a05
(1 row)

postgres=# select rolpassword = 'md5'||md5('1qaz!QAZysla') from pg_authid where rolname='ysla';
 ?column?
----------
 t
(1 row)

三、总结

通过上述测试可以发现，pg_authid系统表里的rolpassword字段的MD5码 ="md5"字符串+ md5(pwd+username)），MD5在理论上是几乎无法破解的，虽然不能反向解析，但是如果获取到了这个MD5处理后的字符串，可以通过撞库方式获取MD5算法处理前的字符，用预先计算好的MD5散列值与已知的散列值进行比较，以查找匹配的明文，从而获取到用户名和密码。但是这种方法需要预先有一定的可能的明文和计算MD5散列值的能力。MD5的认证不是很安全，所以，MD5现在已经被弃用了，发生碰撞的概率是1/（2^128）。

MD5的salt是用户名字符串，密码一致根据算法生成的字符也是一致的。而SCRAM-SHA-256有随机salt的加入，同样的密码, 修改后存储内容也会变化。所以SCRAM-SHA-256更加安全，几乎很难破解，因此比较建议使用SCRAM-SHA-256而不是MD5加密。

SCRAM-SHA-256
$
4096
:
fWK6w0/oDX42HJeYaPkIWA==
$
P7K5YHTAjRW4JHm/6aY4vPA549WepjUCdQzudRm/QtE=
:
GG/TgXZAiSuJqLBa1e7E7q2CFJJU106I9w8iHVB78Kk=

//存储格式：
SCRAM-SHA-256  
$  
  
:  
  
$  
  
:  


postgres=# select rolname,rolpassword from pg_authid where rolname='repl';
 rolname |                                                              rolpassword
---------+---------------------------------------------------------------------------------------------------------------------------------------
 repl    | SCRAM-SHA-256$4096:fWK6w0/oDX42HJeYaPkIWA==$P7K5YHTAjRW4JHm/6aY4vPA549WepjUCdQzudRm/QtE=:GG/TgXZAiSuJqLBa1e7E7q2CFJJU106I9w8iHVB78Kk=
(1 row)

postgres=# SELECT rolname, regexp_replace(rolpassword, '(SCRAM-SHA-256)\$(\d+):([a-zA-Z0-9+/=]+)\$([a-zA-Z0-9+=/]+):([a-zA-Z0-9+/=]+)', '\1$\2:$:') as rolpassword_masked
    FROM pg_authid where rolname='repl';
 rolname |                rolpassword_masked
---------+---------------------------------------------------
 repl    | SCRAM-SHA-256$4096:$:
(1 row)