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/* public domain proof-of-concept code by Solar Designer */

#define _XOPEN_SOURCE /* for crypt(3) */
#include <string.h>
#include <unistd.h>

#include "arch.h"
#include "params.h"
#include "formats.h"

#define FORMAT_LABEL            "crypt"
#define FORMAT_NAME            "generic crypt(3)"
#define ALGORITHM_NAME            "?/" ARCH_BITS_STR

#define BENCHMARK_COMMENT        ""
#define BENCHMARK_LENGTH        0

#define PLAINTEXT_LENGTH        72

#define BINARY_SIZE            128
#define SALT_SIZE            BINARY_SIZE

#define MIN_KEYS_PER_CRYPT        1
#define MAX_KEYS_PER_CRYPT        1

static struct fmt_tests tests[] = {
    {"CCNf8Sbh3HDfQ", "U*U*U*U*"},
    {"CCX.K.MFy4Ois", "U*U***U"},
    {"CC4rMpbg9AMZ.", "U*U***U*"},
    {"XXxzOu6maQKqQ", "*U*U*U*U"},
    {"SDbsugeBiC58A", ""},
    {NULL}
};

static char saved_key[PLAINTEXT_LENGTH + 1];
static char saved_salt[SALT_SIZE];
static char *crypt_out;

static int valid(char *ciphertext)
{
#if 1
    int l = strlen(ciphertext);
    return l >= 13 && l < BINARY_SIZE;
#else
/* Poor load time, but more effective at rejecting bad/unsupported hashes */
    char *r = crypt("", ciphertext);
    int l = strlen(r);
    return
        !strncmp(r, ciphertext, 2) &&
        l == strlen(ciphertext) &&
        l >= 13 && l < BINARY_SIZE;
#endif
}

static void *binary(char *ciphertext)
{
    static char out[BINARY_SIZE];
    strncpy(out, ciphertext, sizeof(out)); /* NUL padding is required */
    return out;
}

static void *salt(char *ciphertext)
{
    static char out[SALT_SIZE];
    int cut = sizeof(out);

#if 1
/* This piece is optional, but matching salts are not detected without it */
    switch (strlen(ciphertext)) {
    case 13:
    case 24:
        cut = 2;
        break;

    case 20:
        if (ciphertext[0] == '_') cut = 9;
        break;

    case 34:
        if (!strncmp(ciphertext, "$1$", 3)) {
            char *p = strchr(ciphertext + 3, '$');
            if (p) cut = p - ciphertext;
        }
        break;

    case 59:
        if (!strncmp(ciphertext, "$2$", 3)) cut = 28;
        break;

    case 60:
        if (!strncmp(ciphertext, "$2a$", 4)) cut = 29;
        break;
    }
#endif

    /* NUL padding is required */
    memset(out, 0, sizeof(out));
    memcpy(out, ciphertext, cut);

    return out;
}

static int binary_hash_0(void *binary)
{
    return ((unsigned char *)binary)[12] & 0xF;
}

static int binary_hash_1(void *binary)
{
    return ((unsigned char *)binary)[12] & 0xFF;
}

static int binary_hash_2(void *binary)
{
    return
        (((unsigned char *)binary)[12] & 0xFF) |
        ((int)(((unsigned char *)binary)[11] & 0xF) << 8);
}

static int get_hash_0(int index)
{
    return (unsigned char)crypt_out[12] & 0xF;
}

static int get_hash_1(int index)
{
    return (unsigned char)crypt_out[12] & 0xFF;
}

static int get_hash_2(int index)
{
    return
        ((unsigned char)crypt_out[12] & 0xFF) |
        ((int)((unsigned char)crypt_out[11] & 0xF) << 8);
}

static int salt_hash(void *salt)
{
    int pos = strlen((char *)salt) - 2;

    return
        (((unsigned char *)salt)[pos] & 0xFF) |
        ((int)(((unsigned char *)salt)[pos + 1] & 3) << 8);
}

static void set_salt(void *salt)
{
    strcpy(saved_salt, salt);
}

static void set_key(char *key, int index)
{
    strcpy(saved_key, key);
}

static char *get_key(int index)
{
    return saved_key;
}

static void crypt_all(int count)
{
    crypt_out = crypt(saved_key, saved_salt);
}

static int cmp_all(void *binary, int count)
{
    return !strcmp((char *)binary, crypt_out);
}

static int cmp_exact(char *source, int index)
{
    return 1;
}

struct fmt_main fmt_crypt = {
    {
        FORMAT_LABEL,
        FORMAT_NAME,
        ALGORITHM_NAME,
        BENCHMARK_COMMENT,
        BENCHMARK_LENGTH,
        PLAINTEXT_LENGTH,
        BINARY_SIZE,
        SALT_SIZE,
        MIN_KEYS_PER_CRYPT,
        MAX_KEYS_PER_CRYPT,
        FMT_CASE | FMT_8_BIT,
        tests
    }, {
        fmt_default_init,
        valid,
        fmt_default_split,
        binary,
        salt,
        {
            binary_hash_0,
            binary_hash_1,
            binary_hash_2
        },
        salt_hash,
        set_salt,
        set_key,
        get_key,
        fmt_default_clear_keys,
        crypt_all,
        {
            get_hash_0,
            get_hash_1,
            get_hash_2
        },
        cmp_all,
        cmp_all,
        cmp_exact
    }
};

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修改Makefile中JOHN_OBJS_MINIMAL的依赖

unshadow.o \
unafs.o \
unique.o \
crypt_fmt.o

执行指令

make clean generic

执行John

sudo unshadow /etc/passwd /etc/shadow> shadowfile
./john shadowfile

执行结果

root@ubuntu:/home/user/Downloads/john-1.8.0# ./john shadowfile
Loaded 6 password hashes with 6 different salts (generic crypt(3) [?/64])
test (test)
aaa (aaa)

暴力破解密码，都需要一个足够强大的字典。

John the Ripper的默认密码字典为run目录下的password.lst。
而密码字典的构造就看自己收集信息的能力了。