逆向中常見的加密算法識別

1、base64及換表

base64主要是將輸入的每3字節（共24bit）按照每六比特分成一組，變成4個小于64的索引值，然后通過一個索引表得到4個可見的字符。索引表為一個64字節的字符串，如果在代碼中發現引用了這個索引表“ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/”，那么基本上可以確定使用了base64。當然，如果和上面的索引表不相符但類型類似，比如：“abcdefghijklmnopqrstuvwxyz0123456789+/ABCDEFGHIJKLMNOPQRSTUVWXYZ”，基本上可以確定使用了base64換表。我們以2020HGame-Week1-advance為例看一下IDA中base64的反編譯偽代碼： 2020HGame-Week1-advance（base64換表）將文件拖入IDA，搜索字符串，出現了標志性的索引表：

TIM截圖20200705094257.png

對字符串交叉引用我們可以看到它的偽代碼：

signed __int64 __fastcall sub_140001EB0(_BYTE *a1, __int64 a2, int a3)
{int v3; // er10__int64 v4; // rax__int64 v5; // rbx_BYTE *v6; // rdi_BYTE *v7; // r9signed __int64 v8; // r11unsigned __int64 v9; // rdxunsigned __int64 v10; // raxchar v11; // cl
?v3 = 0;v4 = a3 - 2;v5 = a2;v6 = a1;v7 = a1;if ( v4 > 0 ){v8 = a2 + 1;v9 = ((unsigned __int64)((unsigned __int64)(v4 - 1) * (unsigned __int128)0xAAAAAAAAAAAAAAABui64 >> 64) >> 1) + 1;v3 = 3 * v9;do{v10 = *(unsigned __int8 *)(v8 - 1);v8 += 3i64;*v7 = aAbcdefghijklmn[v10 >> 2];v7[1] = aAbcdefghijklmn[((unsigned __int64)*(unsigned __int8 *)(v8 - 3) >> 4) | 16i64 * (*(_BYTE *)(v8 - 4) & 3)];v7[2] = aAbcdefghijklmn[4i64 * (*(_BYTE *)(v8 - 3) & 0xF) | ((unsigned __int64)*(unsigned __int8 *)(v8 - 2) >> 6)];v7[3] = aAbcdefghijklmn[*(_BYTE *)(v8 - 2) & 0x3F];v7 += 4;--v9;}while ( v9 );}if ( v3 < a3 ){*v7 = aAbcdefghijklmn[(unsigned __int64)*(unsigned __int8 *)(v3 + v5) >> 2];if ( v3 == a3 - 1 ){v11 = 61;v7[1] = aAbcdefghijklmn[16 * (*(_BYTE *)(v3 + v5) & 3)];}else{v7[1] = aAbcdefghijklmn[((unsigned __int64)*(unsigned __int8 *)(v5 + v3 + 1) >> 4) | 16i64* (*(_BYTE *)(v3 + v5) & 3)];v11 = aAbcdefghijklmn[4 * (*(_BYTE *)(v5 + v3 + 1) & 0xF)];}v7[2] = v11;v7[3] = 61;v7 += 4;}*v7 = 0;return v7 - v6 + 1;
}

2、TEA

TEA算法是一種常見的分組加密算法，密鑰為128比特位，明文為64比特位，主要做了32輪變換，每輪變換中都涉及移位和變換。TEA的源碼如下：

void encrypt(uint32_t *v, uint32_t *k)
{uin32_t v0 = v[0], v1 = v[1], sum = 0, i;uin32_t delta = 0x9e3779b9;uin32_t k0 = k[0], k1 = k[1], k2 = k[2], k3 = k[3];for (i = 0; i <= 32; i++){sum += delta;v0 += ((v1 << 4) + k0) ^ (v1 + sum) ^ ((v1 >> 5) + k1);v1 += ((v0 << 4) + k2) ^ (v0 + sum) ^ ((v0 >> 5) + k3);}v[0] = v0;v[1] = v1;
}
void decrypt(uint32_t *v, uint32_t *k)
{uin32_t v0 = v[0], , v1 = v[1], sum = 0xc6ef3720, i;uin32_t delta = 0x9e3779b9;uin32_t k0 = k[0], k1 = k[1], k2 = k[2], k3 = k[3];for (i = 0; i <= 32; i++){v1 -= ((v0 << 4) + k2) ^ (v0 + sum) ^ ((v0 >> 5) + k3);v0 -= ((v1 << 4) + k0) ^ (v1 + sum) ^ ((v1 >> 5) + k1);}v[0] = v0;v[1] = v1;
}

對TEA的識別也比較容易，在TEA算法中有一個固定的常數0x9e3779b9或者0x61c88647。 <a name="QIJGt"></a>

3、AES

AES也是常見的分組加密算法，多次出現在CTF中。AES的加解密流程如圖所示：

AES加密過程涉及4中操作：字節替代（SubBytes）、行移位（ShiftRows）、列混淆（MixColumns）和輪密鑰加（AddRoundKey）其中，字節替代過程是通過S盒完成一個字節到另外一個字節的映射。S盒和逆S盒具體如下：

static const uint32 FSb[256] = 
{0x63, 0x7c, 0x77, 0x7b, 0xf2, 0x6b, 0x6f, 0xc5, 0x30, 0x01, 0x67, 0x2b, 0xfe, 0xd7, 0xab, 0x76,0xca, 0x82, 0xc9, 0x7d, 0xfa, 0x59, 0x47, 0xf0, 0xad, 0xd4, 0xa2, 0xaf, 0x9c, 0xa4, 0x72, 0xc0,0xb7, 0xfd, 0x93, 0x26, 0x36, 0x3f, 0xf7, 0xcc, 0x34, 0xa5, 0xe5, 0xf1, 0x71, 0xd8, 0x31, 0x15,0x04, 0xc7, 0x23, 0xc3, 0x18, 0x96, 0x05, 0x9a, 0x07, 0x12, 0x80, 0xe2, 0xeb, 0x27, 0xb2, 0x75,0x09, 0x83, 0x2c, 0x1a, 0x1b, 0x6e, 0x5a, 0xa0, 0x52, 0x3b, 0xd6, 0xb3, 0x29, 0xe3, 0x2f, 0x84,0x53, 0xd1, 0x00, 0xed, 0x20, 0xfc, 0xb1, 0x5b, 0x6a, 0xcb, 0xbe, 0x39, 0x4a, 0x4c, 0x58, 0xcf,0xd0, 0xef, 0xaa, 0xfb, 0x43, 0x4d, 0x33, 0x85, 0x45, 0xf9, 0x02, 0x7f, 0x50, 0x3c, 0x9f, 0xa8,0x51, 0xa3, 0x40, 0x8f, 0x92, 0x9d, 0x38, 0xf5, 0xbc, 0xb6, 0xda, 0x21, 0x10, 0xff, 0xf3, 0xd2,0xcd, 0x0c, 0x13, 0xec, 0x5f, 0x97, 0x44, 0x17, 0xc4, 0xa7, 0x7e, 0x3d, 0x64, 0x5d, 0x19, 0x73,0x60, 0x81, 0x4f, 0xdc, 0x22, 0x2a, 0x90, 0x88, 0x46, 0xee, 0xb8, 0x14, 0xde, 0x5e, 0x0b, 0xdb,0xe0, 0x32, 0x3a, 0x0a, 0x49, 0x06, 0x24, 0x5c, 0xc2, 0xd3, 0xac, 0x62, 0x91, 0x95, 0xe4, 0x79,0xe7, 0xc8, 0x37, 0x6d, 0x8d, 0xd5, 0x4e, 0xa9, 0x6c, 0x56, 0xf4, 0xea, 0x65, 0x7a, 0xae, 0x08,0xba, 0x78, 0x25, 0x2e, 0x1c, 0xa6, 0xb4, 0xc6, 0xe8, 0xdd, 0x74, 0x1f, 0x4b, 0xbd, 0x8b, 0x8a,0x70, 0x3e, 0xb5, 0x66, 0x48, 0x03, 0xf6, 0x0e, 0x61, 0x35, 0x57, 0xb9, 0x86, 0xc1, 0x1d, 0x9e,0xe1, 0xf8, 0x98, 0x11, 0x69, 0xd9, 0x8e, 0x94, 0x9b, 0x1e, 0x87, 0xe9, 0xce, 0x55, 0x28, 0xdf,0x8c, 0xa1, 0x89, 0x0d, 0xbf, 0xe6, 0x42, 0x68, 0x41, 0x99, 0x2d, 0x0f, 0xb0, 0x54, 0xbb, 0x16 
};
?

static const uint RSb[256] = 
{ 0x52, 0x09, 0x6a, 0xd5, 0x30, 0x36, 0xa5, 0x38, 0xbf, 0x40, 0xa3, 0x9e, 0x81, 0xf3, 0xd7, 0xfb,0x7c, 0xe3, 0x39, 0x82, 0x9b, 0x2f, 0xff, 0x87, 0x34, 0x8e, 0x43, 0x44, 0xc4, 0xde, 0xe9, 0xcb,0x54, 0x7b, 0x94, 0x32, 0xa6, 0xc2, 0x23, 0x3d, 0xee, 0x4c, 0x95, 0x0b, 0x42, 0xfa, 0xc3, 0x4e,0x08, 0x2e, 0xa1, 0x66, 0x28, 0xd9, 0x24, 0xb2, 0x76, 0x5b, 0xa2, 0x49, 0x6d, 0x8b, 0xd1, 0x25,0x72, 0xf8, 0xf6, 0x64, 0x86, 0x68, 0x98, 0x16, 0xd4, 0xa4, 0x5c, 0xcc, 0x5d, 0x65, 0xb6, 0x92,0x6c, 0x70, 0x48, 0x50, 0xfd, 0xed, 0xb9, 0xda, 0x5e, 0x15, 0x46, 0x57, 0xa7, 0x8d, 0x9d, 0x84,0x90, 0xd8, 0xab, 0x00, 0x8c, 0xbc, 0xd3, 0x0a, 0xf7, 0xe4, 0x58, 0x05, 0xb8, 0xb3, 0x45, 0x06,0xd0, 0x2c, 0x1e, 0x8f, 0xca, 0x3f, 0x0f, 0x02, 0xc1, 0xaf, 0xbd, 0x03, 0x01, 0x13, 0x8a, 0x6b,0x3a, 0x91, 0x11, 0x41, 0x4f, 0x67, 0xdc, 0xea, 0x97, 0xf2, 0xcf, 0xce, 0xf0, 0xb4, 0xe6, 0x73,0x96, 0xac, 0x74, 0x22, 0xe7, 0xad, 0x35, 0x85, 0xe2, 0xf9, 0x37, 0xe8, 0x1c, 0x75, 0xdf, 0x6e,0x47, 0xf1, 0x1a, 0x71, 0x1d, 0x29, 0xc5, 0x89, 0x6f, 0xb7, 0x62, 0x0e, 0xaa, 0x18, 0xbe, 0x1b,0xfc, 0x56, 0x3e, 0x4b, 0xc6, 0xd2, 0x79, 0x20, 0x9a, 0xdb, 0xc0, 0xfe, 0x78, 0xcd, 0x5a, 0xf4,0x1f, 0xdd, 0xa8, 0x33, 0x88, 0x07, 0xc7, 0x31, 0xb1, 0x12, 0x10, 0x59, 0x27, 0x80, 0xec, 0x5f,0x60, 0x51, 0x7f, 0xa9, 0x19, 0xb5, 0x4a, 0x0d, 0x2d, 0xe5, 0x7a, 0x9f, 0x93, 0xc9, 0x9c, 0xef,0xa0, 0xe0, 0x3b, 0x4d, 0xae, 0x2a, 0xf5, 0xb0, 0xc8, 0xeb, 0xbb, 0x3c, 0x83, 0x53, 0x99, 0x61,0x17, 0x2b, 0x04, 0x7e, 0xba, 0x77, 0xd6, 0x26, 0xe1, 0x69, 0x14, 0x63, 0x55, 0x21, 0x0c, 0x7d 
};
?

這里以BUUCTF-re-[GWCTF 2019]re3為例來看看AES加密：

re學習筆記（54）BUUCTF-re-[GWCTF 2019]re3 SMC自修改代碼 | AES加密_buuctf smc-CSDN博客

經過一系列的折騰之后，我們看看AES函數：

也可以發現AES的S盒：byte_4023A0

TIM截圖20200705111640.png

通過識別S盒，可以確定是AES加密 <a name="uSDeR"></a>

4、RC4加密

RC4加密算法屬于流加密算法，包括初始化函數盒加解密函數，函數代碼具體如下：

//初始化函數
void RC4_Init(unsigned char* s, unsigned char* sey, unsigned long Len) {int i = 0, j = 0;//char k[256] = { 0 };unsigned char k[256] = { 0 };unsigned char tmp=0;for (i = 0; i < 256; ++i) {S[i] = i;//初始化S-boxk[i] = key[i % Len];//密鑰填充臨時數組}//打亂S-boxfor (i = 0; i < 256; ++i) {j = (j + s[i] + k[i]) % 256;tmp = s[i];s[i] = s[j];//交換s[i]和s[j]s[j] = tmp;}
}

//加解密
void RC4_Crypt(unsigned char* s, unsigned char* Data, unsigned long Len) {int i = 0, j = 0, t=0;unsigned long k = 0;unsigned char tmp;for (k = 0; k < Len; k++) {i = (i + 1) % 256;j = (j + s[i]) % 256;tmp = s[i];s[i] = s[j];//變換s[i]和s[j]s[j] = tmp;t = (s[i] + s[j]) % 256;Data[n] ^= s[t];}
}

可以看出，初始代碼對字符數組s進行了初始化賦值，且賦值分別遞增，之后有對s進行了256次變換操作。通過識別初始化代碼，可以判斷為RC4算法。 <a name="77ZzK"></a>

5、MD5

MD5消息摘要算法，是一種被廣泛使用的密碼散列函數，可以產生一個128位（16字節）的散列值，用于確保信息傳輸的完整性和一致性。MD5加密的函數大致如下： MD5_CTX md5c; MD5Init(&md5c); MD5UpdateString(&md5c,plain) MD5Final(digest,&md5c); 其中，MD5Init會初始化四個稱作MD5鏈接變量的整形參數。因此如果看到這4個常數0x67452301、0xefcdab89、0x98badcfe、0x10325476，就可以懷疑該函數是否為MD5算法了。MD5Init函數代碼如下：

void MD5Init(MD5_CTX *context) ?
{ ?context->count[0] = 0; ?context->count[1] = 0; ?//Load magic initialization constantscontext->state[0] = 0x67452301; ?context->state[1] = 0xEFCDAB89; ?context->state[2] = 0x98BADCFE; ?context->state[3] = 0x10325476; ?
} ?

六、CRC32、64

BUUCTF-RoarCTF2019-polyre（控制平坦化、反虛假控制流腳本、CRC64）注意文章中的一句話：（CRC64）加密原理實際上就是CRC32算法---輸入一組長度48的字符串，每8個字節分為1組，共6組。對每一組取首位，判斷正負。正值，左移一位；負值，左移一位，再異或0xB0004B7679FA26B3。重復判斷操作64次，得到查表法所用的表。 <a name="7gN0v"></a>