1、base64及换表
base64主要是将输入的每3字节(共24bit)按照每六比特分成一组,变成4个小于64的索引值,然后通过一个索引表得到4个可见的字符。 索引表为一个64字节的字符串,如果在代码中发现引用了这个索引表“ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/”,那么基本上可以确定使用了base64。当然,如果和上面的索引表不相符但类型类似,比如:“abcdefghijklmnopqrstuvwxyz0123456789+/ABCDEFGHIJKLMNOPQRSTUVWXYZ”,基本上可以确定使用了base64换表。 我们以2020HGame-Week1-advance为例看一下IDA中base64的反编译伪代码: 2020HGame-Week1-advance(base64换表) 将文件拖入IDA,搜索字符串,出现了标志性的索引表:
对字符串交叉引用我们可以看到它的伪代码:
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; // r9 signed __int64 v8; // r11 unsigned __int64 v9; // rdx unsigned __int64 v10; // rax char 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算法是一种常见的分组加密算法,密钥为128比特位,明文为64比特位,主要做了32轮变换,每轮变换中都涉及移位和变换。TEA的源码如下:
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; }
对TEA的识别也比较容易,在TEA算法中有一个固定的常数0x9e3779b9或者0x61c88647。 <a name="QIJGt"></a>
3、AES
AES也是常见的分组加密算法,多次出现在CTF中。AES的加解密流程如图所示:
AES加密过程涉及4中操作:字节替代(SubBytes)、行移位(ShiftRows)、列混淆(MixColumns)和轮密钥加(AddRoundKey) 其中,字节替代过程是通过S盒完成一个字节到另外一个字节的映射。S盒和逆S盒具体如下:
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 };
这里以BUUCTF-re-[GWCTF 2019]re3为例来看看AES加密:
re学习笔记(54)BUUCTF-re-[GWCTF 2019]re3 SMC自修改代码 | AES加密_buuctf smc-CSDN博客
经过一系列的折腾之后,我们看看AES函数:
也可以发现AES的S盒:byte_4023A0
通过识别S盒,可以确定是AES加密 <a name="uSDeR"></a>
4、RC4加密
RC4加密算法属于流加密算法,包括初始化函数盒加解密函数,函数代码具体如下:
//初始化函数 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-box k[i] = key[i % Len];//密钥填充临时数组 } //打乱S-box for (i = 0; i < 256; ++i) { j = (j + s[i] + k[i]) % 256; tmp = s[i]; s[i] = s[j];//交换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];//变换s[i]和s[j] s[j] = tmp; t = (s[i] + s[j]) % 256; Data[n] ^= s[t]; } }
可以看出,初始代码对字符数组s进行了初始化赋值,且赋值分别递增,之后有对s进行了256次变换操作。通过识别初始化代码,可以判断为RC4算法。 <a name="77ZzK"></a>
5、MD5
MD5消息摘要算法,是一种被广泛使用的密码散列函数,可以产生一个128位(16字节)的散列值,用于确保信息传输的完整性和一致性。MD5加密的函数大致如下: MD5_CTX md5c; MD5Init(&md5c); MD5UpdateString(&md5c,plain) MD5Final(digest,&md5c); 其中,MD5Init会初始化四个称作MD5链接变量的整形参数。因此如果看到这4个常数0x67452301、0xefcdab89、0x98badcfe、0x10325476,就可以怀疑该函数是否为MD5算法了。MD5Init函数代码如下:
void MD5Init(MD5_CTX *context) { context->count[0] = 0; context->count[1] = 0; //Load magic initialization constants context->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、反虚假控制流脚本、CRC64) 注意文章中的一句话: (CRC64)加密原理实际上就是CRC32算法---输入一组长度48的字符串,每8个字节分为1组,共6组。对每一组取首位,判断正负。正值,左移一位;负值,左移一位,再异或0xB0004B7679FA26B3。重复判断操作64次,得到查表法所用的表。 <a name="7gN0v"></a>
七、SM4加密
BUUCTF-安洵杯 2019-crackMe(hook、SM4加密,base64换表、极其难) 待整理。。。