如何从此加密创建解密
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【中文标题】如何从此加密创建解密【英文标题】：How to create a Decryption From this Encryption 【发布时间】：2020-07-10 19:26:58 【问题描述】：所以我得到了这个任务，我必须使用“encrpypt_chars”函数作为基础创建一个解密函数。据我了解，代码采用字符串的长度和加密密钥，然后使用该加密密钥单独操作存储在字符串中的每个值：
temp_char = OChars[i]; // Get the next char from Original Chars array
我的目标是对解密例程进行类似的操作，但相反，以便我将加密字符串恢复到其原始值。
void encrypt_chars (int length, char EKey)
char temp_char; // Character temporary store
for (int i = 0; i < length; i++) // Encrypt characters one at a time
temp_char = OChars[i]; // Get the next char from Original Chars array
__asm
push eax // stores the "eax" register out onto the stack
push ecx // stores the "ecx" register out onto the stack
push edx // stores the "edx" register out onto the stack
movzx ecx, temp_char // zeroise "ecx" register and move values in "temp_char" varaible to "ecx" register
lea eax, EKey // copies address of values contained within the EKey varaible and moves it into "eax"register
push eax // stores the "eax" register out onto the stack
push ecx // stores the "ecx" register out onto the stack
call encrypt_5 // runs the function called "decryptX"
mov temp_char, dl // move values in "dl" register into "temp_char" variable
add esp, 8 // add 8 to the "esp" register
pop edx // removes the "edx" register from the stack
pop ecx // removes the "ecx" register from the stack
pop eax // removes the "eax" register from the stack
EChars[i] = temp_char; // Store encrypted char in the Encrypted Chars array
return;
// Inputs: register EAX = 32-bit address of Ekey,
// ECX = the character to be encrypted
// (in the low 8-bit field, CL).
// Output: register EDX = the encrypted value of the source character
// (in the low 8-bit field, DL).
__asm
encrypt_5:
push ebp // stores the pointer onto the stack
//
mov ebp, esp // move values in "esp" register into "ebp" register
mov eax, [ebp + 12] // take value from the stack that is 8 bits above
// from the pointer a putting it in the "eax" register
mov ecx, [ebp + 8] // take value from the stack that is 8 bits above
// from the pointer a putting it on ecx
push eax // stores the Ekey address onto the stack
mov al, byte ptr[eax] // move the pointer of the eax register to the al register in bytes
push ecx // move the encrytped charcter value on the stack
and eax, 0x7C // and eax with 0x7c (1111100 in binary)
ror eax, 1 // Rorate eax register value right by 1 byte shift
not eax // not the eax values within the eax register
ror eax, 1 // Rotate eax register value right by 1 byte shift
inc eax // increase the byte value of eax by 1
mov edx, eax // moves the values within eax to edx
pop ecx // removes ecx from the stack
pop eax // removes eax from the stack
mov byte ptr[eax], dl // move dl to the pointer of the eax register in bytes
xor edx, ecx // Exclusive or ecx with edx and stores the value with edx
rol dl, 1 // Rotate dl register value left by 1 byte shift
pop ebp // returning ebp back to the orginal value
ret // end function
//--- End of Assembly code
//*** end of encrypt_chars function
//---------------------------------------------------------------------------------------------------------------
//---------------------------------------------------------------------------------------------------------------
//----------------- DECRYPTION ROUTINES -------------------------------------------------------------------------
//
void decrypt_chars (int length, char EKey)
char temp_char; // Character temporary store
for (int i = 0; i < length; i++) // Encrypt characters one at a time
temp_char = EChars[i]; // Get the next char from Original Chars array
__asm
push eax // stores the "eax" register out onto the stack
push ecx // stores the "ecx" register out onto the stack
push edx // stores the "edx" register out onto the stack
movzx ecx, temp_char // zeroise "ecx" register and move values in "temp_char" varaible to "ecx" register
lea eax, EKey // copies address of values contained within the EKey varaible and moves it into "eax"register
push eax // stores the "eax" register out onto the stack
push ecx // stores the "ecx" register out onto the stack
call decrypt_5 // runs the function called "decryptX"
mov temp_char, dl // move values in "dl" register into "temp_char" variable
add esp, 8 // add 8 to the "esp" register
pop edx // removes the "edx" register from the stack
pop ecx // removes the "ecx" register from the stack
pop eax // removes the "eax" register from the stack
DChars[i] = temp_char; // Store encrypted char in the Encrypted Chars array
return;
// Inputs: register EAX = 32-bit address of Ekey,
// ECX = the character to be encrypted
// (in the low 8-bit field, CL).
// Output: register EDX = the encrypted value of the source character
// (in the low 8-bit field, DL).
__asm
decrypt_5:
push ebp // stores the pointer onto the stack
//
mov ebp, esp // move values in "esp" register into "ebp" register
mov eax, [ebp + 12] // take value from the stack that is 8 bits above
// from the pointer a putting it in the "eax" register
mov ecx, [ebp + 8] // take value from the stack that is 8 bits above
// from the pointer a putting it on ecx
push eax // stores the Ekey address onto the stack
mov al, byte ptr[eax] // move the pointer of the eax register to the al register in bytes
push ecx // move the encrytped charcter value on the stack
and eax, 0x7C // and eax with 0x7c (1111100 in binary)
ror eax, 1 // Rorate eax register value right by 1 byte shift
not eax // not the eax values within the eax register
ror eax, 1 // Rotate eax register value right by 1 byte shift
inc eax // increase the byte value of eax by 1
mov edx, eax // moves the values within eax to edx
pop ecx // removes ecx from the stack
pop eax // removes eax from the stack
mov byte ptr[eax], dl // move dl to the pointer of the eax register in bytes
xor edx, ecx // Exclusive or ecx with edx and stores the value with edx
rol dl, 1 // Rotate dl register value left by 1 byte shift
pop ebp // returning ebp back to the orginal value
ret // end function
//--- End of Assembly code
//*** end of decrypt_chars function
//---------------------------------------------------------------------------------------------------------------
我觉得答案与这部分有关，并以某种方式“反转”它，因为这是唯一改变 Ekey 的部分：
and eax, 0x7C // and eax with 0x7c (1111100 in binary)
ror eax, 1 // Rorate eax register value right by 1 byte shift
not eax // not the eax values within the eax register
ror eax, 1 // Rotate eax register value right by 1 byte shift
inc eax // increase the byte value of eax by 1
请帮我解决这个问题。
【问题讨论】：
将 eax 寄存器值右移 1 个字节。不，它只是 32 位寄存器的 1 位，而不是整个字节。 与***.com/questions/60787907/…类似，唯一需要修改的部分是xor edx, ecx\rol dl, 1。试试ror cl, 1 \ xor dl, cl。
【参考方案1】：
请参考我对x86 assembly encryption to decryption的回复。我用another revision 更新了repo，用你的encrypt_5 和decrypt_5 替换了他们的encrypt_3 和decrypt_3。以下是encrypt_5 中实际相关的特定部分：
mov byte [eax], dl
xor edx, ecx
rol dl, 1
这是decrypt_5 中的反面部分。至于链接的答案，密钥字节计算保持完全相同。 （您错误地将密钥字节计算识别为要反转的部分，而它应该是密钥字节对明文字节的应用。）
mov byte [eax], dl
ror cl, 1
xor edx, ecx
（由于只使用了低字节，xor dl, cl 是xor 行的有效替换。）
【讨论】：
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