> For the complete documentation index, see [llms.txt](https://docs.hex-rays.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.hex-rays.com/developer/idc/idc-examples/analyzing-encrypted-code.md).

# Analyzing encrypted code

This small tutorial demonstrates how to use IDC to decrypt part of a program at analysis time. The sample file is a portion of the Ripper virus.

### 1st step

The binary image of the virus is loaded into IDA and analysis is started at the entry point

![](/files/hHXrW4p5OiAHq9mTFsld)

Obviously, the bytes right after the call don’t make sense, but the call gives us a clue: it is a decryption routine.

![](/files/FJ7dwgnCCqjDs0KGiyCV)

### 2nd step

We create a small IDC program that mimicks the decryption routine.

```
static decrypt(from, size, key ) {
  auto i, x;           // we define the variables
  for ( i=0; i < size; i=i+1 ) { 
    x = Byte(from);    // fetch the byte
    x = (x^key);       // decrypt it
    PatchByte(from,x); // put it back
    from = from + 1;   // next byte
  } 
}
            
```

Save it on disk and press F2 to load it into IDA's interpreter.

![](/files/JKv5WIaP0PG2x1elu0QU)

### 3rd step

Then, we press shift-F2 to call it with the appropriate values. Please note the linear address used for the starting point. Pressing OK executes the statement.

![](/files/VP5UJAqWvjjPRFbHMaBi)

The bytes are now decrypted

![](/files/b0E2ASMLNV08zaYUij6H)

### 4th step

We move the cursor to offset 0x50 and press C to inform IDA that there is now code at that location.

![](/files/rUsDP7OBPKDdqPHS57Nb)

And the code to allocate memory for the virus appears, along with a rather impolite message... The analysis may now resume.


---

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