To generate this patch, we can follow these reasoning steps:

Step 1. Root Cause Analysis:

1. Input Validation:
   - The function takes input parameters 'in', 'inlen', 'out', and 'out_len'.
   - 'cipher_len' is derived from the input 'in' based on different conditions.

2. Buffer Size:
   - A fixed-size buffer 'plaintext' is declared with 4096 bytes.
   - There's a check to ensure 'cipher_len' doesn't exceed the size of 'plaintext'.

3. Cipher Length Adjustment:
   - The 'cipher_len' is potentially reduced in a while loop (line 46-47).

4. Vulnerability Point:
   - At line 53, '*out_len = cipher_len - 2' is executed without checking if 'out_len' can accommodate this value.

5. Potential Overflow:
   - If 'cipher_len - 2' is larger than the actual allocated size for 'out', it could lead to a buffer overflow when writing to 'out' later in the function.

Conclusion: The vulnerability occurs because there's no check to ensure that 'cipher_len - 2' is not larger than the actual size allocated for 'out' before assigning it to '*out_len'.

Step 2. Fixing Strategy:

1. Add a check to ensure '*out_len' is large enough to hold 'cipher_len - 2'.
2. This check should be combined with the existing condition for 'cipher_len == 2'.
3. If either condition fails, the function should likely return an error or handle the case appropriately.

Therefore, the patch modifies the condition to:

```
< 	if (2 == cipher_len)
---
> 	if (2 == cipher_len || *out_len < cipher_len - 2)
```

This patch ensures that '*out_len' is large enough to hold the result before assigning 'cipher_len - 2' to it, preventing potential buffer overflow in subsequent operations using '*out_len'.