Root Cause Analysis:
The CWE-787 (out-of-bounds write) vulnerability exists in the code because the `cipher_len` variable is used to determine the size of the `out` buffer without any bounds checking. This can lead to a buffer overflow if the `cipher_len` is larger than the size of the `out` buffer. This can result in writing data beyond the bounds of the `out` buffer, causing a buffer overflow vulnerability, which is CWE-787.