This tool uses a simple XOR-based obfuscation scheme, as seen in IBM WebSphere.
Each character of your input is XORed with the underscore character '_' (ASCII 0x5F), and the result is then Base64-encoded. To decode, the Base64 string is first decoded to bytes, and then each byte is XORed again with '_' to recover the original text. This is not encryption — just reversible obfuscation.
Example: the word hello becomes {xor}NzozMzA= after XOR obfuscation with '_' and Base64 encoding.