n the case of RC4 stream cipher, the very first step is to initialize the S Vector and T Vector. If the key is HELLO, then the value of T[0x0F] will be

In the RC4 stream cipher, the first step is to initialize the S vector and T vector. The T vector is initially a repetition of the key. If the key is "HELLO", it is represented in ASCII values. The ASCII values for "HELLO" are 72, 69, 76, 76, 79.

The T vector is filled up to 256 values by repeating the key. So, the T vector will look like this: [72, 69, 76, 76, 79, 72, 69, 76, 76, 79, 72, 69, 76, 76, 79, ...] until it reaches 256 values.

The value of T[0x0F] is asking for the 16th value in the T vector (since 0x0F is 15 in decimal and arrays start at 0).

Since the key "HELLO" is 5 characters long, every 5th character in the T vector will start repeating the key from the beginning. Therefore, the 16th value in the T vector will be the first character of the key, which is "H".

In ASCII, "H" is represented by the decimal number 72. So, T[0x0F] will be 72.