Key used by HMAC-SHAx in PBKDF2
The key is the password.

For the first iteration, the salt concatenated with the desired output length in bytes is the input.
U1 = PRF(Password, Salt || INT_32_BE(i))

For subsequent iterations, the HMAC result is the salt.
U2 = PRF(Password, U1)
...
Uc = PRF(Password, Uc-1)