🏷️Sponge function

• sponge construction is any of a class of algorithms with finite internal state that take an input bit stream of any length and produce an output bit stream of any desired length.
  
• 🖇️cf. SHA-3는 keccak-f 순열을 기반으로 한다.

📌Operation

• A sponge function is built from three components
• S: state memory containing b bits,
  • S is divided into two sections:
    • R: the bitrate, one of size r
    • C: the capacity, the remaining part of size c
• f: permutation function (가역적)
• P: a padding function
• P appends enough bits to the input string so that the length of the padded input is a whole multiple of the bitrate, r.
• This means the input is segmented into blocks of r bits.
• Since C is confidential, do not perform XOR operations on C.

absorbing

• S(= R + C) is initialized to zero
• for each r-bit block B of P(string)
• R is replaced with R XOR B
• S is replaced by f(S)

squeezing

• The sponge function output is now ready to be produced "squeezed out".
• repeat
  • output the R portion of S
  • S is replaced by f(S) unless output is full
• If less than r bits remain to be output, then R will be truncated (only part of R will be output).