When A Sponge Helps To Make Things Secure — And Light-weight

When A Sponge Helps To Make Things Secure — And Light-weight

When can you get a hash function and also implement encryption? Well, you can when you have a sponge, and which stores and processes a permutation value. It was first introduced with SHA-3 and where we define a state size (S) of 1600 bits (Keccak-f[1600]). This state (S) is made up from r (rate) and c (capacity). The total bits in the state is thus 1,600 bits. With this we can either use it as a hashing method or as an authenticated encryption method:

A background on the sponge method is here.

The lightweight nature of the sponge method makes it perfect for a lightweight encryption and/or hashing method. One of these is ASCON, and which uses a duplex-sponge mode. For encryption, it can use 64-bit or 128-bit block sizes. Currently, ASCON is one of the finalists for a NIST-defined lightweight encryption method.

ASCON — Authenticated Encryption

ASCON [2] was designed by Christoph Dobraunig, Maria Eichlseder, Florian Mendel and Martin Schläffer from Graz University of Technology, Infineon Technologies, and Radboud University. It is both a lightweight hashing and encryption method. ASCON uses a single lightweight permutation with Sponge-based mode of operation and an SPN (substitution–permutation network) permutation. Overall it has an easy method of implementing within hardware (2.6 gate equivalents) and software. A 5-bit S-box (as used in Keccak’s S-box core) is used to enable a lightweight approach and it has no known side-channel attacks. It can also achieve high throughputs such as throughputs of between 4.9 and 7.3 Gbps. It stores its current state with 320 bits.

Overall we have two C files and a few header files. To compile we can use the gcc compiler:

gcc main.c permutations.c printstate.c  -o ascon.exe

outline of the C code is [1][here]:

#include "api.h"
#include "ascon.h"
#include "crypto_aead.h"
#include "permutations.h"
#include "printstate.h"

#if !ASCON_INLINE_MODE
#undef forceinline
#define forceinline
#endif

#define CRYPTO_BYTES 64

void string2hexString(unsigned char* input, int clen, char* output);
static unsigned char ascii2byte(char *hexstring, unsigned char *bytearray);

int main (int argc, char *argv[]) {

  unsigned long long mlen;
  unsigned long long clen;

  unsigned char plaintext[CRYPTO_BYTES];
  unsigned char cipher[CRYPTO_BYTES]; 
  unsigned char npub[CRYPTO_NPUBBYTES]="";
  unsigned char ad[CRYPTO_ABYTES]="";
  unsigned char nsec[CRYPTO_ABYTES]="";
  
  unsigned char key[CRYPTO_KEYBYTES];

  char pl[CRYPTO_BYTES]="hello";
  char chex[CRYPTO_BYTES]="";
  char keyhex[2*CRYPTO_KEYBYTES+1]="0123456789ABCDEF0123456789ABCDEF";
  char nonce[2*CRYPTO_NPUBBYTES+1]="000000000000111111111111";
   char add[CRYPTO_ABYTES]="";

  if( argc > 1 ) {
      strcpy(pl,argv[1]);
  }
  if( argc > 2 ) {
      strcpy(keyhex,argv[2]);
  }
    if( argc > 3 ) {
      strcpy(nonce,argv[3]);
  }
     if( argc > 4 ) {
      strcpy(add,argv[4]);
  }
  

  strcpy(plaintext,pl);
  strcpy(ad,add);
  ascii2byte(keyhex,key);
  ascii2byte(nonce,npub);

  printf("Ascon light-weight cipher\n");
  printf("Plaintext: %s\n",plaintext);
  printf("Key: %s\n",keyhex);
  printf("Nonce: %s\n",nonce);
  printf("Additional Information: %s\n\n",ad);

  printf("Plaintext: %s\n",plaintext);

  int ret = crypto_aead_encrypt(cipher,&clen,plaintext,strlen(plaintext),ad,strlen(ad),nsec,npub,key);

string2hexString(cipher,clen,chex);

  printf("Cipher: %s, Len: %llu\n",chex, clen);

  ret = crypto_aead_decrypt(plaintext,&mlen,nsec,cipher,clen,ad,strlen(ad),npub,key);

  plaintext[mlen]='\0';
  printf("Plaintext: %s, Len: %llu\n",plaintext, mlen);

  if (ret==0) {
    printf("Success!\n");
  }  
 
	return 0;
} 

forceinline void ascon_loadkey(word_t* K0, word_t* K1, word_t* K2,
                               const uint8_t* k) {
  KINIT(K0, K1, K2);
  if (CRYPTO_KEYBYTES == 20) {
    *K0 = XOR(*K0, KEYROT(WORD_T(0), LOAD(k, 4)));
    k += 4;
  }
  *K1 = XOR(*K1, LOAD(k, 8));
  *K2 = XOR(*K2, LOAD(k + 8, 8));
}

forceinline void ascon_init(state_t* s, const uint8_t* npub, const uint8_t* k) {
  /* load nonce */
  word_t N0 = LOAD(npub, 8);
  word_t N1 = LOAD(npub + 8, 8);
  /* load key */
  word_t K0, K1, K2;
  ascon_loadkey(&K0, &K1, &K2, k);
  /* initialize */
  PINIT(s);
  if (CRYPTO_KEYBYTES == 16 && ASCON_RATE == 8)
    s->x0 = XOR(s->x0, ASCON_128_IV);
  if (CRYPTO_KEYBYTES == 16 && ASCON_RATE == 16)
    s->x0 = XOR(s->x0, ASCON_128A_IV);
  if (CRYPTO_KEYBYTES == 20) s->x0 = XOR(s->x0, ASCON_80PQ_IV);
  if (CRYPTO_KEYBYTES == 20) s->x0 = XOR(s->x0, K0);
  s->x1 = XOR(s->x1, K1);
  s->x2 = XOR(s->x2, K2);
  s->x3 = XOR(s->x3, N0);
  s->x4 = XOR(s->x4, N1);
  P(s, 12);
  if (CRYPTO_KEYBYTES == 20) s->x2 = XOR(s->x2, K0);
  s->x3 = XOR(s->x3, K1);
  s->x4 = XOR(s->x4, K2);
  printstate("initialization", s);
}

forceinline void ascon_adata(state_t* s, const uint8_t* ad, uint64_t adlen) {
  const int nr = (ASCON_RATE == 8) ? 6 : 8;
  if (adlen) {
    /* full associated data blocks */
    while (adlen >= ASCON_RATE) {
      s->x0 = XOR(s->x0, LOAD(ad, 8));
      if (ASCON_RATE == 16) s->x1 = XOR(s->x1, LOAD(ad + 8, 8));
      P(s, nr);
      ad += ASCON_RATE;
      adlen -= ASCON_RATE;
    }
    /* final associated data block */
    word_t* px = &s->x0;
    if (ASCON_RATE == 16 && adlen >= 8) {
      s->x0 = XOR(s->x0, LOAD(ad, 8));
      px = &s->x1;
      ad += 8;
      adlen -= 8;
    }
    *px = XOR(*px, PAD(adlen));
    if (adlen) *px = XOR(*px, LOAD(ad, adlen));
    P(s, nr);
  }
  /* domain separation */
  s->x4 = XOR(s->x4, WORD_T(1));
  printstate("process associated data", s);
}

forceinline void ascon_encrypt(state_t* s, uint8_t* c, const uint8_t* m,
                               uint64_t mlen) {
  const int nr = (ASCON_RATE == 8) ? 6 : 8;
  /* full plaintext blocks */
  while (mlen >= ASCON_RATE) {
    s->x0 = XOR(s->x0, LOAD(m, 8));
    STORE(c, s->x0, 8);
    if (ASCON_RATE == 16) {
      s->x1 = XOR(s->x1, LOAD(m + 8, 8));
      STORE(c + 8, s->x1, 8);
    }
    P(s, nr);
    m += ASCON_RATE;
    c += ASCON_RATE;
    mlen -= ASCON_RATE;
  }
  /* final plaintext block */
  word_t* px = &s->x0;
  if (ASCON_RATE == 16 && mlen >= 8) {
    s->x0 = XOR(s->x0, LOAD(m, 8));
    STORE(c, s->x0, 8);
    px = &s->x1;
    m += 8;
    c += 8;
    mlen -= 8;
  }
  *px = XOR(*px, PAD(mlen));
  if (mlen) {
    *px = XOR(*px, LOAD(m, mlen));
    STORE(c, *px, mlen);
  }
  printstate("process plaintext", s);
}

forceinline void ascon_decrypt(state_t* s, uint8_t* m, const uint8_t* c,
                               uint64_t clen) {
  const int nr = (ASCON_RATE == 8) ? 6 : 8;
  /* full ciphertext blocks */
  while (clen >= ASCON_RATE) {
    word_t cx = LOAD(c, 8);
    s->x0 = XOR(s->x0, cx);
    STORE(m, s->x0, 8);
    s->x0 = cx;
    if (ASCON_RATE == 16) {
      cx = LOAD(c + 8, 8);
      s->x1 = XOR(s->x1, cx);
      STORE(m + 8, s->x1, 8);
      s->x1 = cx;
    }
    P(s, nr);
    m += ASCON_RATE;
    c += ASCON_RATE;
    clen -= ASCON_RATE;
  }
  /* final ciphertext block */
  word_t* px = &s->x0;
  if (ASCON_RATE == 16 && clen >= 8) {
    word_t cx = LOAD(c, 8);
    s->x0 = XOR(s->x0, cx);
    STORE(m, s->x0, 8);
    s->x0 = cx;
    px = &s->x1;
    m += 8;
    c += 8;
    clen -= 8;
  }
  *px = XOR(*px, PAD(clen));
  if (clen) {
    word_t cx = LOAD(c, clen);
    *px = XOR(*px, cx);
    STORE(m, *px, clen);
    *px = CLEAR(*px, clen);
    *px = XOR(*px, cx);
  }
  printstate("process ciphertext", s);
}

forceinline void ascon_final(state_t* s, const uint8_t* k) {
  /* load key */
  word_t K0, K1, K2;
  ascon_loadkey(&K0, &K1, &K2, k);
  /* finalize */
  if (CRYPTO_KEYBYTES == 16 && ASCON_RATE == 8) {
    s->x1 = XOR(s->x1, K1);
    s->x2 = XOR(s->x2, K2);
  }
  if (CRYPTO_KEYBYTES == 16 && ASCON_RATE == 16) {
    s->x2 = XOR(s->x2, K1);
    s->x3 = XOR(s->x3, K2);
  }
  if (CRYPTO_KEYBYTES == 20) {
    s->x1 = XOR(s->x1, KEYROT(K0, K1));
    s->x2 = XOR(s->x2, KEYROT(K1, K2));
    s->x3 = XOR(s->x3, KEYROT(K2, WORD_T(0)));
  }
  P(s, 12);
  s->x3 = XOR(s->x3, K1);
  s->x4 = XOR(s->x4, K2);
  printstate("finalization", s);
}

int crypto_aead_encrypt(unsigned char* c, unsigned long long* clen,
                        const unsigned char* m, unsigned long long mlen,
                        const unsigned char* ad, unsigned long long adlen,
                        const unsigned char* nsec, const unsigned char* npub,
                        const unsigned char* k) {
  state_t s;
  (void)nsec;
  *clen = mlen + CRYPTO_ABYTES;
  /* perform ascon computation */
  ascon_init(&s, npub, k);
  ascon_adata(&s, ad, adlen);
  ascon_encrypt(&s, c, m, mlen);
  ascon_final(&s, k);
  /* set tag */
  STOREBYTES(c + mlen, s.x3, 8);
  STOREBYTES(c + mlen + 8, s.x4, 8);
  return 0;
}

int crypto_aead_decrypt(unsigned char* m, unsigned long long* mlen,
                        unsigned char* nsec, const unsigned char* c,
                        unsigned long long clen, const unsigned char* ad,
                        unsigned long long adlen, const unsigned char* npub,
                        const unsigned char* k) {
  state_t s;
  (void)nsec;
  if (clen < CRYPTO_ABYTES) return -1;
  *mlen = clen = clen - CRYPTO_ABYTES;
  /* perform ascon computation */
  ascon_init(&s, npub, k);
  ascon_adata(&s, ad, adlen);
  ascon_decrypt(&s, m, c, clen);
  ascon_final(&s, k);
  /* verify tag (should be constant time, check compiler output) */
  s.x3 = XOR(s.x3, LOADBYTES(c + clen, 8));
  s.x4 = XOR(s.x4, LOADBYTES(c + clen + 8, 8));
  return NOTZERO(s.x3, s.x4);
}

A sample run is [here]:

ASCON light-weight cipher
Plaintext: abc
Key: 0123456789ABCDEF0123456789ABCDEF
Nonce: AABB000000000111111111111
Additional Information: abc

Plaintext: abc
Cipher: 0A613CD5FA50ABC0F6C5, Len: 19
Plaintext: abc, Len: 3
Success!

The hashing method is here: