encrypt.c

#include "api.h"
#include "crypto_aead.h"
#include <string.h> 
#include "elephant_200.h"

BYTE rotl(BYTE b)
{
    return (b << 1) | (b >> 7);
}

int constcmp(const BYTE* a, const BYTE* b, SIZE length)
{
    BYTE r = 0;

    for (SIZE i = 0; i < length; ++i)
        r |= a[i] ^ b[i];
    return r; 
}

// State should be BLOCK_SIZE bytes long
// Note: input may be equal to output
void lfsr_step(BYTE* output, BYTE* input)
{
    BYTE temp = rotl(input[0]) ^ rotl(input[2]) ^ (input[13] << 1);
    for(SIZE i = 0; i < BLOCK_SIZE - 1; ++i)
        output[i] = input[i + 1];
    output[BLOCK_SIZE - 1] = temp;
}

void xor_block(BYTE* state, const BYTE* block, SIZE size)
{
    for(SIZE i = 0; i < size; ++i)
        state[i] ^= block[i];
}

// Write the ith assocated data block to "output".
// The nonce is prepended and padding is added as required.
// adlen is the length of the associated data in bytes
void get_ad_block(BYTE* output, const BYTE* ad, SIZE adlen, const BYTE* npub, SIZE i)
{
    SIZE len = 0;
    // First block contains nonce
    // Remark: nonce may not be longer then BLOCK_SIZE
    if(i == 0) {
        memcpy(output, npub, CRYPTO_NPUBBYTES);
        len += CRYPTO_NPUBBYTES;
    }

    const SIZE block_offset = i * BLOCK_SIZE - (i != 0) * CRYPTO_NPUBBYTES;
    // If adlen is divisible by BLOCK_SIZE, add an additional padding block
    if(i != 0 && block_offset == adlen) {
        memset(output, 0x00, BLOCK_SIZE);
        output[0] = 0x01;
        return;
    }
    const SIZE r_outlen = BLOCK_SIZE - len;
    const SIZE r_adlen  = adlen - block_offset;
    // Fill with associated data if available
    if(r_outlen <= r_adlen) { // enough AD
        memcpy(output + len, ad + block_offset, r_outlen);
    } else { // not enough AD, need to pad
        if(r_adlen > 0) // ad might be nullptr
            memcpy(output + len, ad + block_offset, r_adlen);
        memset(output + len + r_adlen, 0x00, r_outlen - r_adlen);
        output[len + r_adlen] = 0x01;
    }
}

// Return the ith assocated data block.
// clen is the length of the ciphertext in bytes 
void get_c_block(BYTE* output, const BYTE* c, SIZE clen, SIZE i)
{
    const SIZE block_offset = i * BLOCK_SIZE;
    // If clen is divisible by BLOCK_SIZE, add an additional padding block
    if(block_offset == clen) {
        memset(output, 0x00, BLOCK_SIZE);
        output[0] = 0x01;
        return;
    }
    const SIZE r_clen  = clen - block_offset;
    // Fill with associated data if available
    if(BLOCK_SIZE <= r_clen) { // enough ciphertext
        memcpy(output, c + block_offset, BLOCK_SIZE);
    } else { // not enough ciphertext, need to pad
        if(r_clen > 0) // c might be nullptr
            memcpy(output, c + block_offset, r_clen);
        memset(output + r_clen, 0x00, BLOCK_SIZE - r_clen);
        output[r_clen] = 0x01;
    }
}

// It is assumed that c is sufficiently long
// Also, tag and c should not overlap
void crypto_aead_impl(
    BYTE* c, BYTE* tag, const BYTE* m, SIZE mlen, const BYTE* ad, SIZE adlen,
    const BYTE* npub, const BYTE* k, int encrypt)
{ 
    // Compute number of blocks
    const SIZE nblocks_c  = mlen ? 1 + mlen / BLOCK_SIZE : 0;
    const SIZE nblocks_m  = (!mlen || mlen % BLOCK_SIZE) ? nblocks_c : nblocks_c - 1;
    const SIZE nblocks_ad = 1 + (CRYPTO_NPUBBYTES + adlen) / BLOCK_SIZE;
    const SIZE nb_it = (nblocks_m > nblocks_ad) ? nblocks_m : nblocks_ad + 1;

    // Storage for the expanded key L
    BYTE expanded_key[BLOCK_SIZE] = {0};
    memcpy(expanded_key, k, CRYPTO_KEYBYTES);
    permutation(expanded_key);

    // Buffers for storing previous, current and next mask
    BYTE mask_buffer_1[BLOCK_SIZE] = {0};
    BYTE mask_buffer_2[BLOCK_SIZE] = {0};
    BYTE mask_buffer_3[BLOCK_SIZE] = {0};
    memcpy(mask_buffer_2, expanded_key, CRYPTO_KEYBYTES);

    BYTE* previous_mask = mask_buffer_1;
    BYTE* current_mask = mask_buffer_2;
    BYTE* next_mask = mask_buffer_3;

    // Buffer to store current ciphertext block
    BYTE c_buffer[BLOCK_SIZE];
    
    // Tag buffer and initialization of tag to zero
    BYTE tag_buffer[BLOCK_SIZE] = {0};
    memset(tag, 0, CRYPTO_ABYTES);

    SIZE offset = 0;
    for(SIZE i = 0; i < nb_it; ++i) {
        // Compute mask for the next message
        lfsr_step(next_mask, current_mask);
        
        if(i < nblocks_m) {
            // Compute ciphertext block
            memcpy(c_buffer, npub, CRYPTO_NPUBBYTES);
            memset(c_buffer + CRYPTO_NPUBBYTES, 0, BLOCK_SIZE - CRYPTO_NPUBBYTES);
            xor_block(c_buffer, current_mask, BLOCK_SIZE);
            permutation(c_buffer);
            xor_block(c_buffer, current_mask, BLOCK_SIZE);
            const SIZE r_size = (i == nblocks_m - 1) ? mlen - offset : BLOCK_SIZE;
            xor_block(c_buffer, m + offset, r_size);
            memcpy(c + offset, c_buffer, r_size);
        }

        if(i < nblocks_c) {
            // Compute tag for ciphertext block
            get_c_block(tag_buffer, encrypt ? c : m, mlen, i);
            xor_block(tag_buffer, current_mask, BLOCK_SIZE);
            xor_block(tag_buffer, next_mask, BLOCK_SIZE);
            permutation(tag_buffer);
            xor_block(tag_buffer, current_mask, BLOCK_SIZE);
            xor_block(tag_buffer, next_mask, BLOCK_SIZE);
            xor_block(tag, tag_buffer, CRYPTO_ABYTES);
        }

        // If there is any AD left and i > 0, compute tag for AD block 
        if(i > 0 && i <= nblocks_ad) {
            get_ad_block(tag_buffer, ad, adlen, npub, i - 1);
            xor_block(tag_buffer, previous_mask, BLOCK_SIZE);
            xor_block(tag_buffer, next_mask, BLOCK_SIZE);
            permutation(tag_buffer);
            xor_block(tag_buffer, previous_mask, BLOCK_SIZE);
            xor_block(tag_buffer, next_mask, BLOCK_SIZE);
            xor_block(tag, tag_buffer, CRYPTO_ABYTES);
        }

        // Cyclically shift the mask buffers 
        // Value of next_mask will be computed in the next iteration
        BYTE* const temp = previous_mask;
        previous_mask = current_mask;
        current_mask = next_mask;
        next_mask = temp;

        offset += BLOCK_SIZE;
    }
}

// Remark: c must be at least mlen + CRYPTO_ABYTES long
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)
{ 
    (void)nsec;
    *clen = mlen + CRYPTO_ABYTES;
    BYTE tag[CRYPTO_ABYTES];
    crypto_aead_impl(c, tag, m, mlen, ad, adlen, npub, k, 1);
    memcpy(c + mlen, tag, CRYPTO_ABYTES); 
    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)
{
    (void)nsec;
    if(clen < CRYPTO_ABYTES)
        return -1;
    *mlen = clen - CRYPTO_ABYTES;
    BYTE tag[CRYPTO_ABYTES];
    crypto_aead_impl(m, tag, c, *mlen, ad, adlen, npub, k, 0);
    return (constcmp(c + *mlen, tag, CRYPTO_ABYTES) == 0) ? 0 : -1;
}