91 lines
2.3 KiB
C
91 lines
2.3 KiB
C
#include <stdio.h>
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#include <string.h>
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#include <unistd.h>
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#include <openssl/evp.h>
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#define BYTES_PER_READ 32 * 1024 // 32kb
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#define INITIAL_BUFFER_SIZE 256 * 1024 // 256kb, must be at least 2*BYTES_PER_READ
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void hex2string(char *src, unsigned char **dst_p)
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{
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*dst_p = malloc(strlen(src)/2);
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unsigned char *dst = *dst_p;
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unsigned char *end = dst + (strlen(src)/2);
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unsigned int u;
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while (dst < end && sscanf(src, "%2x", &u) == 1) {
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*dst++ = u;
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src += 2;
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}
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}
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int main(int argc, char **argv)
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{
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unsigned char *gcm_ivkey, *gcm_ct, *gcm_pt;
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int outlen, rv, final_outlen;
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size_t read, actual_size = 0, total_size = INITIAL_BUFFER_SIZE;
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if (argc < 2) {
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printf("Usage: %s <key>\n", argv[0]);
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return 1;
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}
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hex2string(argv[1], &gcm_ivkey);
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gcm_ct = malloc(total_size);
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while ((read = fread(gcm_ct + actual_size, 1, BYTES_PER_READ, stdin)) > 0) {
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actual_size += read;
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if ((actual_size + BYTES_PER_READ) > total_size) {
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total_size = total_size * 1.5;
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gcm_ct = realloc(gcm_ct,total_size);
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}
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}
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if (actual_size < 32) {
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fprintf(stderr, "File too small for decryption\n");
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return 1;
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}
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actual_size -= 16;
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gcm_pt = malloc(actual_size);
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EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();
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/* Select cipher */
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EVP_DecryptInit_ex(ctx, EVP_aes_256_gcm(), NULL, NULL, NULL);
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/* Set IV length, omit for 96 bits */
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EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, 16, NULL);
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/* Specify key and IV */
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EVP_DecryptInit_ex(ctx, NULL, NULL, gcm_ivkey+16, gcm_ivkey);
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/* Decrypt plaintext */
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EVP_DecryptUpdate(ctx, gcm_pt, &outlen, gcm_ct, actual_size);
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/* Set expected tag value. */
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EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_TAG, 16, gcm_ct + actual_size);
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/* Finalise: note get no output for GCM */
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rv = EVP_DecryptFinal_ex(ctx, gcm_pt, &final_outlen);
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EVP_CIPHER_CTX_free(ctx);
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free(gcm_ivkey);
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free(gcm_ct);
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if (rv > 0) {
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// success!
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fwrite(gcm_pt, 1, outlen, stdout);
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free(gcm_pt);
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return 0;
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} else {
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fprintf(stderr, "File integrity check failed\n");
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free(gcm_pt);
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return 1;
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}
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}
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// compile with: gcc aesgcm.c -lcrypto -o aesgcm
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