Add encryption ability to aesgcm.c

README.md

@@ -5,8 +5,10 @@ The anchor is made of 96 characters which represent 48 bytes in HEX. The first 1
 
 The encryption mode is ```aes-256-gcm```. The authentication tag of 16 bytes is appended to the file.
 
-This C project provides sample code on how to decrypt a file. Output is to stdout. Redirect to file or pipe to image viewer.
+This C project provides sample code on how to decrypt and encrypt a file. Output is to stdout. Redirect to file or pipe to image viewer.
 
 Compile with ```gcc aesgcm.c -lcrypto -o aesgcm```, openbrowser.sh also tries to compile it if the executable not already exist.
 
 Set openbrowser.sh as your browser in gajim or another XMPP client to automatically decrypt http uploads encrypted with OMEMO or PGP with Conversations.
+
+You can also encrypt data with aesgcm by sending in `enc` as the second parameter.

aesgcm.c

@@ -3,6 +3,7 @@
 #include <unistd.h>
 #include <openssl/evp.h>
 
+#define TAG_IV_LENGTH 16
 #define BYTES_PER_READ 32 * 1024 // 32kb
 #define INITIAL_BUFFER_SIZE 256 * 1024 // 256kb, must be at least 2*BYTES_PER_READ
 
@@ -22,14 +23,18 @@ void hex2string(char *src, unsigned char **dst_p)
 int main(int argc, char **argv)
 {
     unsigned char *gcm_ivkey, *gcm_ct, *gcm_pt;
-    int outlen, rv, final_outlen;
+    int outlen, rv = 0, final_outlen, decrypt = 1;
     size_t read, actual_size = 0, total_size = INITIAL_BUFFER_SIZE;
 
     if (argc < 2) {
-        printf("Usage: %s <key>\n", argv[0]);
+        fprintf(stderr, "Usage: %s <key> [enc]\n", argv[0]);
         return 1;
     }
 
+    // this means we want to encrypt, not decrypt
+    if (argc > 2 && strcmp("enc", argv[2]) == 0)
+        decrypt = 0;
+
     hex2string(argv[1], &gcm_ivkey);
 
     gcm_ct = malloc(total_size);
@@ -42,34 +47,52 @@ int main(int argc, char **argv)
         }
     }
 
-    if (actual_size < 32) {
+    if (actual_size < (decrypt ? 17 : 1)) {
-        fprintf(stderr, "File too small for decryption\n");
+        fprintf(stderr, "File too small for %scryption\n", decrypt ? "de" : "en");
         return 1;
     }
 
-    actual_size -= 16;
+    if(decrypt)
+        actual_size -= TAG_IV_LENGTH;
 
-    gcm_pt = malloc(actual_size);
+    gcm_pt = malloc(decrypt ? actual_size : (actual_size + TAG_IV_LENGTH));
 
     EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();
 
     /* Select cipher */
+    if(decrypt)
         EVP_DecryptInit_ex(ctx, EVP_aes_256_gcm(), NULL, NULL, NULL);
+    else
+        EVP_EncryptInit_ex(ctx, EVP_aes_256_gcm(), NULL, NULL, NULL);
 
     /* Set IV length, omit for 96 bits */
-    EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, 16, NULL);
+    EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, TAG_IV_LENGTH, NULL);
 
+    if(decrypt) {
         /* Specify key and IV */
-    EVP_DecryptInit_ex(ctx, NULL, NULL, gcm_ivkey+16, gcm_ivkey);
+        EVP_DecryptInit_ex(ctx, NULL, NULL, gcm_ivkey + TAG_IV_LENGTH, gcm_ivkey);
+
+        /* Set expected tag value. */
+        EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_TAG, TAG_IV_LENGTH, gcm_ct + actual_size);
 
         /* Decrypt plaintext */
         EVP_DecryptUpdate(ctx, gcm_pt, &outlen, gcm_ct, actual_size);
 
-    /* Set expected tag value. */
-    EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_TAG, 16, gcm_ct + actual_size);
-
         /* Finalise: note get no output for GCM */
         rv = EVP_DecryptFinal_ex(ctx, gcm_pt, &final_outlen);
+    } else {
+        /* Specify key and IV */
+        EVP_EncryptInit_ex(ctx, NULL, NULL, gcm_ivkey + TAG_IV_LENGTH, gcm_ivkey);
+
+        /* Encrypt plaintext */
+        EVP_EncryptUpdate(ctx, gcm_pt, &outlen, gcm_ct, actual_size);
+
+        /* Finalise: note get no output for GCM */
+        rv = EVP_EncryptFinal_ex(ctx, gcm_pt, &final_outlen);
+
+        /* Get expected tag value. */
+        EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_GET_TAG, TAG_IV_LENGTH, gcm_pt + actual_size);
+    }
 
     EVP_CIPHER_CTX_free(ctx);
     free(gcm_ivkey);
@@ -77,7 +100,7 @@ int main(int argc, char **argv)
 
     if (rv > 0) {
         // success!
-        fwrite(gcm_pt, 1, outlen, stdout);
+        fwrite(gcm_pt, 1, decrypt ? outlen : (outlen + TAG_IV_LENGTH), stdout);
         free(gcm_pt);
         return 0;
     } else {