AES-GCM decryption in C - c

Question

I have token (base64url) that i need to decrypt with aes-gcm. The token contains:
16 bytes for IV, 17 bytes for TAG and the rest is the binary that needs to be decrypted.
I can't seem to be able to figure out, this is my code:
{
unsigned char * source = "BASE64-ENCODED-BINARY";
unsigned char key_raw[] = "KEY";
unsigned char key[2048];
int key_len = 0;
unsigned char output[2048];
int output_len = 0;
unsigned char * plaintext;
int c, r;
size_t out;
int dest_len = 4*(sizeof(source)/3);
// int key_len = 4*(sizeof(key_raw)/3);
unsigned char iv[16];
unsigned char tag[16];
unsigned char content[2048];
int content_len = 0;
b64ud_t s;
EVP_CIPHER_CTX *ctx;
int outlen, tmplen, rv;
unsigned char outbuf[2048];
/* token decode */
base64url_decode_reset(&s);
//memset( output,0, dest_len );
base64url_decode( output, 2048, source, strlen(source), NULL );
/* Just look through the output to get the decode result len */
for(;;)
{
if( output[output_len] )
{
output_len++;
} else {
break;
}
}
printf("decoded-token: [length: %d]\n", output_len );
BIO_dump_fp(stdout, output, output_len);
/* ket decode */
base64url_decode_reset(&s);
memset( key, 0, key_len );
base64url_decode( key, key_len-1, key_raw, strlen(key_raw), NULL );
/* Just look through the key to get the decode result len */
for(;;)
{
if( key[key_len] )
{
key_len++;
} else {
break;
}
}
printf("decoded-key: [Length: %d]\n", key_len );
BIO_dump_fp(stdout, key, key_len);
/*
The token is composed like so:
[16bytes IV] + [16 bytes TAG] + [Encrypted Message]
*/
printf("getting 16 bytes out of the decode output and storing them in IV\n");
for(int v=0;v<16;v++)
{
iv[v] = output[v];
}
printf("getting the NEXT 16 bytes out of the decode output and storing them in TAG\n");
for(int v=16;v<32;v++)
{
tag[v-16] = output[v];
}
printf("Just count haw many non-00 bytes remain and store it in content_len\n");
for(int i=32;i<output_len;i++)
{
if(output[i])
{
content_len++;
} else {
break;
}
}
printf("%d\n", content_len);
printf("We now use content_len and get the remaining bytes and store them in content\n");
for(int v=0; v<content_len;v++)
{
content[v] = output[v+32];
}
printf( "iv:\n" );
BIO_dump_fp(stdout, iv, sizeof(iv));
printf("tag:\n" );
BIO_dump_fp(stdout, tag, sizeof(tag));
printf("content :\n" );
BIO_dump_fp(stdout, content, content_len);
printf("AES GCM Decrypt:\n");
unsigned char * key_final;
key_final = key;
unsigned char * ciphertext;
ciphertext = content;
printf("Ciphertext:\n");
BIO_dump_fp(stdout, content, content_len);
ctx = EVP_CIPHER_CTX_new();
/* Select cipher */
EVP_DecryptInit_ex(ctx, EVP_aes_256_gcm(), NULL, NULL, NULL);
/* Set IV length, omit for 96 bits */
EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_IVLEN, sizeof(iv), NULL);
/* Specify key and IV */
EVP_DecryptInit_ex(ctx, NULL, NULL, key, iv);
/* Zero or more calls to specify any AAD */
//EVP_DecryptUpdate(ctx, NULL, &outlen, gcm_aad, sizeof(gcm_aad));
/* Decrypt plaintext */
EVP_DecryptUpdate(ctx, outbuf, &outlen, ciphertext, content_len);
/* Output decrypted block */
printf("Plaintext:\n");
BIO_dump_fp(stdout, outbuf, outlen);
/* Set expected tag value. */
EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_TAG, sizeof(tag), (void *)tag);
/* Finalise: note get no output for GCM */
rv = EVP_DecryptFinal_ex(ctx, outbuf, &outlen);
/*
* Print out return value. If this is not successful authentication
* failed and plaintext is not trustworthy.
*/
printf("outbuf: %s", outbuf);
printf("Tag Verify %s\n", rv > 0 ? "Successful!" : "Failed!");
EVP_CIPHER_CTX_free(ctx);
return 0;
}
My output is:
decoded-token: [length: 57]
0000 - ae 3f d9 92 46 54 39 93-31 64 e7 ce 98 ba 44 50 .?..FT9.1d....DP
0010 - 1d ec 89 4e ee e9 18 d9-15 e3 3d b3 e8 1b ff 10 ...N......=.....
0020 - 91 e7 a5 85 28 50 09 88-cc 85 d9 3e 82 05 19 a5 ....(P.....>....
0030 - 87 f4 b2 d2 2f e5 7f 24-fd ..../..$.
decoded-key: [Length: 33]
0000 - 0e 0b e4 0a b9 32 04 d4-b2 f7 21 cf d5 8c e7 c9 .....2....!.....
0010 - cd 83 90 74 c8 51 76 8e-e8 d9 44 c3 80 92 ab 40 ...t.Qv...D....#
0020 - e3 .
doing iv
doing tag
getting content length: 25
copying content length to content var
iv:
0000 - ae 3f d9 92 46 54 39 93-31 64 e7 ce 98 ba 44 50 .?..FT9.1d....DP
tag:
0000 - 1d ec 89 4e ee e9 18 d9-15 e3 3d b3 e8 1b ff 10 ...N......=.....
content :
0000 - 91 e7 a5 85 28 50 09 88-cc 85 d9 3e 82 05 19 a5 ....(P.....>....
0010 - 87 f4 b2 d2 2f e5 7f 24-fd ..../..$.
AES GCM Decrypt:
Ciphertext:
0000 - 91 e7 a5 85 28 50 09 88-cc 85 d9 3e 82 05 19 a5 ....(P.....>....
0010 - 87 f4 b2 d2 2f e5 7f 24-fd ..../..$.
Plaintext:
0000 - f3 6d 72 13 d9 dd 5b a3-b6 af 73 8d a2 93 8b f7 .mr...[...s.....
0010 - 0e 9e 2a 87 6c 82 84 bd-46 ..*.l...F
outbuf: �mr��[���s������*�l���FTag Verify Failed!
I can't be 100% I'm doing it right, but anyway i'm getting Verify Failed!
Help is GREATLY appreciated!

Answer 1

I'm not very familiar with AES, but this code looks suspicious:
int dest_len = 4*(sizeof(source)/3);
because source is defined as:
unsigned char * source = "BASE64-ENCODED-BINARY";
and in consequence sizeof(source) gives you the size of a pointer.
Try:
int dest_len = 4*(strlen(source)/3);
or define source as:
const unsigned char source[] = "BASE64-ENCODED-BINARY";