c - C中的AES-GCM解密

Question

我有需要用 aes-gcm 解密的 token (base64url)。 token 包含:

16 个字节用于 IV，17 个字节用于 TAG，其余是需要解密的二进制文件。

我似乎无法弄清楚，这是我的代码:

{
    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;
}

我的输出是:

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!

我不能 100% 我做对了，但无论如何我得到验证失败!

非常感谢帮助!

Answer 1

我对 AES 不是很熟悉，但是这段代码看起来很可疑:

int dest_len = 4*(sizeof(source)/3);

因为 source定义为:

unsigned char * source = "BASE64-ENCODED-BINARY";

结果 sizeof(source)给你一个指针的大小。

尝试:

int dest_len = 4*(strlen(source)/3);

或定义 source作为:

const unsigned char source[] = "BASE64-ENCODED-BINARY";