Add support for playing Audible AAXC (.aaxc) files [PATCH v4]

The AAXC container format is the same as the (already supported) Audible
AAX format but it uses a different encryption scheme.

Note: audible_key and audible_iv values are variable (per file) and are
externally fed.

It is possible to extend https://github.com/mkb79/Audible to derive the
audible_key and audible_key values.

Relevant code:

def decrypt_voucher(deviceSerialNumber, customerId, deviceType, asin, voucher):
    buf = (deviceType + deviceSerialNumber + customerId + asin).encode("ascii")
    digest = hashlib.sha256(buf).digest()
    key = digest[0:16]
    iv = digest[16:]

    # decrypt "voucher" using AES in CBC mode with no padding
    cipher = AES.new(key, AES.MODE_CBC, iv)
    plaintext = cipher.decrypt(voucher).rstrip(b"\x00")  # improve this!
    return json.loads(plaintext)

The decrypted "voucher" has the required audible_key and audible_iv
values.

Update (Nov-2020): This patch has now been tested by multiple folks -
details at the following URL:

https://github.com/mkb79/Audible/issues/3

Signed-off-by: Vesselin Bontchev <vesselin.bontchev@yandex.com>

libavformat/isom.h

@@ -286,6 +286,10 @@ typedef struct MOVContext {
     int activation_bytes_size;
     void *audible_fixed_key;
     int audible_fixed_key_size;
+    void *audible_key;
+    int audible_key_size;
+    void *audible_iv;
+    int audible_iv_size;
     struct AVAES *aes_decrypt;
     uint8_t *decryption_key;
     int decryption_key_len;

libavformat/mov.c

@@ -1075,6 +1075,30 @@ fail:
     return ret;
 }
 
+static int mov_aaxc_crypto(MOVContext *c)
+{
+    if (c->audible_key_size != 16) {
+        av_log(c->fc, AV_LOG_FATAL, "[aaxc] audible_key value needs to be 16 bytes!\n");
+        return AVERROR(EINVAL);
+    }
+
+    if (c->audible_iv_size != 16) {
+        av_log(c->fc, AV_LOG_FATAL, "[aaxc] audible_iv value needs to be 16 bytes!\n");
+        return AVERROR(EINVAL);
+    }
+
+    c->aes_decrypt = av_aes_alloc();
+    if (!c->aes_decrypt) {
+        return AVERROR(ENOMEM);
+    }
+
+    memcpy(c->file_key, c->audible_key, 16);
+    memcpy(c->file_iv, c->audible_iv, 16);
+    c->aax_mode = 1;
+
+    return 0;
+}
+
 // Audible AAX (and AAX+) bytestream decryption
 static int aax_filter(uint8_t *input, int size, MOVContext *c)
 {
@@ -1123,6 +1147,11 @@ static int mov_read_ftyp(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     av_dict_set(&c->fc->metadata, "compatible_brands",
                 comp_brands_str, AV_DICT_DONT_STRDUP_VAL);
 
+    // Logic for handling Audible's .aaxc files
+    if (!strcmp(type, "aaxc")) {
+        mov_aaxc_crypto(c);
+    }
+
     return 0;
 }
 
@@ -8170,6 +8199,10 @@ static const AVOption mov_options[] = {
         AV_OPT_TYPE_BOOL, { .i64 = 0 }, 0, 1, .flags = FLAGS },
     { "activation_bytes", "Secret bytes for Audible AAX files", OFFSET(activation_bytes),
         AV_OPT_TYPE_BINARY, .flags = AV_OPT_FLAG_DECODING_PARAM },
+    { "audible_key", "AES-128 Key for Audible AAXC files", OFFSET(audible_key),
+        AV_OPT_TYPE_BINARY, .flags = AV_OPT_FLAG_DECODING_PARAM },
+    { "audible_iv", "AES-128 IV for Audible AAXC files", OFFSET(audible_iv),
+        AV_OPT_TYPE_BINARY, .flags = AV_OPT_FLAG_DECODING_PARAM },
     { "audible_fixed_key", // extracted from libAAX_SDK.so and AAXSDKWin.dll files!
         "Fixed key used for handling Audible AAX files", OFFSET(audible_fixed_key),
         AV_OPT_TYPE_BINARY, {.str="77214d4b196a87cd520045fd20a51d67"},