fix: ncm decryption

um_crypto/ncm/src/header.rs

@@ -1,20 +1,24 @@
-use crate::{content_key, NetEaseCryptoError as Error};
+use crate::{content_key, metadata, NetEaseCryptoError as Error};
 use byteorder::{ByteOrder, LE};
 
 const CRC32: crc::Crc<u32> = crc::Crc::<u32>::new(&crc::CRC_32_ISO_HDLC);
 
-struct NCMFile {
+#[derive(Debug, PartialEq, Clone)]
+pub struct NCMFile {
+    audio_rc4_key_stream: [u8; 256],
+
+    /// NCM File format version, 0x01
     pub ncm_version: u8,
+    /// CloudMusic client version (guess)
     pub client_version: u8,
-    /// Encrypted content key
+    /// Encrypted content key.
     pub content_key: Vec<u8>,
     /// Encrypted metadata.
     pub metadata: Vec<u8>,
-    /// Cover image 1
+    /// Cover image 1, usually jpg.
     pub image1: Option<Vec<u8>>,
     /// Cover image 2, format unknown
     pub image2: Option<Vec<u8>>,
-    audio_rc4_key_stream: [u8; 256],
     pub audio_data_offset: usize,
 }
 
@@ -34,6 +38,7 @@ fn build_audio_rc4_key_stream(enciphered_content_key: &[u8]) -> Result<[u8; 256]
 
     let mut key = [0u8; 256];
     for (i, k) in key.iter_mut().enumerate() {
+        let i = (i + 1) & 0xff;
         let j = s[i].wrapping_add(i as u8);
         let idx = s[i].wrapping_add(s[j as usize]);
         *k = s[idx as usize];
@@ -129,35 +134,66 @@ impl NCMFile {
             audio_data_offset: offset,
         })
     }
+
+    pub fn get_metadata(&self) -> Result<Vec<u8>, Error> {
+        metadata::decrypt(&self.metadata)
+    }
+
+    /// Decrypt audio data.
+    ///
+    /// # Arguments
+    ///
+    /// * `data`: The data to decrypt
+    /// * `offset`: Offset of the data in file, subtract `self.audio_data_offset`
+    ///
+    /// returns: Result<(), NetEaseCryptoError>
+    pub fn decrypt<T>(&self, data: &mut T, offset: usize) -> Result<(), Error>
+    where
+        T: AsMut<[u8]>,
+    {
+        let key_stream = self.audio_rc4_key_stream.iter().cycle().skip(offset & 0xff);
+        for (datum, &key) in data.as_mut().iter_mut().zip(key_stream) {
+            *datum ^= key;
+        }
+
+        Ok(())
+    }
 }
 
 #[test]
 fn test_load_ncm() -> Result<(), Error> {
     let ncm_header = include_bytes!("__fixture__/ncm_test1.bin");
     let ncm = NCMFile::new(ncm_header)?;
-    let sbox = [
+    let key_stream = [
-        0x08, 0x67, 0x20, 0xF0, 0x5C, 0xAC, 0xAF, 0x1B, 0x74, 0x0D, 0x26, 0x40, 0xBE, 0x85, 0x61,
+        0x67, 0x20, 0xF0, 0x5C, 0xAC, 0xAF, 0x1B, 0x74, 0x0D, 0x26, 0x40, 0xBE, 0x85, 0x61, 0x45,
-        0x45, 0x0D, 0xA8, 0x69, 0xAE, 0x78, 0xEC, 0xB8, 0x14, 0x79, 0x53, 0xC4, 0x74, 0xF2, 0x9F,
+        0x0D, 0xA8, 0x69, 0xAE, 0x78, 0xEC, 0xB8, 0x14, 0x79, 0x53, 0xC4, 0x74, 0xF2, 0x9F, 0xE1,
-        0xE1, 0x18, 0xE2, 0xF9, 0xC0, 0x7D, 0x1E, 0x5A, 0xBA, 0x4A, 0xB3, 0x11, 0x36, 0xD9, 0xAA,
+        0x18, 0xE2, 0xF9, 0xC0, 0x7D, 0x1E, 0x5A, 0xBA, 0x4A, 0xB3, 0x11, 0x36, 0xD9, 0xAA, 0xD2,
-        0xD2, 0x13, 0xEE, 0x92, 0x45, 0x4B, 0x57, 0xE6, 0xF1, 0x13, 0x90, 0xE9, 0xE5, 0x2F, 0xBE,
+        0x13, 0xEE, 0x92, 0x45, 0x4B, 0x57, 0xE6, 0xF1, 0x13, 0x90, 0xE9, 0xE5, 0x2F, 0xBE, 0x6E,
-        0x6E, 0x16, 0x01, 0xBF, 0x10, 0x81, 0x5C, 0x68, 0x1C, 0xE3, 0xEB, 0xDF, 0x5A, 0xC7, 0xC3,
+        0x16, 0x01, 0xBF, 0x10, 0x81, 0x5C, 0x68, 0x1C, 0xE3, 0xEB, 0xDF, 0x5A, 0xC7, 0xC3, 0x3F,
-        0x3F, 0x9C, 0xD7, 0x28, 0x3A, 0x81, 0x2B, 0x1F, 0xF3, 0x6F, 0x27, 0x15, 0x7E, 0x53, 0xD3,
+        0x9C, 0xD7, 0x28, 0x3A, 0x81, 0x2B, 0x1F, 0xF3, 0x6F, 0x27, 0x15, 0x7E, 0x53, 0xD3, 0xF0,
-        0xF0, 0xFA, 0x50, 0x9F, 0xC7, 0xF0, 0x7F, 0xA6, 0xA7, 0x24, 0x80, 0x87, 0x39, 0x55, 0xA5,
+        0xFA, 0x50, 0x9F, 0xC7, 0xF0, 0x7F, 0xA6, 0xA7, 0x24, 0x80, 0x87, 0x39, 0x55, 0xA5, 0x0B,
-        0x0B, 0x2B, 0x8B, 0x00, 0x19, 0x82, 0xA8, 0x40, 0xA5, 0x77, 0x9C, 0x93, 0x3B, 0xEE, 0x27,
+        0x2B, 0x8B, 0x00, 0x19, 0x82, 0xA8, 0x40, 0xA5, 0x77, 0x9C, 0x93, 0x3B, 0xEE, 0x27, 0x70,
-        0x70, 0x0F, 0xA2, 0xEB, 0xA5, 0x6F, 0x58, 0xFB, 0xEB, 0x57, 0x1B, 0xC3, 0x61, 0x10, 0x6D,
+        0x0F, 0xA2, 0xEB, 0xA5, 0x6F, 0x58, 0xFB, 0xEB, 0x57, 0x1B, 0xC3, 0x61, 0x10, 0x6D, 0x95,
-        0x95, 0x2C, 0xAA, 0xC6, 0x58, 0x88, 0xD5, 0x9E, 0x33, 0x9A, 0x5A, 0xD1, 0xB5, 0xD2, 0x17,
+        0x2C, 0xAA, 0xC6, 0x58, 0x88, 0xD5, 0x9E, 0x33, 0x9A, 0x5A, 0xD1, 0xB5, 0xD2, 0x17, 0x44,
-        0x44, 0xD6, 0xDB, 0xAB, 0xED, 0x7C, 0xFC, 0x5C, 0x37, 0xB9, 0x16, 0xB8, 0xA5, 0x77, 0xFB,
+        0xD6, 0xDB, 0xAB, 0xED, 0x7C, 0xFC, 0x5C, 0x37, 0xB9, 0x16, 0xB8, 0xA5, 0x77, 0xFB, 0x58,
-        0x58, 0x35, 0x36, 0xE7, 0x51, 0xD0, 0x03, 0xBB, 0x2A, 0x88, 0x24, 0x84, 0x1D, 0x28, 0xB1,
+        0x35, 0x36, 0xE7, 0x51, 0xD0, 0x03, 0xBB, 0x2A, 0x88, 0x24, 0x84, 0x1D, 0x28, 0xB1, 0xE1,
-        0xE1, 0xA0, 0xA5, 0xA0, 0xDD, 0x15, 0x66, 0xBB, 0x4F, 0x7E, 0xBC, 0x99, 0x76, 0x1A, 0xD2,
+        0xA0, 0xA5, 0xA0, 0xDD, 0x15, 0x66, 0xBB, 0x4F, 0x7E, 0xBC, 0x99, 0x76, 0x1A, 0xD2, 0xEF,
-        0xEF, 0xBE, 0xD9, 0x79, 0xA0, 0x33, 0xD5, 0x96, 0x6C, 0xD9, 0xEF, 0x42, 0x7E, 0x11, 0x45,
+        0xBE, 0xD9, 0x79, 0xA0, 0x33, 0xD5, 0x96, 0x6C, 0xD9, 0xEF, 0x42, 0x7E, 0x11, 0x45, 0x1F,
-        0x1F, 0x99, 0x96, 0x1A, 0x90, 0x0C, 0x75, 0xBC, 0x01, 0xE2, 0xC8, 0xEF, 0x16, 0x98, 0x9A,
+        0x99, 0x96, 0x1A, 0x90, 0x0C, 0x75, 0xBC, 0x01, 0xE2, 0xC8, 0xEF, 0x16, 0x98, 0x9A, 0x09,
-        0x09, 0xEB, 0xD8, 0xA1, 0xEA, 0x62, 0xE7, 0x92, 0x20, 0x8F, 0x6F, 0x72, 0xDE, 0x14, 0xFE,
+        0xEB, 0xD8, 0xA1, 0xEA, 0x62, 0xE7, 0x92, 0x20, 0x8F, 0x6F, 0x72, 0xDE, 0x14, 0xFE, 0xBF,
-        0xBF, 0xCD, 0xBA, 0x97, 0xDE, 0xA3, 0x3B, 0x67, 0xD4, 0x0B, 0xF4, 0xD3, 0x97, 0x25, 0xBA,
+        0xCD, 0xBA, 0x97, 0xDE, 0xA3, 0x3B, 0x67, 0xD4, 0x0B, 0xF4, 0xD3, 0x97, 0x25, 0xBA, 0xCA,
-        0xCA,
+        0x08,
     ];
-    assert_eq!(ncm.audio_rc4_key_stream, sbox);
+    assert_eq!(ncm.audio_rc4_key_stream, key_stream);
     assert_eq!(ncm.image1, Some(b"img#1".to_vec()));
     assert_eq!(ncm.image2, Some(b"IMAGE#2".to_vec()));
 
+    let mut audio_data = ncm_header[ncm.audio_data_offset..].to_vec();
+    ncm.decrypt(&mut audio_data, 0)?;
+    let actual = vec![
+        0x49, 0x44, 0x33, 0x03, 0x00, 0x00, 0x00, 0x00, 0x01, 0x73, 0x54, 0x50, 0x45, 0x31, 0x00,
+    ];
+    assert_eq!(audio_data, actual);
+
     Ok(())
 }

um_crypto/ncm/src/lib.rs

@@ -34,4 +34,6 @@ pub enum NetEaseCryptoError {
     MetadataDecryptError(UnpadError),
     #[error("Metadata: Decode metadata failed: {0}")]
     MetadataDecodeError(base64::DecodeError),
+    #[error("Metadata: Invalid prefix on final json: {0}")]
+    MetadataInvalidJsonPrefix(String),
 }

um_crypto/ncm/src/metadata.rs

@@ -35,6 +35,12 @@ where
     let metadata = aes
         .decrypt_padded::<Pkcs7>(&mut data[..])
         .map_err(NetEaseCryptoError::MetadataDecryptError)?;
+    let metadata = match metadata.strip_prefix(b"music:") {
+        None => Err(NetEaseCryptoError::MetadataInvalidJsonPrefix(
+            String::from_utf8_lossy(metadata).to_string(),
+        ))?,
+        Some(stripped) => stripped,
+    };
     Ok(Vec::from(metadata))
 }
 
@@ -100,6 +106,6 @@ fn test_decrypt_metadata() -> Result<(), NetEaseCryptoError> {
         0x25, 0x1B, 0x37, 0x48, 0x57, 0x53, 0x2A, 0x22, 0x3A, 0x00, 0x34, 0x53, 0x24, 0x12,
     ];
     let key = decrypt(enciphered_metadata)?;
-    assert_eq!(&key[..6], b"music:");
+    assert_eq!(key[0], b'{');
     Ok(())
 }