[kgm] feat: add decipher self-test on startup

um_crypto/kgm/src/header.rs

@@ -11,6 +11,8 @@ pub struct Header {
     pub key_slot: u32,
     pub decrypt_test_data: [u8; 0x10],
     pub file_key: [u8; 0x10],
+
+    challenge_data: [u8; 0x10],
 }
 
 impl Header {
@@ -25,6 +27,8 @@ impl Header {
 
         let mut magic = [0u8; 0x10];
         magic.copy_from_slice(&buffer[..0x10]);
+        let challenge_data = get_challenge_data(&magic)?;
+
         let offset_to_data = LE::read_u32(&buffer[0x10..0x14]) as usize;
         let crypto_version = LE::read_u32(&buffer[0x14..0x18]);
         let key_slot = LE::read_u32(&buffer[0x18..0x1C]);
@@ -40,6 +44,7 @@ impl Header {
             key_slot,
             decrypt_test_data,
             file_key,
+            challenge_data,
         })
     }
 
@@ -54,6 +59,37 @@ impl Header {
             3 => Box::from(DecipherV3::new(self, slot_key)?),
             version => Err(KugouError::UnsupportedCipherVersion(version))?,
         };
+
+        let mut test_data = self.decrypt_test_data;
+        decipher.decrypt(&mut test_data, 0);
+        if self.challenge_data != test_data {
+            Err(KugouError::SelfTestFailed)?;
+        }
+
         Ok(decipher)
     }
 }
+
+fn get_challenge_data(magic_header: &[u8; 0x10]) -> Result<[u8; 0x10], KugouError> {
+    match magic_header {
+        &KGM_HEADER => Ok(KGM_TEST_DATA),
+        &VPR_HEADER => Ok(VPR_TEST_DATA),
+        _ => Err(KugouError::NotKGMFile)?,
+    }
+}
+
+pub const KGM_HEADER: [u8; 16] = [
+    0x7C, 0xD5, 0x32, 0xEB, 0x86, 0x02, 0x7F, 0x4B, 0xA8, 0xAF, 0xA6, 0x8E, 0x0F, 0xFF, 0x99, 0x14,
+];
+
+pub const KGM_TEST_DATA: [u8; 16] = [
+    0x38, 0x85, 0xED, 0x92, 0x79, 0x5F, 0xF8, 0x4C, 0xB3, 0x03, 0x61, 0x41, 0x16, 0xA0, 0x1D, 0x47,
+];
+
+pub const VPR_HEADER: [u8; 16] = [
+    0x05, 0x28, 0xBC, 0x96, 0xE9, 0xE4, 0x5A, 0x43, 0x91, 0xAA, 0xBD, 0xD0, 0x7A, 0xF5, 0x36, 0x31,
+];
+
+pub const VPR_TEST_DATA: [u8; 16] = [
+    0x1D, 0x5A, 0x05, 0x34, 0x0C, 0x41, 0x8D, 0x42, 0x9C, 0x83, 0x92, 0x6C, 0xAE, 0x16, 0xFE, 0x56,
+];

um_crypto/kgm/src/lib.rs

@@ -14,6 +14,12 @@ pub enum KugouError {
 
     #[error("Unsupported cipher version: {0}")]
     UnsupportedCipherVersion(u32),
+
+    #[error("Not KGM File (magic mismatch)")]
+    NotKGMFile,
+
+    #[error("Unsupported cipher (self-test failed)")]
+    SelfTestFailed,
 }
 
 pub trait Decipher {

um_crypto/kgm/src/v2.rs

@@ -15,9 +15,8 @@ impl DecipherV2 {
 
 impl Decipher for DecipherV2 {
     fn decrypt(&self, buffer: &mut [u8], offset: usize) {
-        let key_stream = self.key.iter().cycle().skip(offset % self.key.len());
-        for (datum, &k) in buffer.iter_mut().zip(key_stream) {
-            *datum ^= k;
+        for (datum, offset) in buffer.iter_mut().zip(offset..) {
+            *datum ^= self.key[offset % self.key.len()];
         }
     }
 }

um_crypto/kgm/src/v3.rs

@@ -25,34 +25,27 @@ impl DecipherV3 {
 
         Ok(Self { slot_key, file_key })
     }
+
+    fn offset_key(offset: usize) -> u8 {
+        let offset_key = (offset as u32).to_ne_bytes();
+        offset_key[0] ^ offset_key[1] ^ offset_key[2] ^ offset_key[3]
+    }
 }
 
 impl Decipher for DecipherV3 {
     fn decrypt(&self, buffer: &mut [u8], offset: usize) {
-        let slot_key_stream = self
-            .slot_key
-            .iter()
-            .cycle()
-            .skip(offset % self.slot_key.len());
-        let file_key_stream = self
-            .file_key
-            .iter()
-            .cycle()
-            .skip(offset % self.file_key.len());
+        for (datum, offset) in buffer.iter_mut().zip(offset..) {
+            let offset_key = Self::offset_key(offset);
 
-        let mut offset = offset as u32;
-        let key_stream = slot_key_stream.zip(file_key_stream);
-        for (datum, (&slot_key, &file_key)) in buffer.iter_mut().zip(key_stream) {
-            let offset_key = offset.to_ne_bytes().iter().fold(0, |acc, &x| acc ^ x);
+            let file_key = self.file_key[offset % self.file_key.len()];
+            let slot_key = self.slot_key[offset % self.slot_key.len()];
 
             let mut temp = *datum;
             temp ^= file_key;
-            temp ^= temp.wrapping_shl(4);
+            temp ^= temp << 4;
             temp ^= slot_key;
             temp ^= offset_key;
             *datum = temp;
-
-            offset = offset.wrapping_add(1);
         }
     }
 }

um_crypto/kuwo/src/kwm_v1.rs

@@ -11,10 +11,8 @@ const KEY: [u8; 0x20] = [
 impl CipherV1 {
     pub fn new(resource_id: u32) -> Self {
         let mut key = KEY;
-        for (k, r) in key
-            .iter_mut()
-            .zip(resource_id.to_string().as_bytes().iter().cycle())
-        {
+        let resource_id = resource_id.to_string();
+        for (k, &r) in key.iter_mut().zip(resource_id.as_bytes().iter().cycle()) {
             *k ^= r;
         }
 
@@ -26,10 +24,9 @@ impl CipherV1 {
         T: AsMut<[u8]> + ?Sized,
     {
         let data = data.as_mut();
-        let key_stream = self.key.iter().cycle().skip(offset % self.key.len());
 
-        for (datum, key) in data.iter_mut().zip(key_stream) {
-            *datum ^= *key;
+        for (datum, offset) in data.iter_mut().zip(offset..) {
+            *datum ^= self.key[offset % self.key.len()];
         }
     }
 }

um_crypto/ncm/src/header.rs

@@ -182,9 +182,8 @@ impl NCMFile {
     where
         T: AsMut<[u8]> + ?Sized,
     {
-        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;
+        for (datum, offset) in data.as_mut().iter_mut().zip(offset..) {
+            *datum ^= self.audio_rc4_key_stream[offset % self.audio_rc4_key_stream.len()];
         }
     }
 }