[kgm] feat #2: basic kgm support

um_crypto/kgm/src/header.rs

@@ -0,0 +1,59 @@
+use crate::v2::DecipherV2;
+use crate::v3::DecipherV3;
+use crate::{Decipher, KugouError};
+use byteorder::{ByteOrder, LE};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Header {
+    pub magic: [u8; 0x10],
+    pub offset_to_data: usize,
+    pub crypto_version: u32,
+    pub key_slot: u32,
+    pub decrypt_test_data: [u8; 0x10],
+    pub file_key: [u8; 0x10],
+}
+
+impl Header {
+    pub fn from_buffer<T>(buffer: T) -> Result<Self, KugouError>
+    where
+        T: AsRef<[u8]>,
+    {
+        let buffer = buffer.as_ref();
+        if buffer.len() < 0x3c {
+            Err(KugouError::HeaderTooSmall(0x3c))?;
+        }
+
+        let mut magic = [0u8; 0x10];
+        magic.copy_from_slice(&buffer[..0x10]);
+        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]);
+        let mut decrypt_test_data = [0u8; 0x10];
+        decrypt_test_data.copy_from_slice(&buffer[0x1c..0x2c]);
+        let mut file_key = [0u8; 0x10];
+        file_key.copy_from_slice(&buffer[0x2c..0x3c]);
+
+        Ok(Self {
+            magic,
+            offset_to_data,
+            crypto_version,
+            key_slot,
+            decrypt_test_data,
+            file_key,
+        })
+    }
+
+    pub fn make_decipher(&self) -> Result<Box<dyn Decipher>, KugouError> {
+        let slot_key: &[u8] = match self.key_slot {
+            1 => b"l,/'",
+            slot => Err(KugouError::UnsupportedKeySlot(slot))?,
+        };
+
+        let decipher: Box<dyn Decipher> = match self.crypto_version {
+            2 => Box::from(DecipherV2::new(self, slot_key)?),
+            3 => Box::from(DecipherV3::new(self, slot_key)?),
+            version => Err(KugouError::UnsupportedCipherVersion(version))?,
+        };
+        Ok(decipher)
+    }
+}