[kgm] feat #2: basic kgm support

um_crypto/kgm/src/v3.rs

@@ -0,0 +1,48 @@
+use crate::header::Header;
+use crate::{Decipher, KugouError};
+
+pub struct DecipherV3 {
+    slot_key: [u8; 16],
+    file_key: [u8; 17],
+}
+
+impl DecipherV3 {
+    fn hash_key<T: AsRef<[u8]>>(data: T) -> [u8; 16] {
+        let digest = umc_utils::md5(data);
+        let mut result = [0u8; 16];
+        for (result, digest) in result.rchunks_exact_mut(2).zip(digest.chunks_exact(2)) {
+            result[0] = digest[0];
+            result[1] = digest[1];
+        }
+        result
+    }
+
+    pub fn new(header: &Header, slot_key: &[u8]) -> Result<Self, KugouError> {
+        let slot_key = Self::hash_key(slot_key);
+
+        let mut file_key = [0x6b; 17];
+        file_key[..16].copy_from_slice(&Self::hash_key(header.file_key));
+
+        Ok(Self { slot_key, file_key })
+    }
+}
+
+impl Decipher for DecipherV3 {
+    fn decrypt(&self, buffer: &mut [u8], offset: usize) {
+        let slot_key_stream = self.slot_key.iter().cycle().skip(offset);
+        let file_key_stream = self.file_key.iter().cycle().skip(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 mut temp = *datum;
+            temp ^= file_key;
+            temp ^= temp.wrapping_shl(4);
+            temp ^= slot_key;
+            temp ^= offset.to_ne_bytes().iter().fold(0, |acc, &x| acc ^ x);
+            *datum = temp;
+
+            offset = offset.wrapping_add(1);
+        }
+    }
+}