[xiami] feat: implement xiami decipher

um_crypto/xiami/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "umc_xiami"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+thiserror = "1.0.63"

um_crypto/xiami/src/lib.rs

@@ -0,0 +1,45 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum XiamiError {
+    #[error("header too small, require at least {0} bytes")]
+    HeaderTooSmall(usize),
+
+    #[error("not a xiami file")]
+    NotXiamiFile,
+}
+
+pub struct XiamiFile {
+    pub copy_len: usize,
+    pub format: [u8; 4],
+    key: u8,
+}
+
+impl XiamiFile {
+    pub fn from_header(buffer: &[u8]) -> Result<Self, XiamiError> {
+        if buffer.len() < 0x10 {
+            Err(XiamiError::HeaderTooSmall(0x10))?;
+        }
+
+        let (format, copy_len, key) = match buffer[..0x10] {
+            [b'i', b'f', b'm', b't', f1, f2, f3, f4, 0xfe, 0xfe, 0xfe, 0xfe, a, b, c, key] => {
+                let copy_len = (a as usize) | ((b as usize) << 8) | ((c as usize) << 16);
+                let format = [f1, f2, f3, f4];
+                (format, copy_len, key.wrapping_sub(1))
+            }
+            _ => Err(XiamiError::NotXiamiFile)?,
+        };
+
+        Ok(Self {
+            copy_len,
+            format,
+            key,
+        })
+    }
+
+    pub fn decrypt(&self, buffer: &mut [u8]) {
+        for b in buffer.iter_mut() {
+            *b = self.key.wrapping_sub(*b);
+        }
+    }
+}