[kgm] feat #2: basic kgm support

.idea/lib_um_crypto.iml

@@ -10,6 +10,7 @@
       <sourceFolder url="file://$MODULE_DIR$/um_crypto/utils/src" isTestSource="false" />
       <sourceFolder url="file://$MODULE_DIR$/um_crypto/ncm/src" isTestSource="false" />
       <sourceFolder url="file://$MODULE_DIR$/um_audio/src" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/um_crypto/kgm/src" isTestSource="false" />
       <excludeFolder url="file://$MODULE_DIR$/target" />
       <excludeFolder url="file://$MODULE_DIR$/um_wasm_loader/dist" />
       <excludeFolder url="file://$MODULE_DIR$/um_wasm_loader/pkg" />

Cargo.lock

@@ -74,6 +74,15 @@ version = "0.22.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "72b3254f16251a8381aa12e40e3c4d2f0199f8c6508fbecb9d91f575e0fbb8c6"
 
+[[package]]
+name = "block-buffer"
+version = "0.10.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3078c7629b62d3f0439517fa394996acacc5cbc91c5a20d8c658e77abd503a71"
+dependencies = [
+ "generic-array",
+]
+
 [[package]]
 name = "block-padding"
 version = "0.3.3"
@@ -216,6 +225,16 @@ version = "0.1.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "56254986775e3233ffa9c4d7d3faaf6d36a2c09d30b20687e9f88bc8bafc16c8"
 
+[[package]]
+name = "digest"
+version = "0.10.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9ed9a281f7bc9b7576e61468ba615a66a5c8cfdff42420a70aa82701a3b1e292"
+dependencies = [
+ "block-buffer",
+ "crypto-common",
+]
+
 [[package]]
 name = "either"
 version = "1.13.0"
@@ -303,6 +322,16 @@ version = "0.4.22"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a7a70ba024b9dc04c27ea2f0c0548feb474ec5c54bba33a7f72f873a39d07b24"
 
+[[package]]
+name = "md-5"
+version = "0.10.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d89e7ee0cfbedfc4da3340218492196241d89eefb6dab27de5df917a6d2e78cf"
+dependencies = [
+ "cfg-if",
+ "digest",
+]
+
 [[package]]
 name = "minicov"
 version = "0.3.5"
@@ -441,6 +470,7 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "clap",
+ "umc_kgm",
  "umc_kuwo",
  "umc_ncm",
  "umc_qmc",
@@ -462,6 +492,16 @@ dependencies = [
  "wasm-bindgen-test",
 ]
 
+[[package]]
+name = "umc_kgm"
+version = "0.1.0"
+dependencies = [
+ "byteorder",
+ "itertools",
+ "thiserror",
+ "umc_utils",
+]
+
 [[package]]
 name = "umc_kuwo"
 version = "0.1.0"
@@ -507,6 +547,8 @@ name = "umc_utils"
 version = "0.1.0"
 dependencies = [
  "base64",
+ "itertools",
+ "md-5",
 ]
 
 [[package]]

um_cli/Cargo.toml

@@ -7,6 +7,7 @@ edition = "2021"
 anyhow = "1.0.86"
 clap = { version = "4.5.17", features = ["derive"] }
 umc_kuwo = { path = "../um_crypto/kuwo" }
-umc_qmc = { path = "../um_crypto/qmc" }
+umc_kgm = { path = "../um_crypto/kgm" }
 umc_ncm = { path = "../um_crypto/ncm" }
+umc_qmc = { path = "../um_crypto/qmc" }
 umc_utils = { path = "../um_crypto/utils" }

um_cli/src/cmd/kgm.rs

@@ -0,0 +1,44 @@
+use crate::Cli;
+use clap::Args;
+use std::fs::File;
+use std::io::{Read, Seek, SeekFrom, Write};
+use std::path::PathBuf;
+use umc_kgm::header::Header;
+
+/// Decrypt a KGM/VPR file (Kugou Music)
+#[derive(Args)]
+pub struct ArgsKGM {
+    /// Path to output file, e.g. /export/Music/song.flac
+    #[arg(short, long)]
+    output: PathBuf,
+
+    /// Path to input file, e.g. /export/Music/song.kgm
+    #[arg(name = "input")]
+    input: PathBuf,
+}
+
+impl ArgsKGM {
+    pub fn run(&self, cli: &Cli) -> anyhow::Result<i32> {
+        let mut file_input = File::open(&self.input)?;
+        let mut header = [0u8; 0x40];
+        file_input.read_exact(&mut header)?;
+        let kgm_header = Header::from_buffer(&mut header)?;
+        let decipher = kgm_header.make_decipher()?;
+        file_input.seek(SeekFrom::Start(kgm_header.offset_to_data as u64))?;
+
+        let mut offset = 0usize;
+        let mut buffer = vec![0u8; cli.buffer_size].into_boxed_slice();
+        let mut file_output = File::create(&self.output)?;
+        while let Ok(n) = file_input.read(&mut buffer) {
+            if n == 0 {
+                break;
+            }
+
+            decipher.decrypt(&mut buffer[..n], offset);
+            file_output.write_all(&buffer[..n])?;
+            offset += n;
+        }
+
+        Ok(0)
+    }
+}

um_cli/src/cmd/mod.rs

@@ -1,5 +1,6 @@
 use clap::Subcommand;
 
+pub mod kgm;
 pub mod ncm;
 pub mod qmc1;
 pub mod qmc2;
@@ -12,4 +13,6 @@ pub enum Commands {
     QMCv2(qmc2::ArgsQMCv2),
     #[command(name = "ncm")]
     NCM(ncm::ArgsNCM),
+    #[command(name = "kgm")]
+    KGM(kgm::ArgsKGM),
 }

um_cli/src/cmd/qmc2.rs

@@ -30,7 +30,7 @@ pub struct ArgsQMCv2 {
     info_only: bool,
 }
 
-fn read_ekey(ekey: &str) -> Result<Box<[u8]>> {
+fn read_ekey(ekey: &str) -> Result<Vec<u8>> {
     let mut external_file = false;
     let mut decrypt_ekey = true;
 
@@ -54,7 +54,7 @@ fn read_ekey(ekey: &str) -> Result<Box<[u8]>> {
     let ekey = ekey.trim();
     let ekey = match decrypt_ekey {
         true => umc_qmc::ekey::decrypt(ekey)?,
-        false => base64::decode(ekey)?.into_boxed_slice(),
+        false => base64::decode(ekey)?,
     };
     Ok(ekey)
 }

um_cli/src/main.rs

@@ -30,6 +30,7 @@ fn run_command(cli: &Cli) -> Result<i32> {
         Some(Commands::QMCv1(cmd)) => cmd.run(&cli),
         Some(Commands::QMCv2(cmd)) => cmd.run(&cli),
         Some(Commands::NCM(cmd)) => cmd.run(&cli),
+        Some(Commands::KGM(cmd)) => cmd.run(&cli),
         None => {
             // https://github.com/clap-rs/clap/issues/3857#issuecomment-1161796261
             todo!("implement a sensible default command, similar to um/cli");

um_crypto/kgm/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "umc_kgm"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+byteorder = "1.5.0"
+itertools = "0.13.0"
+thiserror = "1.0.63"
+umc_utils = { path = "../utils" }

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)
+    }
+}

um_crypto/kgm/src/lib.rs

@@ -0,0 +1,21 @@
+pub mod header;
+pub mod v2;
+mod v3;
+
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum KugouError {
+    #[error("Header too small, need at least {0} bytes.")]
+    HeaderTooSmall(usize),
+
+    #[error("Unsupported key slot: {0}")]
+    UnsupportedKeySlot(u32),
+
+    #[error("Unsupported cipher version: {0}")]
+    UnsupportedCipherVersion(u32),
+}
+
+pub trait Decipher {
+    fn decrypt(&self, buffer: &mut [u8], offset: usize);
+}

um_crypto/kgm/src/v2.rs

@@ -0,0 +1,31 @@
+use crate::header::Header;
+use crate::{Decipher, KugouError};
+
+pub struct DecipherV2 {
+    key: [u8; 4],
+}
+
+impl DecipherV2 {
+    pub fn new(_header: &Header, slot_key: &[u8]) -> Result<Self, KugouError> {
+        let mut key = [0u8; 4];
+        key.copy_from_slice(slot_key);
+        Ok(Self { key })
+    }
+}
+
+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;
+        }
+    }
+}
+
+#[test]
+fn test_v2_init() -> Result<(), KugouError> {
+    let hdr_v2 = Header::from_buffer(include_bytes!("__fixtures__/kgm_v2_hdr.bin"))?;
+    DecipherV2::new(&hdr_v2)?;
+
+    Ok(())
+}

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);
+        }
+    }
+}

um_crypto/utils/Cargo.toml

@@ -5,3 +5,5 @@ edition = "2021"
 
 [dependencies]
 base64 = "0.22.1"
+itertools = "0.13.0"
+md-5 = "0.10.6"

um_crypto/utils/src/lib.rs

@@ -1 +1,3 @@
 pub mod base64;
+mod md5;
+pub use md5::md5;

um_crypto/utils/src/md5.rs

@@ -0,0 +1,5 @@
+use md5::{Digest, Md5};
+
+pub fn md5<T: AsRef<[u8]>>(buffer: T) -> [u8; 16] {
+    Md5::digest(buffer).into()
+}