sm2: add SM2PKE support (#1069)

Adds support for the SM2 public key encryption algorithm defined in 
China's national standard GBT.32918.4-2016 (a.k.a. SM2-4)

Closes #1067

Co-authored-by: Tony Arcieri <[email protected]>

sm2/Cargo.toml

@@ -13,7 +13,7 @@ homepage = "https://github.com/RustCrypto/elliptic-curves/tree/master/sm2"
 repository = "https://github.com/RustCrypto/elliptic-curves"
 readme = "README.md"
 categories = ["cryptography", "no-std"]
-keywords = ["crypto", "ecc", "shangmi", "signature"]
+keywords = ["crypto", "ecc", "shangmi", "signature", "encryption"]
 edition = "2021"
 rust-version = "1.73"
 
@@ -33,13 +33,14 @@ proptest = "1"
 rand_core = { version = "0.6", features = ["getrandom"] }
 
 [features]
-default = ["arithmetic", "dsa", "pem", "std"]
+default = ["arithmetic", "dsa", "pke", "pem", "std"]
 alloc = ["elliptic-curve/alloc"]
 std = ["alloc", "elliptic-curve/std", "signature?/std"]
 
 arithmetic = ["dep:primeorder", "elliptic-curve/arithmetic"]
 bits = ["arithmetic", "elliptic-curve/bits"]
 dsa = ["arithmetic", "dep:rfc6979", "dep:signature", "dep:sm3"]
+pke = ["arithmetic", "dep:sm3"]
 getrandom = ["rand_core/getrandom"]
 pem = ["elliptic-curve/pem", "pkcs8"]
 pkcs8 = ["elliptic-curve/pkcs8"]

sm2/README.md

@@ -33,7 +33,7 @@ The SM2 cryptosystem is composed of three distinct algorithms:
 
 - [x] **SM2DSA**: digital signature algorithm defined in [GBT.32918.2-2016], [ISO.IEC.14888-3] (SM2-2)
 - [ ] **SM2KEP**: key exchange protocol defined in [GBT.32918.3-2016] (SM2-3)
-- [ ] **SM2PKE**: public key encryption algorithm defined in [GBT.32918.4-2016] (SM2-4)
+- [x] **SM2PKE**: public key encryption algorithm defined in [GBT.32918.4-2016] (SM2-4)
 
 ## Minimum Supported Rust Version
 

sm2/src/arithmetic/field.rs

@@ -34,10 +34,10 @@ use core::{
     iter::{Product, Sum},
     ops::{AddAssign, MulAssign, Neg, SubAssign},
 };
-use elliptic_curve::ops::Invert;
 use elliptic_curve::{
     bigint::Limb,
     ff::PrimeField,
+    ops::Invert,
     subtle::{Choice, ConstantTimeEq, CtOption},
 };
 

sm2/src/lib.rs

@@ -31,6 +31,9 @@ extern crate alloc;
 #[cfg(feature = "dsa")]
 pub mod dsa;
 
+#[cfg(feature = "pke")]
+pub mod pke;
+
 #[cfg(feature = "arithmetic")]
 mod arithmetic;
 #[cfg(feature = "dsa")]

sm2/src/pke.rs

@@ -0,0 +1,178 @@
+//! SM2 Encryption Algorithm (SM2) as defined in [draft-shen-sm2-ecdsa § 5].
+//!
+//! ## Usage
+//!
+//! NOTE: requires the `sm3` crate for digest functions and the `primeorder` crate for prime field operations.
+//!
+//! The `DecryptingKey` struct is used for decrypting messages that were encrypted using the SM2 encryption algorithm.
+//! It is initialized with a `SecretKey` or a non-zero scalar value and can decrypt ciphertexts using the specified decryption mode.
+#![cfg_attr(feature = "std", doc = "```")]
+#![cfg_attr(not(feature = "std"), doc = "```ignore")]
+//! # fn example() -> Result<(), Box<dyn std::error::Error>> {
+//! use rand_core::OsRng; // requires 'getrandom` feature
+//! use sm2::{
+//!     pke::{EncryptingKey, Mode},
+//!     {SecretKey, PublicKey}
+//!
+//! };
+//!
+//! // Encrypting
+//! let secret_key = SecretKey::random(&mut OsRng); // serialize with `::to_bytes()`
+//! let public_key = secret_key.public_key();
+//! let encrypting_key = EncryptingKey::new_with_mode(public_key, Mode::C1C2C3);
+//! let plaintext = b"plaintext";
+//! let ciphertext = encrypting_key.encrypt(plaintext)?;
+//!
+//! use sm2::pke::DecryptingKey;
+//! // Decrypting
+//! let decrypting_key = DecryptingKey::new_with_mode(secret_key.to_nonzero_scalar(), Mode::C1C2C3);
+//! assert_eq!(decrypting_key.decrypt(&ciphertext)?, plaintext);
+//!
+//! // Encrypting ASN.1 DER
+//! let ciphertext = encrypting_key.encrypt_der(plaintext)?;
+//!
+//! // Decrypting ASN.1 DER
+//! assert_eq!(decrypting_key.decrypt_der(&ciphertext)?, plaintext);
+//!
+//! Ok(())
+//! # }
+//!  ```
+//!
+//!
+//!
+
+use core::cmp::min;
+
+use crate::AffinePoint;
+
+#[cfg(feature = "alloc")]
+use alloc::vec;
+
+use elliptic_curve::{
+    bigint::{Encoding, Uint, U256},
+    pkcs8::der::{
+        asn1::UintRef, Decode, DecodeValue, Encode, Length, Reader, Sequence, Tag, Writer,
+    },
+};
+
+use elliptic_curve::{
+    pkcs8::der::{asn1::OctetStringRef, EncodeValue},
+    sec1::ToEncodedPoint,
+    Result,
+};
+use sm3::digest::DynDigest;
+
+#[cfg(feature = "arithmetic")]
+mod decrypting;
+#[cfg(feature = "arithmetic")]
+mod encrypting;
+
+#[cfg(feature = "arithmetic")]
+pub use self::{decrypting::DecryptingKey, encrypting::EncryptingKey};
+
+/// Modes for the cipher encoding/decoding.
+#[derive(Clone, Copy, Debug)]
+pub enum Mode {
+    /// old mode
+    C1C2C3,
+    /// new mode
+    C1C3C2,
+}
+/// Represents a cipher structure containing encryption-related data (asn.1 format).
+///
+/// The `Cipher` structure includes the coordinates of the elliptic curve point (`x`, `y`),
+/// the digest of the message, and the encrypted cipher text.
+pub struct Cipher<'a> {
+    x: U256,
+    y: U256,
+    digest: &'a [u8],
+    cipher: &'a [u8],
+}
+
+impl<'a> Sequence<'a> for Cipher<'a> {}
+
+impl<'a> EncodeValue for Cipher<'a> {
+    fn value_len(&self) -> elliptic_curve::pkcs8::der::Result<Length> {
+        UintRef::new(&self.x.to_be_bytes())?.encoded_len()?
+            + UintRef::new(&self.y.to_be_bytes())?.encoded_len()?
+            + OctetStringRef::new(self.digest)?.encoded_len()?
+            + OctetStringRef::new(self.cipher)?.encoded_len()?
+    }
+
+    fn encode_value(&self, writer: &mut impl Writer) -> elliptic_curve::pkcs8::der::Result<()> {
+        UintRef::new(&self.x.to_be_bytes())?.encode(writer)?;
+        UintRef::new(&self.y.to_be_bytes())?.encode(writer)?;
+        OctetStringRef::new(self.digest)?.encode(writer)?;
+        OctetStringRef::new(self.cipher)?.encode(writer)?;
+        Ok(())
+    }
+}
+
+impl<'a> DecodeValue<'a> for Cipher<'a> {
+    type Error = elliptic_curve::pkcs8::der::Error;
+
+    fn decode_value<R: Reader<'a>>(
+        decoder: &mut R,
+        header: elliptic_curve::pkcs8::der::Header,
+    ) -> core::result::Result<Self, Self::Error> {
+        decoder.read_nested(header.length, |nr| {
+            let x = UintRef::decode(nr)?.as_bytes();
+            let y = UintRef::decode(nr)?.as_bytes();
+            let digest = OctetStringRef::decode(nr)?.into();
+            let cipher = OctetStringRef::decode(nr)?.into();
+            Ok(Cipher {
+                x: Uint::from_be_bytes(zero_pad_byte_slice(x)?),
+                y: Uint::from_be_bytes(zero_pad_byte_slice(y)?),
+                digest,
+                cipher,
+            })
+        })
+    }
+}
+
+/// Performs key derivation using a hash function and elliptic curve point.
+fn kdf(hasher: &mut dyn DynDigest, kpb: AffinePoint, c2: &mut [u8]) -> Result<()> {
+    let klen = c2.len();
+    let mut ct: i32 = 0x00000001;
+    let mut offset = 0;
+    let digest_size = hasher.output_size();
+    let mut ha = vec![0u8; digest_size];
+    let encode_point = kpb.to_encoded_point(false);
+
+    while offset < klen {
+        hasher.update(encode_point.x().ok_or(elliptic_curve::Error)?);
+        hasher.update(encode_point.y().ok_or(elliptic_curve::Error)?);
+        hasher.update(&ct.to_be_bytes());
+
+        hasher
+            .finalize_into_reset(&mut ha)
+            .map_err(|_e| elliptic_curve::Error)?;
+
+        let xor_len = min(digest_size, klen - offset);
+        xor(c2, &ha, offset, xor_len);
+        offset += xor_len;
+        ct += 1;
+    }
+    Ok(())
+}
+
+/// XORs a portion of the buffer `c2` with a hash value.
+fn xor(c2: &mut [u8], ha: &[u8], offset: usize, xor_len: usize) {
+    for i in 0..xor_len {
+        c2[offset + i] ^= ha[i];
+    }
+}
+
+/// Converts a byte slice to a fixed-size array, padding with leading zeroes if necessary.
+pub(crate) fn zero_pad_byte_slice<const N: usize>(
+    bytes: &[u8],
+) -> elliptic_curve::pkcs8::der::Result<[u8; N]> {
+    let num_zeroes = N
+        .checked_sub(bytes.len())
+        .ok_or_else(|| Tag::Integer.length_error())?;
+
+    // Copy input into `N`-sized output buffer with leading zeroes
+    let mut output = [0u8; N];
+    output[num_zeroes..].copy_from_slice(bytes);
+    Ok(output)
+}