sm2: unify default Mode for encryption and decryption (#1668)

Previously, EncryptingKey and DecryptingKey used inconsistent default modes,
C1C2C3 for EncryptingKey vs C1C3C2 for DecryptingKey.
Introduced DEFAULT_MODE to standardize the default behavior across both
structures.

Author: ivila

sm2/src/pke.rs

@@ -75,6 +75,10 @@ pub enum Mode {
     /// new mode
     C1C3C2,
 }
+
+/// The standard data layout used for SM2 encryption/decryption.
+/// C1C3C2 is the default per modern specifications.
+const DEFAULT_MODE: Mode = 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`),

sm2/src/pke/decrypting.rs

@@ -18,7 +18,7 @@ use primeorder::PrimeField;
 
 use sm3::{Digest, Sm3, digest::DynDigest};
 
-use super::{Cipher, Mode, encrypting::EncryptingKey, kdf, vec};
+use super::{Cipher, DEFAULT_MODE, Mode, encrypting::EncryptingKey, kdf, vec};
 /// Represents a decryption key used for decrypting messages using elliptic curve cryptography.
 #[derive(Clone)]
 pub struct DecryptingKey {
@@ -30,7 +30,7 @@ pub struct DecryptingKey {
 impl DecryptingKey {
     /// Creates a new `DecryptingKey` from a `SecretKey` with the default decryption mode (`C1C3C2`).
     pub fn new(secret_key: SecretKey) -> Self {
-        Self::new_with_mode(secret_key.to_nonzero_scalar(), Mode::C1C3C2)
+        Self::new_with_mode(secret_key.to_nonzero_scalar(), DEFAULT_MODE)
     }
 
     /// Creates a new `DecryptingKey` from a non-zero scalar and sets the decryption mode.
@@ -59,7 +59,7 @@ impl DecryptingKey {
 
     /// Create a signing key from a non-zero scalar.
     pub fn from_nonzero_scalar(secret_scalar: NonZeroScalar) -> Result<Self> {
-        Ok(Self::new_with_mode(secret_scalar, Mode::C1C3C2))
+        Ok(Self::new_with_mode(secret_scalar, DEFAULT_MODE))
     }
 
     /// Serialize as bytes.

sm2/src/pke/encrypting.rs

@@ -22,7 +22,7 @@ use sm3::{
     digest::{Digest, DynDigest},
 };
 
-use super::{Cipher, Mode};
+use super::{Cipher, DEFAULT_MODE, Mode};
 /// Represents an encryption key used for encrypting messages using elliptic curve cryptography.
 #[derive(Clone, Debug)]
 pub struct EncryptingKey {
@@ -31,9 +31,9 @@ pub struct EncryptingKey {
 }
 
 impl EncryptingKey {
-    /// Initialize [`EncryptingKey`] from PublicKey
+    /// Initialize [`EncryptingKey`] from PublicKey with the default encryption mode (`C1C3C2`).
     pub fn new(public_key: PublicKey) -> Self {
-        Self::new_with_mode(public_key, Mode::C1C2C3)
+        Self::new_with_mode(public_key, DEFAULT_MODE)
     }
 
     /// Initialize [`EncryptingKey`] from PublicKey and set Encryption mode