[crypto] replace enum constants with constexpr (#6845)

src/core/crypto/aes_ccm.hpp

@@ -59,18 +59,15 @@ namespace Crypto {
 class AesCcm
 {
 public:
-    enum
-    {
-        kMinTagLength = 4,                  ///< Minimum tag length (in bytes).
-        kMaxTagLength = AesEcb::kBlockSize, ///< Maximum tag length (in bytes).
-        kNonceSize    = 13,                 ///< Size of IEEE 802.15.4 Nonce (in bytes).
-    };
+    static constexpr uint8_t kMinTagLength = 4;                  ///< Minimum tag length (in bytes).
+    static constexpr uint8_t kMaxTagLength = AesEcb::kBlockSize; ///< Maximum tag length (in bytes).
+    static constexpr uint8_t kNonceSize    = 13;                 ///< Size of IEEE 802.15.4 Nonce (in bytes).
 
     /**
      * This enumeration type represent the encryption vs decryption mode.
      *
      */
-    enum Mode
+    enum Mode : uint8_t
     {
         kEncrypt, // Encryption mode.
         kDecrypt, // Decryption mode.

src/core/crypto/aes_ecb.hpp

@@ -55,10 +55,7 @@ namespace Crypto {
 class AesEcb
 {
 public:
-    enum
-    {
-        kBlockSize = 16, ///< AES-128 block size (bytes).
-    };
+    static constexpr uint8_t kBlockSize = 16; ///< AES-128 block size (bytes).
 
     /**
      * Constructor to initialize the mbedtls_aes_context.

src/core/crypto/ecdsa.hpp

@@ -64,15 +64,13 @@ namespace Ecdsa {
 class P256
 {
 public:
-    enum : uint16_t
-    {
-        kFieldBitLength = 256, ///< Prime field bit length used by the P-256 curve.
-    };
+    static constexpr uint16_t kFieldBitLength = 256; ///< Prime field bit length used by the P-256 curve.
 
-    enum : uint8_t
-    {
-        kMpiSize = kFieldBitLength / 8, ///< Max bytes in binary representation of an MPI (multi-precision int).
-    };
+    /**
+     * Max bytes in binary representation of an MPI (multi-precision int).
+     *
+     */
+    static constexpr uint8_t kMpiSize = kFieldBitLength / 8;
 
     class PublicKey;
     class KeyPair;
@@ -91,10 +89,7 @@ public:
         friend class PublicKey;
 
     public:
-        enum : uint8_t
-        {
-            kSize = 2 * kMpiSize, ///< Size of the signature in bytes (two times the curve MPI size).
-        };
+        static constexpr uint8_t kSize = 2 * kMpiSize; ///< Signature size in bytes (two times the curve MPI size).
 
         /**
          * This method returns the signature as a byte array.
@@ -128,10 +123,11 @@ public:
     class KeyPair
     {
     public:
-        enum : uint8_t
-        {
-            kMaxDerSize = 125, ///< Max buffer size (in bytes) for representing the key-pair in DER format.
-        };
+        /**
+         * Max buffer size (in bytes) for representing the key-pair in DER format.
+         *
+         */
+        static constexpr uint8_t kMaxDerSize = 125;
 
         /**
          * This constructor initializes a `KeyPair` as empty (no key).
@@ -237,10 +233,7 @@ public:
         friend class KeyPair;
 
     public:
-        enum
-        {
-            kSize = kMpiSize * 2, ///< Size of the public key in bytes.
-        };
+        static constexpr uint8_t kSize = kMpiSize * 2; ///< Size of the public key in bytes.
 
         /**
          * This method gets the pointer to the buffer containing the public key (as an uncompressed curve point).

src/core/crypto/pbkdf2_cmac.hpp

@@ -50,10 +50,7 @@ namespace Pbkdf2 {
  *
  */
 
-enum : uint16_t
-{
-    kMaxSaltLength = 30, ///< Max SALT length: salt prefix (6) + extended panid (8) + network name (16)
-};
+constexpr uint16_t kMaxSaltLength = 30; ///< Max SALT length: salt prefix (6) + extended panid (8) + network name (16)
 
 /**
  * This function performs PKCS#5 PBKDF2 using CMAC (AES-CMAC-PRF-128).

src/core/crypto/sha256.hpp

@@ -73,10 +73,7 @@ public:
     class Hash : public otCryptoSha256Hash, public Clearable<Hash>, public Equatable<Hash>
     {
     public:
-        enum : uint8_t
-        {
-            kSize = OT_CRYPTO_SHA256_HASH_SIZE, ///< SHA-256 hash size (bytes)
-        };
+        static const uint8_t kSize = OT_CRYPTO_SHA256_HASH_SIZE; ///< SHA-256 hash size (bytes)
 
         /**
          * This method returns a pointer to a byte array containing the hash value.