Implement KeySizeInBytes on AesGcm and AesCcm.

src/libraries/Common/src/System/Security/Cryptography/AesGcm.cs

@@ -31,6 +31,14 @@ public sealed partial class AesGcm : IDisposable
         /// </value>
         public int? TagSizeInBytes { get; }
 
+        /// <summary>
+        /// Gets the size of the key, in bytes.
+        /// </summary>
+        /// <value>
+        /// The size of the key, in bytes.
+        /// </value>
+        public int KeySizeInBytes { get; }
+
         /// <summary>
         /// Gets the tag sizes, in bytes, supported by this instance.
         /// </summary>
@@ -78,6 +86,7 @@ public AesGcm(ReadOnlySpan<byte> key, int tagSizeInBytes)
                 throw new ArgumentException(SR.Cryptography_InvalidTagLength, nameof(tagSizeInBytes));
             }
 
+            KeySizeInBytes = key.Length;
             TagSizeInBytes = tagSizeInBytes;
             ImportKey(key);
         }

src/libraries/Common/tests/System/Security/Cryptography/AesGcmTests.cs

@@ -60,6 +60,39 @@ public static void InvalidKeyLength(int keyLength)
             Assert.Throws<CryptographicException>(() => new AesGcm(key.AsSpan(), AesGcm.TagByteSizes.MinSize));
         }
 
+        [Theory]
+        [InlineData(16)]
+        [InlineData(24)]
+        [InlineData(32)]
+        public static void KeySizeInBytes(int keyLength)
+        {
+            byte[] key = new byte[keyLength];
+
+#if NET
+#pragma warning disable SYSLIB0053
+            using (var aesGcm = new AesGcm(key))
+            {
+                Assert.Equal(keyLength, aesGcm.KeySizeInBytes);
+            }
+
+            using (var aesGcm = new AesGcm(key.AsSpan()))
+            {
+                Assert.Equal(keyLength, aesGcm.KeySizeInBytes);
+            }
+#pragma warning restore SYSLIB0053
+#endif
+
+            using (var aesGcm = new AesGcm(key, AesGcm.TagByteSizes.MinSize))
+            {
+                Assert.Equal(keyLength, aesGcm.KeySizeInBytes);
+            }
+
+            using (var aesGcm = new AesGcm(key.AsSpan(), AesGcm.TagByteSizes.MinSize))
+            {
+                Assert.Equal(keyLength, aesGcm.KeySizeInBytes);
+            }
+        }
+
         [Theory]
         [MemberData(nameof(GetInvalidNonceSizes))]
         public static void InvalidNonceSize(int nonceSize)

src/libraries/System.Security.Cryptography/ref/System.Security.Cryptography.cs

@@ -71,6 +71,7 @@ public sealed partial class AesCcm : System.IDisposable
         public AesCcm(byte[] key) { }
         public AesCcm(System.ReadOnlySpan<byte> key) { }
         public static bool IsSupported { get { throw null; } }
+        public int KeySizeInBytes { get { throw null; } }
         public static System.Security.Cryptography.KeySizes NonceByteSizes { get { throw null; } }
         public static System.Security.Cryptography.KeySizes TagByteSizes { get { throw null; } }
         public void Decrypt(byte[] nonce, byte[] ciphertext, byte[] tag, byte[] plaintext, byte[]? associatedData = null) { }
@@ -144,6 +145,7 @@ public AesGcm(byte[] key, int tagSizeInBytes) { }
         public AesGcm(System.ReadOnlySpan<byte> key) { }
         public AesGcm(System.ReadOnlySpan<byte> key, int tagSizeInBytes) { }
         public static bool IsSupported { get { throw null; } }
+        public int KeySizeInBytes { get { throw null; } }
         public static System.Security.Cryptography.KeySizes NonceByteSizes { get { throw null; } }
         public static System.Security.Cryptography.KeySizes TagByteSizes { get { throw null; } }
         public int? TagSizeInBytes { get { throw null; } }

src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/AesCcm.cs

@@ -14,11 +14,20 @@ public sealed partial class AesCcm : IDisposable
         public static KeySizes NonceByteSizes { get; } = new KeySizes(7, 13, 1);
         public static KeySizes TagByteSizes { get; } = new KeySizes(4, 16, 2);
 
+        /// <summary>
+        /// Gets the size of the key, in bytes.
+        /// </summary>
+        /// <value>
+        /// The size of the key, in bytes.
+        /// </value>
+        public int KeySizeInBytes { get; }
+
         public AesCcm(ReadOnlySpan<byte> key)
         {
             ThrowIfNotSupported();
 
             AesAEAD.CheckKeySize(key.Length);
+            KeySizeInBytes = key.Length;
             ImportKey(key);
         }
 
@@ -29,6 +38,7 @@ public AesCcm(byte[] key)
             ArgumentNullException.ThrowIfNull(key);
 
             AesAEAD.CheckKeySize(key.Length);
+            KeySizeInBytes = key.Length;
             ImportKey(key);
         }
 

src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/AesGcm.cs

@@ -18,6 +18,7 @@ public AesGcm(ReadOnlySpan<byte> key)
             ThrowIfNotSupported();
 
             AesAEAD.CheckKeySize(key.Length);
+            KeySizeInBytes = key.Length;
             ImportKey(key);
         }
 

src/libraries/System.Security.Cryptography/tests/AesCcmTests.cs

@@ -51,6 +51,25 @@ public static void InvalidKeyLength(int keyLength)
             Assert.Throws<CryptographicException>(() => new AesCcm(key));
         }
 
+        [Theory]
+        [InlineData(16)]
+        [InlineData(24)]
+        [InlineData(32)]
+        public static void KeySizeInBytes(int keyLength)
+        {
+            byte[] key = new byte[keyLength];
+
+            using (var aesCcm = new AesCcm(key))
+            {
+                Assert.Equal(keyLength, aesCcm.KeySizeInBytes);
+            }
+
+            using (var aesCcm = new AesCcm(key.AsSpan()))
+            {
+                Assert.Equal(keyLength, aesCcm.KeySizeInBytes);
+            }
+        }
+
         [Theory]
         [MemberData(nameof(GetInvalidNonceSizes))]
         public static void InvalidNonceSize(int nonceSize)