refactor!: modernize base types and interfaces (#87)

Signed-off-by: Shiwei Zhang <[email protected]>

src/OrasProject.Oras/Content/Digest.cs

@@ -0,0 +1,51 @@
+// Copyright The ORAS Authors.
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+using OrasProject.Oras.Exceptions;
+using System;
+using System.Security.Cryptography;
+using System.Text.RegularExpressions;
+
+namespace OrasProject.Oras.Content;
+
+internal static class Digest
+{
+    private const string digestRegexPattern = @"[a-z0-9]+(?:[.+_-][a-z0-9]+)*:[a-zA-Z0-9=_-]+";
+    private static readonly Regex digestRegex = new Regex(digestRegexPattern, RegexOptions.Compiled);
+
+    /// <summary>
+    /// Verifies the digest header and throws an exception if it is invalid.
+    /// </summary>
+    /// <param name="digest"></param>
+    internal static string Validate(string digest)
+    {
+        if (string.IsNullOrEmpty(digest) || !digestRegex.IsMatch(digest))
+        {
+            throw new InvalidDigestException($"Invalid digest: {digest}");
+        }
+        return digest;
+    }
+
+    /// <summary>
+    /// Generates a SHA-256 digest from a byte array.
+    /// </summary>
+    /// <param name="content"></param>
+    /// <returns></returns>
+    internal static string ComputeSHA256(byte[] content)
+    {
+        using var sha256 = SHA256.Create();
+        var hash = sha256.ComputeHash(content);
+        var output = $"sha256:{BitConverter.ToString(hash).Replace("-", "")}";
+        return output.ToLower();
+    }
+}