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src/Attestor/StellaOps.Attestor.Envelope/DssePreAuthenticationEncoding.cs

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+using System;
+using System.Buffers;
+using System.Globalization;
+using System.Text;
+
+namespace StellaOps.Attestor.Envelope;
+
+/// <summary>
+/// Computes DSSE pre-authentication encoding (PAE) for payload signing.
+/// </summary>
+public static class DssePreAuthenticationEncoding
+{
+    private static readonly byte[] Prefix = Encoding.ASCII.GetBytes("DSSEv1");
+    private static readonly byte[] Space = new byte[] { (byte)' ' };
+
+    public static byte[] Compute(string payloadType, ReadOnlySpan<byte> payload)
+    {
+        if (payloadType is null)
+        {
+            throw new ArgumentNullException(nameof(payloadType));
+        }
+
+        var payloadTypeBytes = Encoding.UTF8.GetBytes(payloadType);
+        var payloadTypeLength = Encoding.ASCII.GetBytes(payloadTypeBytes.Length.ToString(CultureInfo.InvariantCulture));
+        var payloadLength = Encoding.ASCII.GetBytes(payload.Length.ToString(CultureInfo.InvariantCulture));
+
+        var buffer = new ArrayBufferWriter<byte>();
+        Write(buffer, Prefix);
+        Write(buffer, Space);
+        Write(buffer, payloadTypeLength);
+        Write(buffer, Space);
+        Write(buffer, payloadTypeBytes);
+        Write(buffer, Space);
+        Write(buffer, payloadLength);
+        Write(buffer, Space);
+        Write(buffer, payload);
+
+        return buffer.WrittenSpan.ToArray();
+    }
+
+    private static void Write(ArrayBufferWriter<byte> writer, ReadOnlySpan<byte> bytes)
+    {
+        var span = writer.GetSpan(bytes.Length);
+        bytes.CopyTo(span);
+        writer.Advance(bytes.Length);
+    }
+}