Add electronic signature

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/esign/tsa.go

@@ -0,0 +1,83 @@
+// Copyright (c) 2025 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package esign
+
+import (
+	"bytes"
+	"context"
+	"crypto"
+	"fmt"
+	"io"
+	"net/http"
+
+	"github.com/digitorus/timestamp"
+	"go.gearno.de/kit/httpclient"
+)
+
+// TSAClient sends RFC 3161 timestamp requests to a Trusted Timestamp Authority.
+type TSAClient struct {
+	URL        string
+	HTTPClient *http.Client
+}
+
+// Timestamp sends an RFC 3161 TimeStampReq via HTTP POST to the TSA.
+// The data parameter is the raw bytes to timestamp (typically the seal hex
+// string as UTF-8 bytes). CreateRequest internally computes SHA-256(data)
+// to build the MessageImprint. Returns the raw DER-encoded TimeStampResp bytes.
+func (c *TSAClient) Timestamp(ctx context.Context, data []byte) ([]byte, error) {
+	tsReq, err := timestamp.CreateRequest(
+		bytes.NewReader(data),
+		&timestamp.RequestOptions{
+			Hash:         crypto.SHA256,
+			Certificates: true,
+		},
+	)
+	if err != nil {
+		return nil, fmt.Errorf("esign: cannot create timestamp request: %w", err)
+	}
+
+	httpClient := c.HTTPClient
+	if httpClient == nil {
+		httpClient = httpclient.DefaultPooledClient()
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, c.URL, bytes.NewReader(tsReq))
+	if err != nil {
+		return nil, fmt.Errorf("esign: cannot build TSA HTTP request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/timestamp-query")
+
+	resp, err := httpClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("esign: TSA request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("esign: TSA returned HTTP %d", resp.StatusCode)
+	}
+
+	respBytes, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("esign: cannot read TSA response: %w", err)
+	}
+
+	// Validate the response: checks PKIStatus and parses the signed TSTInfo.
+	if _, err := timestamp.ParseResponse(respBytes); err != nil {
+		return nil, fmt.Errorf("esign: invalid TSA response: %w", err)
+	}
+
+	return respBytes, nil
+}