age: add ExampleDecryptReaderAt with zip.NewReader

age.go

@@ -284,8 +284,8 @@ func Decrypt(src io.Reader, identities ...Identity) (io.Reader, error) {
 // DecryptReaderAt takes an underlying [io.ReaderAt] and its total encrypted
 // size, and returns a ReaderAt of the decrypted plaintext and the plaintext
 // size. These can be used for example to instantiate an [io.SectionReader],
-// which implements [io.Reader] and [io.Seeker]. Note that ReaderAt by
-// definition disregards the seek position of src.
+// which implements [io.Reader] and [io.Seeker], or for [zip.NewReader].
+// Note that ReaderAt by definition disregards the seek position of src.
 //
 // The ReadAt method of the returned ReaderAt can be called concurrently.
 // The ReaderAt will internally cache the most recently decrypted chunk.

age_test.go

@@ -5,10 +5,12 @@
 package age_test
 
 import (
+	"archive/zip"
 	"bytes"
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"log"
 	"os"
 	"slices"
@@ -191,6 +193,40 @@ func TestEncryptDecryptScrypt(t *testing.T) {
 	}
 }
 
+func ExampleDecryptReaderAt() {
+	identity, err := age.ParseX25519Identity(privateKey)
+	if err != nil {
+		log.Fatalf("Failed to parse private key: %v", err)
+	}
+
+	f, err := os.Open("testdata/example.zip.age")
+	if err != nil {
+		log.Fatalf("Failed to open file: %v", err)
+	}
+	stat, err := f.Stat()
+	if err != nil {
+		log.Fatalf("Failed to stat file: %v", err)
+	}
+
+	r, size, err := age.DecryptReaderAt(f, stat.Size(), identity)
+	if err != nil {
+		log.Fatalf("Failed to open encrypted file: %v", err)
+	}
+
+	z, err := zip.NewReader(r, size)
+	if err != nil {
+		log.Fatalf("Failed to open zip: %v", err)
+	}
+	contents, err := fs.ReadFile(z, "example.txt")
+	if err != nil {
+		log.Fatalf("Failed to read file from zip: %v", err)
+	}
+
+	fmt.Printf("File contents: %q\n", contents)
+	// Output:
+	// File contents: "Black lives matter."
+}
+
 func TestParseIdentities(t *testing.T) {
 	tests := []struct {
 		name      string