1 files changed, 91 insertions, 0 deletions
diff --git a/internal/cryptokit/age.go b/internal/cryptokit/age.go
new file mode 100644
index 0000000..525b1fa
--- /dev/null
+++ b/internal/cryptokit/age.go
@@ -0,0 +1,91 @@
+package cryptokit
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+
+	"filippo.io/age"
+	"filippo.io/age/agessh"
+	agearmor "filippo.io/age/armor"
+)
+
+const maxCryptoMessageBytes = 64 << 20
+
+func parseAgeRecipient(value string) (age.Recipient, error) {
+	value = strings.TrimSpace(value)
+	if recipient, err := age.ParseX25519Recipient(value); err == nil {
+		return recipient, nil
+	}
+	if recipient, err := agessh.ParseRecipient(value); err == nil {
+		return recipient, nil
+	}
+	return nil, errors.New("unsupported age recipient: expected native X25519 or SSH Ed25519/RSA")
+}
+
+func parseAgeIdentity(value string) (age.Identity, error) {
+	value = strings.TrimSpace(value)
+	if identity, err := age.ParseX25519Identity(value); err == nil {
+		return identity, nil
+	}
+	if identity, err := agessh.ParseIdentity([]byte(value)); err == nil {
+		return identity, nil
+	}
+	return nil, errors.New("unsupported age identity: expected native X25519 or SSH Ed25519/RSA private key")
+}
+
+// EncryptAge encrypts to a user-provided native age X25519 recipient or SSH
+// Ed25519/RSA recipient and returns ASCII-armored age text.
+func EncryptAge(message []byte, recipientKey string) ([]byte, error) {
+	recipient, err := parseAgeRecipient(recipientKey)
+	if err != nil {
+		return nil, fmt.Errorf("parse age recipient: %w", err)
+	}
+	var output bytes.Buffer
+	armored := agearmor.NewWriter(&output)
+	writer, err := age.Encrypt(armored, recipient)
+	if err != nil {
+		_ = armored.Close()
+		return nil, fmt.Errorf("create age encryption: %w", err)
+	}
+	if _, err := writer.Write(message); err != nil {
+		_ = writer.Close()
+		_ = armored.Close()
+		return nil, fmt.Errorf("write age message: %w", err)
+	}
+	if err := writer.Close(); err != nil {
+		_ = armored.Close()
+		return nil, fmt.Errorf("close age message: %w", err)
+	}
+	if err := armored.Close(); err != nil {
+		return nil, fmt.Errorf("close age armor: %w", err)
+	}
+	return output.Bytes(), nil
+}
+
+// DecryptAge decrypts an armored or binary age message with a user-provided
+// native X25519 identity or an unencrypted SSH Ed25519/RSA private key.
+func DecryptAge(message []byte, identityKey string) ([]byte, error) {
+	identity, err := parseAgeIdentity(identityKey)
+	if err != nil {
+		return nil, fmt.Errorf("parse age identity: %w", err)
+	}
+	var input io.Reader = bytes.NewReader(message)
+	if bytes.HasPrefix(bytes.TrimSpace(message), []byte(agearmor.Header)) {
+		input = agearmor.NewReader(input)
+	}
+	reader, err := age.Decrypt(input, identity)
+	if err != nil {
+		return nil, fmt.Errorf("create age decryption: %w", err)
+	}
+	plaintext, err := io.ReadAll(io.LimitReader(reader, maxCryptoMessageBytes+1))
+	if err != nil {
+		return nil, fmt.Errorf("read age message: %w", err)
+	}
+	if len(plaintext) > maxCryptoMessageBytes {
+		return nil, errors.New("decrypted age message exceeds the 64 MiB limit")
+	}
+	return plaintext, nil
+}