送受信時のメールの暗号化

S/MIME is used to support enhanced encryption in transit, and automatically encrypts your outgoing emails if it can.

注: 下記の手順で暗号化状態を確認するには、お使いのアカウントで S/MIME を有効にしている必要があります。

Check if a message you're sending is encrypted 

 
  1. Start composing a message.
  2. Add recipients to the "To" field.
  3. To the right of your recipients, you'll see a lock icon that shows the level of encryption that is supported by your message's recipients. If there are multiple users with various encryption levels, the icon will show the lowest encryption status.
  4. To change your S/MIME settings or learn more about your recipient's level of encryption, click the lock, then View details.

Check if a message you received is encrypted


  1. メールを開きます。
  2. Android デバイスの場合: [詳細を表示] 次に [セキュリティの詳細を表示] をタップします。
    iPhone または iPad の場合
    : [詳細を表示] をタップします。
  3. 表示される鍵アイコンの色は、メールの送信時に使用された暗号化のレベルを示します。

What the encryption icons mean

When you're sending or receiving messages, you can see the level of encryption a message has. The color of the icon will change based on the level of encryption.

  • Green (S/MIME enhanced encryption) 暗号化あり. Suitable for your most sensitive information. S/MIME encrypts all outgoing messages if we have the recipient's public key. Only the recipient with the corresponding private key can decrypt this message.
  • Gray (TLS - standard encryption) TLS(標準的な暗号化). Suitable for most messages. TLS (Transport Layer Security) is used for messages exchanged with other email services who don't support S/MIME.
  • Red (no encryption) 暗号化なし. Unencrypted mail which is not secure. Past messages sent to the recipient's domain are used to predict whether the message you're sending won't be reliably encrypted.

I see the red lock icon

If you're writing a message and see the red lock icon, consider removing these addresses or deleting the confidential information. To see which address is unencrypted, click View Details.

If you received a message with the red lock icon and the message contained particularly sensitive content, let the sender know and they can contact their email service provider.

Learn more about encryption

Why some emails might not be encrypted

If the person you’re emailing is using an email service that doesn’t encrypt all messages using S/MIME or TLS, their emails might not be secure. However, messages are encrypted in S/MIME whenever possible.

For S/MIME to work, to either sign or receive S/MIME encrypted mail, a user must have a valid S/MIME cert from a trusted root.

S/MIME (enhanced encryption)

S/MIME is a long standing protocol which allows encrypted and signed messages to be sent using standard mail delivery SMTP.

It uses public key cryptography to:

  • Encrypt the message on send and decrypt the message on receipt with a suitable private key to keep message content private.
  • Sign on send and verify the signature on receipt to authenticate and protect integrity.

注: メールの配信時にユーザーの鍵がアップロードされていない場合、メールを復号化できません。詳しくは、証明書のアップロードについての記事をご覧ください。

TLS (standard encryption)

Opportunistic TLS (STARTTLS) is a protocol that helps provide privacy between communicating applications and their users during email delivery. When a server and client communicate, TLS ensures that no third party can overhear or tamper with any messages.

For delivery TLS to work, the email delivery services of both the sender and the receiver always have to use TLS.

詳しくは、TLS メール暗号化方式についての記事をご覧ください。

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