Security Method Using Digital Signatures

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Cryptography provides two primary security methods you can use with email: digital signatures and encryption.  If you’re planning to take the Security+ exam, you should understand that these are separate processes, but you can digitally sign and encrypt the same email.

For example, can you answer this question?

Q. Lenny and Carl work in an organization that includes a PKI. Carl needs to send a digitally signed file to Lenny. What does Carl use in this process?

A. Carl’s public key

B. Carl’s private key

C. Lenny’s public key

D. Lenny’s private key

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation is available at the end of this post.

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Signing Email with Digital Signatures

Digital signatures are similar in concept to handwritten signatures on printed documents that identify individuals, but they provide more security benefits. A digital signature is an encrypted hash of a message, encrypted with the sender’s private key. If the recipient of a digitally signed email can decrypt the hash, it provides the following three security benefits:

  • Authentication. This identifies the sender of the email. Email recipients have assurances the email actually came from who it appears to be coming from. For example, if an executive digitally signs an email, recipients know it came from the executive and not from an attacker impersonating the executive.
  • Non-repudiation. The sender cannot later deny sending the message. This is sometimes required with online transactions. For example, imagine if Homer sends an order to sell stocks using a digitally signed email. If the stocks increase after his sale completes, he can’t deny the transaction.
  • Integrity. This provides assurances that the message has not been modified or corrupted. Recipients know that the message they received is the same as the sent message.

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Digital signatures are much easier to grasp if you understand some other cryptography concepts. As a short review, these concepts are:

  • Hashing. Digital signatures start by creating a hash of the message. A hash is simply a number created by performing an algorithm on the message.
  • Certificates. Digital signatures need certificates, and certificates include the sender’s public key.
  • Public/private keys. In a digital signature, the sender uses the sender’s private key to encrypt the hash of the message. The recipient uses the sender’s public key to decrypt the hash of the message.

The figure shows an overview of this process. In the figure, Lisa is sending a message to Bart with a digital signature. Note that the message “I passed” is not secret. If it was, Lisa would encrypt it, which is a completely separate process. The focus in this explanation is only the digital signature.

Security Method Using Digital Signatures

Digital signature process

Lisa creates her message in an email program, such as Microsoft Outlook. Once Microsoft Outlook is configured, all she has to do is click a button to digitally sign the message. Here is what happens when she clicks the button:

  1. The application hashes the message.
  2. The application retrieves Lisa’s private key and encrypts the hash using this private key.
  3. The application sends both the encrypted hash and the unencrypted message to Bart.

When Bart’s system receives the message, it verifies the digital signature using the following steps:

  1. Bart’s system retrieves Lisa’s public key, which is in Lisa’s public certificate. In some situations, Lisa may have sent Bart a copy of her certificate with her public key. In domain environments, Bart’s system can automatically retrieve Lisa’s certificate from a network location.
  2. The email application on Bart’s system decrypts the encrypted hash with Lisa’s public key.
  3. The application calculates the hash on the received message.
  4. The application compares the decrypted hash with the calculated hash.

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If the calculated hash of the received message is the same as the encrypted hash of the digital signature, it validates several important checks:

  • Authentication. Lisa sent the message. The public key can only decrypt something encrypted with the private key, and only Lisa has the private key. If the decryption succeeded, Lisa’s private key must have encrypted the hash. On the other hand, if another key was used to encrypt the hash, Lisa’s public key could not decrypt it. In this case, Bart will see an error indicating a problem with the digital signature.
  • Non-repudiation. Lisa cannot later deny sending the message. Only Lisa has her private key and if the public key decrypted the hash, the hash must have been encrypted with her private key. Non-repudiation is valuable in online transactions.
  • Integrity. Because the hash of the sent message matches the hash of the received message, the message has maintained integrity. It hasn’t been modified.

Remember this

A digital signature is an encrypted hash of a message. The sender’s private key encrypts the hash of the message to create the digital signature. The recipient decrypts the hash with the sender’s public key. If successful, it provides authentication, non-repudiation, and integrity. Authentication identifies the sender. Integrity verifies the message has not been modified. Non-repudiation prevents senders from later denying they sent an email.

At this point, you might be thinking, if we do all of this, why not just encrypt the message, too? The answer is resources. It doesn’t take much processing power to encrypt 256 bits in a SHA-256 hash. In contrast, it would take quite a bit of processing power to encrypt a lengthy email and its attachments. However, if you need to ensure confidentiality of the email, you can encrypt it.


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Q. Lenny and Carl work in an organization that includes a PKI. Carl needs to send a digitally signed file to Lenny. What does Carl use in this process?

A. Carl’s public key

B. Carl’s private key

C. Lenny’s public key

D. Lenny’s private key

Answer is B. Carl uses his private key to digitally sign the file.

Lenny uses Carl’s public key to decrypt the digital signature.

Lenny’s keys are not used in this scenario.

See Chapter 10 of the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide for more information cryptographic protocols.

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