Digital Signatures: What Are They and How to Use Them?

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You may have come across the term “digital signatures” online. You may even have digitally signed emails or documents yourself. However, a digital signature is not the same as an electronic signature.

While an electronic signature can be a simple PIN, a digital signature is a “virtual fingerprint” that proves the authenticity of the author.

In this guide, we will cover everything you need to know about digital signatures, including:

  • what a digital signature is;
  • what sets it apart from electronic signatures;
  • how to send your first digitally signed emails;

… and much more! So without further ado, let’s explore.

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What Is a Digital Signature?

First things first, let’s define what a digital signature is.

A digital signature is a type of electronic signature that can be used to authenticate the person who signed a document.

In a sense, a digital signature is a “virtual fingerprint”. Once a document is signed with it, it becomes an embedded part of the document.

Digital signatures serve three essential roles:

  • sender verification: the sender is who they claim to be;
  • integrity: the message was not altered during transit;
  • non-repudiation: the sender cannot deny having sent the message.
A digitally-signed email in the Mailfence mobile app
A digitally-signed email in the Mailfence mobile app

This makes digital signatures more secure than handwritten signatures. This is because there is no reliable way of verifying a handwritten signature, much less a way to know if the document was not tampered with.

Handwritten signatures are unique to every person (handwriting style, speed of writing etc.). However, once a signature is put to paper, it is easily forgeable with tracing paper.

Handwritten signatures therefore pose a repudiation problem. One can simply claim that someone copied their signature, and it would be difficult to prove otherwise.

This is why we need digital signatures 👇

Digital Signature and Electronic Signature: What’s the Difference?

We mentioned earlier that a digital signature is a type of electronic signature. It’s important to differentiate between those two properly, as not every e-signature is automatically a digital signature.

An electronic signature is any type of expression, not just a signature itself. It can be a PIN code, password, or even a simple image of your signature:

Example of a simple electronic signature
Example of a simple electronic signature

Here’s a quick comparison between electronic signatures and digital signatures:

FeatureElectronic signatureDigital Signature
PurposeVerifying the documentProtecting the document
VerificationCannot be verifiedCan be verified
Third-party regulationTypically not regulatedRegulated by the CA (PKI) or “web of trust” (OpenPGP)
CryptographyDoesn’t use cryptographic algorithmsCreated using cryptographic algorithms
AdvantageAble to authenticate the senderEasier to use
Comparison table

How Do Digital Signatures Work?

We’ve now established what a digital signature is, and why it is a superior form of electronic signature.

Let’s now dive into how they actually work.

Firstly, a digital signature only requires the sender (the signer) to have cryptographic keys (a private key and a public key). The sender signs the message locally on their device (using their own private key). The receiver then verifies it on their device using the sender’s public key. The process works as follows:

  1. Alice (sender) generates a key pair and shares her public key with Bob (a one-time pre-requisite).
  2. Alice signs the message using her private key on her device and sends the message to Bob.
  3. Bob receives the signed message on his device and verifies the signature using Alice’s public key.

Make sure to check out this guide on private and public keys if you need a refresher.

Digitally signing an email with Mailfence
Digitally signing an email with Mailfence

Digital signatures on a technical level

On a more technical level, sending a digitally-signed email will follow these steps:

  • signing starts with taking a cryptographic hash of the original message;
  • the hash is encrypted with the sender’s private key and sent with the original message;
  • the recipient receives both the original message and its encrypted hash;
  • he then decrypts the encrypted hash using the sender’s public key and matches it with the hash of the original message;
  • if both of the hashes match, the digital signature is verified.
Digital Signature process
Source: Wikipedia

To fully understand how digital signatures work, we need to get a little technical. Specifically, we need to talk about two different protocols that digital signatures are based on.

The first one is PKI or “Public Key Infrastructure“. This means that the public key that Alice was using to sign her message to Bob has to be certified by a CA or a “Certificate Authority” (such as Verisign).

In other words, the CA vouches for the key, and the key validates the sender – making the CA a trust anchor.

However, a centralized authority (i.e. Certificate Authority) can not always be trusted (e.g., due to sociopolitical reasons) or can be compromised.

An alternative trust model is Web of Trust or Pretty Good Privacy (PGP).

In the “web of trust” model, each user has a set of “trusted” public keys that verify the sender to the recipient, instead of everyone relying on a single CA.

Are Digital Signatures the Same as End-to-End Encryption?

Although the concepts are similar, they are not the same.

The most notable difference is that with end-to-end encryption, both parties need their own private/public keypair. However, you can send a digitally signed email to somebody who hasn’t generated a keypair.

Here’s a quick summary:

End-to-end encryptionDigital Signatures
Used to encode sensitive information in an email or documentUsed to verify the authenticity of the document or email message
The public key is used to encrypt the data in the email or documentThe public key is used to verify the sender’s signature
The private key is used to decrypt the data in the email by the recipientThe private key is used to sign the document by the sender
The encrypted message is verified only after it is decrypted using the recipient’s private keyThe digital signature is verified only if it remains unaltered between the sender and the recipient
Requires PGP or similar encryption protocolThe certificate authority (CA) provides a digital certificate
Differences between E2EE and digital signatures

If you want to dive deeper into end-to-end encryption, check out our comprehensive guide.

However, digital signatures can be combined with end-to-end encryption. This provides increased security, as we’ll explore in the following section.

How to Send a Digitally Signed Emails

Now that you know what a digital signature is, how do you send one?

The exact process will differ with each email provider. To get started, the easiest is to create a free Mailfence account and follow these steps.

First, generate your key pair, and share your public key with the recipient. Next:

  • on our web interface, compose your email and enter the recipient’s email address. Then, click the dropdown arrow next to “Send”, and select “Sign and Send”. Enter your password, and click OK.
  • on our mobile app, when composing a message, tap the “Lock” icon at the top-right of the screen. Select “Signed with OpenPGP”.

That’s it – it’s that simple!

By the way, if you haven’t yet, make sure to download our mobile app on the App Store or Google Play store!

As mentioned previously, you can combine E2E encryption with digital signatures. This provides increased security to your communications. With Mailfence, if you decide to E2E encrypt your email, it will also be automatically signed:

Combining end-to-end encryption with digital signatures
Combining end-to-end encryption with digital signatures

Digital Signatures Best Practices

Before wrapping up this guide, let’s go over some best practices when sending digitally-signed emails.

Tip #1: Use digital signatures when sending emails

Digitally signing emails with attachments reassures the receiver of the email. In addition, this assures that both the content and the attachment have not been tampered with during transit. This increases the legal value of the email.

Tip #2: Use a strong key when digitally signing

Use a 4096 bit (or at least a 2048 bit) length-based private key to sign a digital message. Mailfence always generates a 4096bit RSA key by default.

Tip #3: Use a sub key when signing

We’ve covered in this guide how encryption and signing are two different operations.

Therefore they demand two different key management approaches. You may want a signing key to be valid for a long time so people around the world can verify signatures from the past. As for your encryption key, you will want to rotate it earlier and easily revoke/expire the old one.

Hence, keeping your key-pair externally as a ‘master keypair’ (for e.g., in a flash drive) and using a sub-key as a ‘local key’ for signing messages is a good practice.

This good article provides a list of steps that you should follow to achieve this using GPG. You can then import that local keypair in your Mailfence account keystore to use it seamlessly around all of your devices.

Note: 1) Keypair containing only sub-keys (for signing and encryption) and a dummy private key packet (a GNU extension to OpenPGP) may not be supported by all OpenPGP-compliant programs. 2) Keeping the primary secret key on removable media makes signing (or certifying) other GPG keys problematic since the primary secret key must be loaded in order to do the signing. 3) Using expiration dates with the subkeys (recommended) means that the subkeys must be extended prior to expiration, or new subkeys issued if they are allowed to expire. 4) Also, people who use your public key (i.e., to verify something you have signed) may receive errors about your key being expired if they do not regularly update their GPG keyrings with public key servers.

Tip #4: Ask friends and colleagues to sign your public key

Having trust signatures on your key will enhance its legitimacy. Ask your friends/colleagues to sign your key.  You should sign their public key in exchange.

Alternatively, is an operational model that maps your identity to your public keys, and vice versa.

Tip #5: Sign your message for each of your recipients

Yes, include the sender and recipient(s) email address with timestamps in your digital signature! (by adding it to the body of the message).  You can also use a service that provides sufficient protection against replay attacks (nonce, session tokens, timestamp, etc).

Tip #6: Pay caution while forwarding a digitally signed message

Let’s take the case of a digitally signed message by Alice to Bob now forwarded to Carol.  Only the identity of the original sender: Alice is proved to the recipient. No assumption can be made by Carol that Bob who forwarded the message is the owner of the digital signature/or is the right sender of the message. Unless the forwarded message is signed by Bob himself.

Tip #7: Check the fingerprint of your sender’s key in advance

If possible, get the fingerprint of your sender’s key in advance (preferably in person) – or if your sender is a known figure, by doing an online search.

Additionally, always verify the keypair used for signing a message with the fingerprint.

Automatic verification (matching decrypted hash with the hash of the original message) is one thing. But it will not protect you from impersonation attacks, where an attacker may create a fake identity of the sender and use it to fool the recipients. Thus, a fingerprint should always be your sole cryptographic factor for verifying the public keys of all your senders.

Tip #8: Make sure the signing key hasn’t been revoked or expired

Make sure the key that has been used to sign a particular message is not revoked or expired! This is a big one! Applications often fail to display this clearly to the user.  At Mailfence, every such case is timely and properly displayed.

However, if the signing of a message happened before the expiration/or revocation date of the signing key – the digital signature remains valid.

Tip #9: Export messages with digital signatures on them locally

Since account compromises are more common these days it’s better to keep a local copy of messages of high importance or value by exporting them to your machine.

In addition, you can improve their security by locally encrypting them as well.

A digital signature can be used to hold someone liable as the author of a certain message. However, the precise legal value of digital signatures depends on where the signature was been made and the local laws.

Nonetheless, there are always certain terms that need to be agreed upon before using digital signatures in a legal context. One issue is the possibility that the private key of the signer gets compromised.   What should be done in such a case to verify whether it was a real compromise or done deliberately?

Note: if you don’t want non-repudiation in exceptional cases, then use deniable authentication based schemes!

Last Word on Digital Signatures

This wraps up this guide on digital signatures!

OpenPGP digital signatures are quite common because they do not require you to have the public key of the recipient. At Mailfence, we’ve made the process of digital signing very simple. If you use the above-mentioned OpenPGP digital signature best practices, you will further leverage your usage of digital signing.

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Picture of M Salman Nadeem

M Salman Nadeem

Salman works as an Information Security Analyst for Mailfence. His areas of interest include cryptography, security architecture and design, access control, and operations security. You can follow him on LinkedIn @mohammadsalmannadeem.

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