Digital signatures are a combination of public key cryptography and hashing.

Encryption and decryption address problems of eavesdropping, but they do not address tampering and impersonation. However, public-key cryptography does address the problems of tampering and impersonation.

You can use your private key for encryption and your public key for decryption. Although this is not desirable when you are encrypting sensitive data, it is an important part of digitally signing any data. Rather than encrypting the data itself, you can create a one-way hash of the data and then use your private key to encrypt the hash. The encrypted hash, along with other information like the hashing algorithm, is known as a digital signature.

shows a simplified view of how you can use a digital signature to validate the integrity of signed data.

Figure 1. Digital signature validating data integrity

shows the original data and the digital signature transferred to the recipient. The digital signature is basically a one-way hash (or message digest) of the original data that was encrypted with the signer's private key. To validate the data's integrity, the recipient first uses the signer's public key to decrypt the digital signature. The recipient then uses the same hashing algorithm that generated the original hash to generate a new one-way hash of the same data. Information about the hashing algorithm used is sent with the digital signature; this is not shown in the figure. Finally, the recipient compares the two hash values. If they match, the data has not changed since it was signed. If the hashes do not match, the data may have been tampered with since it was first signed or the digital signature may have been created with a private key that does not correspond to the public key presented by the signer.

If the hashes match, the recipient can be assured that the public key used to decrypt the digital signature corresponds to the private key used to create the digital signature. However, confirming the identity of the signer also requires some way of confirming that the public key truly belongs to a particular person or other entity. Digital certificates and authentication are used in this case.

You can compare the significance of a digital signature to that of your handwritten signature. Once you have signed data, it is difficult to deny doing so later. This assumes that the private key was not compromised or out of the owner's control. Digital signatures make it difficult for the signer to deny having signed the data.

The terms digital signature and electronic signature are sometimes confused or used interchangeably. While digital signatures are a form of electronic signature, not all electronic signatures are digital signatures. Electronic signatures—also called e-signatures—are any sound, symbol, or process that shows the intent to sign something. This could be a scan of your hand-written signature, a stamp, or a recorded verbal confirmation. An electronic signature could even be your typed name on the signature line of a document.

What is a digital signature?

A digital signature—a type of electronic signature—is a mathematical algorithm routinely used to validate the authenticity and integrity of a message (e.g., an email, a credit card transaction, or a digital document). Digital signatures create a virtual fingerprint that is unique to a person or entity and are used to identify users and protect information in digital messages or documents. In emails, the email content itself becomes part of the digital signature. Digital signatures are significantly more secure than other forms of electronic signatures.

Why would you use a digital signature?

Digital signatures increase the transparency of online interactions and develop trust between customers, business partners, and vendors.

How do digital signatures work?

Familiarize yourself with the following terms to better understand how digital signatures work:

• Hash function – A hash function (also called a “hash”) is a fixed-length string of numbers and letters generated from a mathematical algorithm and an arbitrarily sized file such as an email, document, picture, or other type of data. This generated string is unique to the file being hashed and is a one-way function— a computed hash cannot be reversed to find other files that may generate the same hash value. Some of the more popular hashing algorithms in use today are Secure Hash Algorithm-1 (SHA-1), the Secure Hashing Algorithm-2 family (SHA-2 and SHA-256), and Message Digest 5 (MD5).
• Public key cryptography – Public key cryptography (also known as asymmetric encryption) is a cryptographic method that uses a key pair system. One key, called the public key, encrypts the data. The other key, called the private key, decrypts the data. Public key cryptography can be used several ways to ensure confidentiality, integrity, and authenticity. Public key cryptography can
  • Ensure integrity by creating a digital signature of the message using the sender’s private key. This is done by hashing the message and encrypting the hash value with their private key. By doing this, any changes to the message will result in a different hash value.
  • Ensure confidentiality by encrypting the entire message with the recipient’s public key. This means that only the recipient, who is in possession of the corresponding private key, can read the message.
  • Verify the user’s identity using the public key and checking it against a certificate authority.
• Public key infrastructure (PKI) – PKI consists of the policies, standards, people, and systems that support the distribution of public keys and the identity validation of individuals or entities with digital certificates and a certificate authority.
• Certificate authority (CA) – A CA is a trusted third party that validates a person’s identity and either generates a public/private key pair on their behalf or associates an existing public key provided by the person to that person. Once a CA validates someone’s identity, they issue a digital certificate that is digitally signed by the CA. The digital certificate can then be used to verify a person associated with a public key when requested.
• Digital certificates – Digital certificates are analogous to driver licenses in that their purpose is to identify the holder of a certificate. Digital certificates contain the public key of the individual or organization and are digitally signed by a CA. Other information about the organization, individual, and CA can be included in the certificate as well.
• Pretty Good Privacy (PGP)/OpenPGP – PGP/OpenPGP is an alternative to PKI. With PGP/OpenPGP, users “trust” other users by signing certificates of people with verifiable identities. The more interconnected these signatures are, the higher the likelihood of verifying a particular user on the internet. This concept is called the “Web of Trust.”

Digital signatures work by proving that a digital message or document was not modified—intentionally or unintentionally—from the time it was signed. Digital signatures do this by generating a unique hash of the message or document and encrypting it using the sender’s private key. The hash generated is unique to the message or document, and changing any part of it will completely change the hash.

Once completed, the message or digital document is digitally signed and sent to the recipient. The recipient then generates their own hash of the message or digital document and decrypts the sender’s hash (included in the original message) using the sender’s public key. The recipient compares the hash they generate against the sender’s decrypted hash; if they match, the message or digital document has not been modified and the sender is authenticated.

Why should you use PKI or PGP with digital signatures?

Using digital signatures in conjunction with PKI or PGP strengthens them and reduces the possible security issues connected to transmitting public keys by validating that the key belongs to the sender, and verifying the identity of the sender. The security of a digital signature is almost entirely dependent on how well the private key is protected. Without PGP or PKI, proving someone’s identity or revoking a compromised key is impossible; this could allow malicious actors to impersonate someone without any method of confirmation.

Through the use of a trusted third party, digital signatures can be used to identify and verify individuals and ensure the integrity of the message.

As paperless, online interactions are used more widely, digital signatures can help you secure and safeguard the integrity of your data. By understanding and using digital signatures, you can better protect your information, documents, and transactions.

What combines public key cryptography with a cryptographic hash?

How are digital signatures different from public key cryptography?

Which type of cryptography is also called public key cryptography?

How digital signature is used with hash function to secure message authentication?