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Asymmetric and symmetric encryptions are the types of encryptions used in cryptography. There is only one key involved in symmetric encryption, which is used for both encryption and decryption. The key has to be shared among the parties involved who wish to encrypt and decrypt data. In comparison, asymmetric encryption uses two separate keys which are related to each other mathematically. The keys are called Private keys and public keys. Generally, a certificate is associated with a public key, which holds the information about the public key owner. The certificate contains information such as name, organization name, algorithms used, etc. Even though symmetric and asymmetric encryption seems similar, symmetric encryption is comparably faster than asymmetric encryption; thus, performance-wise, asymmetric encryption is slower, which is why symmetric encryption is often used in conjunction with asymmetric encryption. We will explore more about this on the use cases discussed below.
As discussed above, symmetric encryption uses the same key for encryption and decryption, so the sender would somehow send the key to the receiver to decrypt the encrypted data. The key involved has to be protected and transferred securely. If the key is lost, then the data cannot be decrypted, and if the key gets compromised, that will compromise the encryption.
Thus, symmetric keys are transferred among parties using asymmetric encryption, ensuring that the symmetric key remains encrypted. Two different keys are now involved in encrypting and decrypting the data.
Symmetric encryption is comparably much faster than asymmetric encryption, which is why it is still used massively today.
Asymmetric encryption uses two separate keys which are mathematically involved with each other. The first key is called the private key. The private key is the one that is heavily protected. Often this key remains in an HSM or an air-gapped computer to ensure the protection of the private key. The public key is derived from the private key and can be distributed. A certificate is often created with a public key containing information about the key’s owner and a few details about the key itself.
The key pair relies on prime numbers of extended length. The public and private keys are computed simultaneously, using the same mathematical operation, namely trapdoor functions. The most characteristic of trapdoor functions is that they’re simple to calculate in one direction, nevertheless troublesome calculating in the reverse order. Using a private key, we can find the public key, but the private key cannot be obtained using the public key.
Even though asymmetric encryption provides more protection to the keys, they are comparatively slower than symmetric encryption. For this reason, asymmetric encryption is often used to exchange the secret key, which can be used to establish symmetric encryption for faster data transfer and make encryption and decryption of the data faster.
Symmetric Encryption uses one key for encryption and decryption, which is why it is best used to encrypt and decrypt local data. Some of the use cases involving symmetric encryption can be:
- Data at rest: Data at rest refers to data stored in a physical drive and is not transferred among devices. This data can be stored in hard drives, SSD, flash drives, etc. Data at rest often involves data that can be valuable to attackers as it usually contains customer information, employee information, trade secrets, Intellectual Properties, etc. To be able to protect this data adequately, organizations use symmetric encryption.
One effective way to encrypt all the data in a physical drive is to encrypt the drive itself. This is called whole disk or full disk encryption, which has fewer benefits than partial or folder encryption. Many files (such as Word files) create a temporary file while it is opened. This temporary file can remain unencrypted. Many Linux distribution does provide full disk encryption while installing the operating system, and Microsoft delivers BitLocker Drive Encryption for Windows. Full drive encryption does leave the boot volume unencrypted for the operating system to boot, but every other volume, including swap space and temporary files, remains encrypted.
- Banking and Payment Industry: The banking and Payment Industry is one of the most secure industries, and there are many compliances involved in keeping it safe. Nevertheless, it is also one of the busiest industries, which requires them to be fast. Symmetric encryption provides fast encryption and decryption of a large amount of data, which includes the transaction. It can often contain Personal Identifiable Information (PII), which needs to be protected for being compliant with PCI DSS and preventing Identity Theft. They also need to ensure the identity of the user and the authenticity of the transactions fast. This is one of the reasons why the banking industry uses symmetric encryption.
Asymmetric Encryption is slower than symmetric encryption, so it is used for small amounts of data, such as exchanging secret keys or providing digital identities.
As organizations move to the digital age, digital signatures become crucial to identifying authentic data and ensuring someone’s identity. Digital signatures provide that the data involved has not been modified and if the data (which can include PDF, applications, etc.) is authentic. The recipient of signed data will use a digital signature to demonstrate to a 3rd party that the claimed individual generated the signature. This is often called non-repudiation since the individual cannot simply repudiate the signature at a later time.
The following steps explain how a digital signature is used to exchange information between a sender and a receiver:
- First, the data that would be transferred is hashed using some hashing algorithms such as SHA-256. Hashing is a one-way function and produces a unique value for unique input. The original data cannot be obtained using the hash value. This ensures data integrity, as if the data is modified, the corresponding hash value will also change.
- The hash is then encrypted using the sender’s private key. This creates a digital signature.
- The digital signature would now be attached and sent to the sender.
- After receiving the data and the digital signature, the signature is decrypted using the sender’s public key, which provides the hash value generated on the first step.
- The receiver then hashes the obtained data.
- If the hash obtained on step 5 is equal to the hash receiver got on step 4, it ensured data integrity. If, however, the hashes do not match, then the data has been modified.
Digital signatures are meant to be used in emails, transferring data, distributing software, and other applications, requiring data integrity and ensuring the authenticity of the data origin.
Use case of Asymmetric and Symmetric Encryption
Asymmetric and symmetric encryption is often used in combination with each other to maintain a balance of performance, secure transfer, identification, etc. A single one may not be able to achieve alone.
Use case 1: Messaging Applications
Many messaging applications such as WhatsApp, telegram, Signal provide end-to-end encryption to provide confidentiality of the users involved and authenticate users to communicate with each other securely.
In end-to-end encryption, the messages and calls are encrypted so anyone apart from the users would not obtain plaintext information. Only the data is encrypted, but the headers, trailers, and routing information of the messages remain unencrypted.
One of the best features is that even if the key is somehow compromised from the user’s physical device, that key cannot be used to decrypt any previously encrypted text.
To be able to achieve this, both symmetric and asymmetric encryptions are used. Asymmetric encryption is used to initiate the conversation among the users, which involves exchanging secret keys for symmetric encryption. After the communication is established and a secret key is exchanged, symmetric encryption is used for the whole duration of the communication.
First, when the application is installed on the user’s end, the key pair is created. The user’s public key is registered and stored in the application server, but the private key remains in the user’s device. The user who wants to initiate the conversation obtains the receiver’s public key from the application server. Using the public key, the sender sends an encrypted message to the receiver. The encrypted message contains parameters to establish a symmetric session among the parties involved. The receiver would use their private key to decrypt the message and develop symmetric encryption between the sender and the receiver. Once the session has been established, clients exchange messages protected with a Message Key using AES256 in CBC mode for encryption and HMAC-SHA256 for authentication. The encrypted session would be recreated only when the application is re-installed, or the device is changed.
Use case 2: HTTPS
HTTPS (Hypertext Transfer Protocol Secure) is a secure version of the HTTP protocol that uses the SSL/TLS protocol for encryption and authentication. HTTPS is specified by RFC 2818 (May 2000) and uses port 443 by default instead of HTTP’s port 80.
The HTTPS protocol makes it possible for website users to transmit sensitive data such as credit card numbers, banking information, and login credentials securely over the internet. For this reason, HTTPS is essential for securing online activities such as shopping, banking, and remote work. HTTPS is now the standard protocol for all websites, whether they exchange sensitive data with users.
An HTTPS connection between a client and a server uses both symmetric and asymmetric encryption. Like the previous one, asymmetric encryption is first used to establish communication and exchange secrets, and then symmetric encryption is used for the rest of the communication. The key used for symmetric encryption is called session keys. Session keys are randomly generated when a session is created and are used for that particular session only. The steps involved in HTTPS are:
- When the client tried to connect to the server, the server first sends a TLS certificate. The respective CA would verify the certificate to ensure the authenticity of the certificate and the server involved.
- The certificate also contains cipher suits and the maximum TLS version supported.
- After the certificate is verified and the algorithms and TLS versions are set to be the maximum among the client and the server, the client generates a session key. The session key is encrypted using the server’s public key and is sent to the server.
- The server decrypts the session key using its private key.
- The session key is then used for symmetric encryption, and data is exchanged among the server and the client using the session key.
- The session continues with only symmetric encryption.
Symmetric encryption is indeed one of the fastest encryption techniques, but the secret key needs to be exchanged securely to be effective. Asymmetric encryption is thus used to exchange the key involved for symmetric encryption. In both use cases, asymmetric encryption is used briefly to exchange parameters and establish symmetric encryption used for the rest of the communication. Being slow and resource exhaustive, asymmetric encryption is only used to cover the shortcomings of symmetric encryption. Thus, both of them are used together to achieve ideal secure communication, maintain privacy, achieve authenticity, data integrity, and proper authentication.