ECDSA and RSA are two of the world’s most widely adopted asymmetric algorithms. However, both these algorithms are significantly different when it comes to the way they function and how their keys are generated.
In this blog post, we will briefly examine the differences between RSA and ECDSA certificates to help you understand what they are, how they work, and to explore their advantages and drawbacks.
Cryptographic certificates overview
TLS certificates are used to secure network communications and establish the identity of websites over the internet, as well as the identity of resources on private networks. Public certificates that you request through Certificate Authorities (CAs), cloud computing providers such as Amazon Web Services (AWS) and Google Cloud Platform (GCP) or hosting providers.
Private certificates are issued through certificate authorities, which are typically to organizations for use in internal or private networks.
Both public and private certificates can help customers identify resources on networks and secure communication between these resources. Public certificates identify resources on the public internet, whereas private certificates do the same for private networks. One key difference is that applications and browsers trust public certificates by default, but an administrator must explicitly configure applications and devices to trust private certificates.
RSA and ECDSA primer
RSA and ECDSA are two widely used public-key cryptographic algorithms—algorithms that use two different keys to encrypt and decrypt data. In the case of TLS, a public key is used to encrypt data, and a private key is used to decrypt data. Public key (or asymmetric key) algorithms are not as computationally efficient as symmetric key algorithms like AES. For this reason, public key algorithms like RSA and ECDSA are primarily used to exchange secrets between two parties initiating a TLS connection. These secrets are then used by both parties to decipher the same symmetric key that actually encrypts the data in transit.
RSA stands for Rivest, Shamir, and Adleman: the researchers who first publicly described this algorithm in 1977. The basic functionality of RSA relies on the idea that large prime numbers are very difficult to efficiently factor. ECDSA, or Elliptic Curve Digital Signature Algorithm, is based on certain unique mathematical properties of elliptic curves that make them very useful for cryptographic operations. The cryptographic utility of ECDSA comes from a concept called the discrete logarithm problem.
Considerations when choosing between RSA and ECDSA
What are the important differences between RSA and ECDSA certificates? When should you choose ECDSA certificates to encrypt network traffic? In this section, we’ll examine the security and performance considerations that help to determine whether ECDSA or RSA certificates are the best choice for your workload.
Security
In cryptography, security is measured as the computational work it takes to exhaust all possible values of a symmetric key in an ideal cipher. An ideal cipher is a theoretical algorithm that has no weaknesses, so you must try every possible key to discover which is the correct key. This is similar to the idea of “brute forcing” a password: trying every possible character combination to find the correct password.
Let’s imagine you have a 112-bit key ideal cipher, which means it would take 2112 tries to exhaust the key space—we would say this cipher has a 112-bit security strength. However, it is important to realize that security strength and key length are not always equal—meaning that an encryption key with a length of 112 bits will not always have a 112-bit security strength.
ECDSA provides higher security strength for lower computational cost. ECDSA P-256, for example, provides 128-bit security strength and is equivalent to an RSA 3072 key. Meanwhile, ECDSA P-384 provides 192-bit security strength, equivalent to the key associated with an RSA 7680 certificate. In other words, an ECDSA P-384 key would require 2192 tries to exhaust the key space.
The following table provides an in-depth comparison of the different security strengths for RSA key lengths and ECDSA curve types.
Performance
ECDSA provides a higher security strength (for a given key length) than RSA but does not add performance overhead. For example, ECDSA P-256 is as performant as RSA 2048 while providing security strength that is comparable to RSA 3072.
ECDSA certificates also have up to a 50% smaller certificate size when compared to RSA certificates, and are therefore more suitable to protect data-in-transit over low bandwidth or for applications with limited memory and storage, such as Internet of Things (IoT) devices.
Take a look at the following certificate examples; you can see the size difference between RSA and ECDSA certificates.
Consider a small IoT sensor device that tracks temperature in an office building. This device typically has very low storage capacity and compute power, so the smaller ECDSA certificate will be easier to process and store. In the case of an IoT device, you might not be able to store the entire RSA certificate chain on the device due to memory limitations and the larger size of RSA certificates. This can make it more difficult to validate the chain of trust for that certificate.
Using ECDSA, customers can take advantage of the smaller size of the certificates (and the certificate trust chain) and store the entire chain of trust on the IoT device itself, enabling the IoT device to more easily validate the certificate.
In general, you should consider using ECDSA certificates wherever possible, because they provide stronger security (for a given key length) compared to RSA, without impacting performance. ECDSA certificates are strongly recommended for applications that need to securely send data over low-bandwidth connections, or when you are using IoT devices that might not have much memory or computational power to store and process the larger certificate sizes that RSA offers.
