by Nick Naziridis / June 8, 2018
Lately, there have been numerous discussions on the pros and cons of RSA
and ECDSA, in the crypto community. For the uninitiated, they are two of
the most widely-used digital signature algorithms, but even for the more tech
savvy, it can be quite difficult to keep up with the facts. This article is an
attempt at a simplifying comparison of the two algorithms. Although, this is not
a deeply technical essay, the more impatient reader can check the end of the
article for a quick TL;DR table with the summary of the discussion.
ECDSA vs RSA
ECDSA and RSA are algorithms used by public key cryptography systems,
to provide a mechanism for authentication. Public key cryptography is the
science of designing cryptographic systems that employ pairs of keys: a public
key (hence the name) that can be distributed freely to anyone, along with a
corresponding private key, which is only known to its owner. Authentication
refers to the process of verifying that a message, signed with a public key, was
created by the holder of a specific private key. Algorithms used for
authentication are collectively known as digital signature algorithms .
Such algorithms rely on complex mathematical problems that are relatively simple
to compute one way, although quite impractical to reverse. This means that for
an attacker to forge a digital signature, without any knowledge of the private
key, they must solve intractable mathematical problems, such as integer
factorization, for which there are no known efficient solutions .
On that account, since there are no efficient solutions available for the
underlying mathematical problems, evaluation of cryptographic algorithms can
only occur with respect to their implementation details in conjunction with the
level of security they provide. For this purpose, this section presents a
comparison of RSA and ECDSA using five (or six) quantifying metrics. Each metric
is introduced in its own section, along with its significance for anyone that is
trying to decide between the two algorithms.
RSA has been the industry standard for public key cryptography for many years
now. Most SSL/TLS certificates were (and still are) being signed with RSA keys.
Although most CAs have, by now, implemented support for ECDSA-based
certificates, this long-lived adoption has led to many legacy systems only
supporting RSA. Therefore, if a vendor requires backwards-compatibility with old
client software, they are forced to use certificates signed with RSA. Nowadays,
though, most modern clients have implemented support for ECDSA, which will
probably remove this compatibility constraint in the near future.
RSA was first standardised for SSL/TLS in 1994 , while ECDSA was introduced
in the specification of TLS v1.2 in 2008 . This age difference indicates a
disparity in the maturity of the standards that describe the best practices for
each algorithm. Although, RSA standards have been extensively researched and
audited, ECDSA has not seen that much attention. Recently, advocacy of this
algorithm by major CAs and its adoption in most modern SSL/TLS clients has
resulted in more extensive research being published, but it still remains a
relatively new scheme. This leaves room for undiscovered design flaws or
erroneous implementations being disclosed in the future.
Key-size to security-level ratio
Security level  is a metric in cryptography, referring to the strength
of a cryptographic primitive or function. It is commonly measured in “bits” that
denote the number of operations an attacker needs to perform to compromise its
security. This metric can provide a quantifying method to compare the efficacy
of various cryptosystems. It should be emphasized that public key size is also
measured in bits, but it is an entirely different concept, referring to the
physical size of the key.
In this regard, a common RSA 2048-bit public key provides a security level of
112 bits. However, ECDSA requires only 224-bit sized public keys to provide the
same 112-bit security level. This striking difference in key size has two
significant implications. Smaller key sizes require less bandwidth to set up an
SSL/TLS stream, which means that ECDSA certificates are ideal for mobile
applications. Moreover, such certificates can be stored into devices with much
more limiting memory constraints, a fact that allows m/TLS stacks to be
implemented in IoT devices without allocating many resources. Published
research, even shows that ECDSA is more efficient  to implement in embedded
Performance & time complexity
Algorithms are abstract recipes describing a method to achieve a certain goal.
In computer science, their performance is measured by counting the number of
elementary operations that are necessary to reach this predetermined ending
condition. Such metric is called time complexity. Since different input
sizes require different number of operations, time complexity is usually
expressed as a function of input size.
Both the algorithms in question, perform about the same time-consuming
mathematical operations, such as divisions and multiplications. Thus, input size
(which in this case is the size of their keys) remains the most significant
factor affecting their performance. Comparing the two algorithms, needs to be
distinguished between signing a message and verifying a signature. In most
practical implementations, RSA seems to be significantly faster than ECDSA in
verifying signatures, though it is slower while signing.
Things get complicated for higher security levels. For example, in the most
common configuration of a security level of 112 bits, RSA requires 2048-bit
versus ECDSA needing 224-bit keys. In the next common level of 128 bits, RSA
requires a 3072-bit key, while ECDSA only 256 bits. This results in RSA’s
performance to decline dramatically, whereas ECDSA is only slightly affected. As
a consequence of this scaling issue, although RSA seems more performant at the
moment, the continuous increase in security requirements could very well render
ECDSA the de-facto solution in the future.
Shor’s algorithm  is a well known algorithm for breaking RSA keys using
quantum computers. Since there are no (public) practical implementations of a
such a machine, the following is a conjecture on the future of public key
cryptography. At the moment of this writing, the best implementation of Shor’s
algorithm can defeat a 15-bit key RSA encryption. Although this does not sound
concerning, as more and more research is directed towards quantum computing, RSA
could be getting into serious trouble at any time.
Advocates for ECDSA should not be quick to celebrate though, because elliptic
curve cryptography is also vulnerable  to a modified version of Shor’s
algorithm. Consequently, if both ciphers can be broken by a quantum computer,
the only objective metric is the complexity required to implement such an
attack. According to public research, RSA 2048-bit keys require 4098 qubits
(and 5.2 trillion Tofolli gates) to be defeated, whereas ECDSA 256-bit keys
require only 2330 qubits (and 126 billion Tofolli gates). Hence, RSA is more
expensive to break, using a theoretical quantum machine.
Although, this comparison is, by no means comprehensive, it is apparent that
RSA has rightfully gained its position as the leading digital signature
algorithm for most certificate applications. However, since technology is always
advancing in more unpredictable ways, security awareness and needs are also
increasing. A little more than ten years ago, embedded device security was
fiction and nowadays secure communications is a must-have for any real-world
application. As a result, even if ECDSA is relatively young, it is anyone’s
guess if it will replace RSA as the standard for authentication in SSL/TLS
If you, the reader, still cannot decide which algorithm to choose, there are
solutions for supporting both ECDSA and RSA (as a fallback mechanism), until the
crypto community settles on a winner. Check this article section for a future