RSA Key Size Estimator

Why Key Size Matters

In public key cryptography, larger keys generally offer greater protection against brute-force attacks. The RSA algorithm relies on the difficulty of factoring a large number into its prime components. If that number—known as the modulus—is sufficiently large, current hardware cannot efficiently derive the private key from the public one. This calculator relates security levels expressed in bits to practical key sizes, highlighting how quickly requirements grow as threat models evolve.

Security level is often expressed in “bits of security,” which loosely corresponds to the base-2 logarithm of the number of operations required to defeat the system. A 128-bit security level implies roughly $2^{128}$ operations to brute force—far beyond what modern computers can achieve. Symmetric algorithms such as AES use bits of security directly in their key length, but RSA requires much larger keys to reach equivalent strength because integer factorization is a more complex problem.

Mapping Security to Key Size

Organizations such as NIST publish guidelines that map symmetric security levels to recommended RSA key sizes. These mappings may vary slightly across publications, but they all show a significant jump in key length as security requirements grow. Choosing an appropriate key size ensures that encrypted data remains safe for the lifetime of the information it protects.

Complement this tool with the Password Strength Checker when assessing user authentication, explore network-level resilience via the Quantum Key Distribution Secure Distance Calculator, and plan for algorithm transitions using the Quantum-Safe Cryptography Migration Timeline.

Estimating Brute-Force Time

Although RSA keys are rarely brute forced by exhaustively testing every possibility, you can approximate how long such a naive attack might take. If a key offers $S$ bits of security, an attacker would theoretically need $2^S$ operations to break it. If the attacker can perform $R$ operations per second, then the time required is $\frac{2^S}{R}$ seconds. Taking the base-10 logarithm keeps the numbers manageable and avoids overflow when estimating astronomical timelines.

For instance, aiming for 128 bits of security against an adversary capable of one trillion attempts per second produces a brute-force timeline with a base-10 logarithm near 25 years. That equates to ${10}^{25}$ years—longer than the age of the universe. This demonstrates why even 128-bit security remains robust for the foreseeable future. Nevertheless, technology advances quickly, so key size recommendations should be reviewed periodically.

Long-Term Considerations

When selecting a key size, consider the lifespan of the data you are protecting. If you are encrypting information that must remain confidential for decades, opting for a larger key may be prudent. Larger keys increase computational overhead during encryption and decryption, which can slow down performance on resource-constrained devices. The estimator allows you to experiment with various scenarios so you can balance security requirements against processing time.

Another factor is post-quantum cryptography. Quantum computers capable of running Shor’s algorithm could factor large integers exponentially faster than classical machines, rendering RSA insecure. While practical quantum computers at the scale required for such attacks do not exist today, organizations planning for the distant future should evaluate post-quantum alternatives. Use this estimator as a baseline while you monitor emerging standards and hybrid migration strategies.