RSA Key Size Calculator

About the RSA Key Size Calculator

RSA key size directly determines the security level of RSA encryption and digital signatures. However, RSA key bits do not map one-to-one with security bits — RSA-2048 provides only about 112 bits of security, not 2048. This is because RSA can be attacked more efficiently than brute force using number field sieve algorithms for factoring large numbers.

This calculator shows the equivalent security strength in bits for any RSA key size, along with the corresponding AES and ECC key sizes that provide equal protection. It helps certificate administrators, security architects, and developers choose appropriate RSA key sizes and plan migrations to stronger keys or alternative algorithms like ECC.

When This Page Helps

RSA key size recommendations evolve as computing power and cryptanalysis improve. Understanding the actual security level helps you avoid both under-provisioning (RSA-1024 is broken) and over-provisioning (RSA-8192 is extremely slow). It gives the concrete numbers needed for certificate planning, migration work, and security reviews.

How to Use the Inputs

1. Enter an RSA key size in bits (common baselines: 1024, 2048, 3072, 4096).
2. View the equivalent security strength in symmetric bits.
3. See the corresponding AES and ECC key sizes.
4. Check whether the key size lines up with the planning baseline shown on the page.
5. Review the security status: deprecated, acceptable, recommended, or high-security.

Example Calculation

Tips & Best Practices

• RSA-2048 remains a common baseline; plan migration to RSA-3072 or ECC-256 for longer-lived deployments.
• Certificate Authorities stopped issuing RSA-1024 certificates years ago.
• RSA-4096 is often used for CA root certificates due to their 20+ year lifespans.
• Consider switching to ECC (P-256 or P-384) for dramatic performance improvements.
• RSA key generation time grows roughly cubically with key size.
• For SSH keys, ED25519 provides 128-bit security with tiny 256-bit keys.

RSA Security Level Mapping

The security of RSA depends on the difficulty of factoring large numbers using the General Number Field Sieve (GNFS). NIST publishes recommended key sizes based on projected advances in both algorithmic techniques and hardware capabilities.

RSA Key Size Baselines

RSA-1024 is deprecated because it provides only about 80-bit security. RSA-2048 remains the most common deployment baseline and provides roughly 112-bit security. Teams planning for longer-lived keys often move to RSA-3072 (128-bit) or higher.

Performance Considerations

RSA operations are computationally expensive. Encryption and signature verification are relatively fast (public key operations), but decryption and signing (private key operations) scale roughly with the cube of the key size. RSA-4096 private key operations are approximately 8× slower than RSA-2048.

The Case for ECC Migration

ECC P-256 provides 128-bit security with 256-bit keys, matching RSA-3072 while being 10–20× faster for signing operations. Most modern TLS implementations prefer ECC certificates, and mainstream browsers and servers have supported ECDSA for years.

Frequently Asked Questions

• Is RSA-2048 still safe?

  RSA-2048 remains acceptable in many environments and is commonly treated as the lower end of modern deployment baselines. For long-lived keys or highly sensitive data, many teams prefer RSA-3072 or higher. If large-scale quantum computers become practical, RSA of any size would be broken.

• Why doesn't RSA-2048 mean 2048-bit security?

  RSA can be attacked using the General Number Field Sieve (GNFS) algorithm, which is much faster than brute force. GNFS has sub-exponential complexity, meaning each doubling of key size adds less than a doubling of security. This is fundamentally different from symmetric algorithms like AES.

• What RSA key size should I use for a new certificate?

  For shorter-lived certificates, RSA-2048 is often acceptable. For longer validity periods, many teams use RSA-3072 or RSA-4096. Better yet, consider ECC P-256 or P-384 certificates, which provide equal or better security with dramatically better performance.

• How long does RSA key generation take?

  RSA-2048 generation takes about 0.1–0.5 seconds. RSA-4096 takes 1–5 seconds. RSA-8192 can take 30–60 seconds or more depending on the hardware. This is a one-time cost but significant for applications generating many keys.

• Will quantum computers break RSA?

  Yes, Shor's algorithm on a sufficiently powerful quantum computer can factor RSA keys of any size in polynomial time. Organizations with long-lived trust anchors should already be planning for eventual post-quantum migration.

• Should I use RSA-8192 for maximum security?

  RSA-8192 is rarely justified. It provides approximately 200-bit security but is extremely slow for key generation and operations. AES-256 or ECC-384 provide comparable or better security with vastly better performance. Use RSA-4096 as the practical maximum if RSA is required.