GCD for RSA Key Check

Summary

Enter two large integers (e.g. RSA moduli or primes) and instantly see their GCD — if it is greater than 1, they share a factor and the key is weak.

How it works

1. Enter both integers A and B in the input fields.
2. Click "Compute GCD" to run the Euclidean algorithm using JavaScript BigInt.
3. The result panel shows GCD(A, B), the number of algorithm steps, and a security verdict.
4. If GCD = 1, the numbers are coprime — no shared prime factor.
5. If GCD > 1, the shared factor is displayed along with a warning.

Use cases

• Audit a batch of RSA public keys for shared prime factors (ROCA / batch GCD attack).
• Verify that two RSA moduli generated on the same weak RNG are not related.
• Educational demo of the Euclidean algorithm on cryptographic-sized numbers.
• Quick sanity-check before deploying an RSA key pair to production.
• Detect entropy failures in embedded devices that reuse prime factors.
• Check whether p and q derived from the same seed share a GCD > 1.

Frequently Asked Questions