Generate RSA public/private key pairs using the Web Crypto API with configurable key sizes.
This tool generates an RSA public/private key pair from a key size you choose, using the browser's Web Crypto API rather than a server, so nothing is uploaded and nothing can be intercepted in transit. The primes and exponents come from the browser's own secure random machinery. What surprises most people: generation is deliberately random, so every click produces a different pair, and the private key exists only in the page. Close the tab before saving it and it is gone permanently. The tool creates keys only - it does not encrypt, sign, or issue certificates.
A concrete input and expected output from the current implementation.
Input
Key size: 2048 bits
Expected output
-----BEGIN PUBLIC KEY----- (392 base64 characters encoding the 294-byte SPKI structure; different every run) -----END PUBLIC KEY----- -----BEGIN PRIVATE KEY----- (PKCS#8 encoding of the private key; different every run) -----END PRIVATE KEY-----
RSA key generation is random by design, so the base64 bodies cannot be predicted and differ on every click; only the envelope is deterministic. Web Crypto exports private keys as PKCS#8 and public keys as SPKI, which is why the headers read PRIVATE KEY and PUBLIC KEY rather than the older PKCS#1 RSA PRIVATE KEY.
The page calls the Web Crypto generateKey function with the modulus length you selected, and the browser hunts for two large primes whose product becomes the shared modulus. The public exponent is always 65537, the only value major browsers accept. Prime search is probabilistic and gets slower as keys grow: a 2048-bit pair appears almost instantly, while 4096-bit generation can take a few seconds of CPU time.
The generated pair exists in the page as CryptoKey objects. Exporting turns them into PEM text: PKCS#8 for the private key, which begins with BEGIN PRIVATE KEY, and SPKI for the public key, which begins with BEGIN PUBLIC KEY. Because Web Crypto exports PKCS#8, the private key never carries the older PKCS#1 header BEGIN RSA PRIVATE KEY that some legacy tools expect; OpenSSL's rsa command converts between the two formats.
No. Generation runs entirely in the browser through the Web Crypto API, and the page makes no network calls, so the private key never leaves your machine unless you copy or download it yourself. The flip side: nobody can recover it for you. If you lose the file or close the tab first, the key is unrecoverable, so back it up somewhere safe.
Prime search is probabilistic, and the search space grows with the modulus size, so 4096-bit generation can consume seconds of CPU while 2048-bit is near-instant. The extra size buys a modest margin against future attacks, but 2048-bit remains the accepted baseline; the far more common failure is a lost or shared private key.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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