RSA Key Pair Generator

Security Notes

• Keep your private key secret. Anyone with the private key can decrypt messages or forge signatures.
• Share only your public key — it is designed to be public.
• This tool generates keys for testing and development. For production use, manage keys with a dedicated secret manager (AWS KMS, HashiCorp Vault, etc.).

What the Output Looks Like

PEM gives the public key as SubjectPublicKeyInfo (-----BEGIN PUBLIC KEY-----) and the private key as unencrypted PKCS#8 (-----BEGIN PRIVATE KEY-----). OpenSSL 3.6 reads both directly (Private-Key: (2048 bit, 2 primes)). Older software that expects PKCS#1 (-----BEGIN RSA PRIVATE KEY-----) needs a conversion; see the commands below.

JWK (RFC 7517) is what the browser’s Web Crypto API exports. A public key generated with RSA-OAEP looks like this (modulus shortened):

{
  "alg": "RSA-OAEP-256",
  "e": "AQAB",
  "ext": true,
  "key_ops": ["encrypt"],
  "kty": "RSA",
  "n": "6mqtvNrIisYdKt30jcmQN_za5t3OYQJ-8uhu_DkGEF5iZE4SrseP1CkxnD1yDEdgdD3E…"
}

With RSASSA-PKCS1-v1_5 the public JWK has "alg": "RS256" and "key_ops": ["verify"]. The output has no kid or use; add a kid before publishing the key in a JWKS endpoint, because JWT headers refer to keys by kid.

Use the Keys With OpenSSH and OpenSSL

These commands were run on the downloaded files with OpenSSH on macOS and OpenSSL 3.6.1. ssh-keygen refuses a private key file that others can read (“Permissions 0644 … are too open”), so restrict it first:

chmod 600 private-key.pem

# OpenSSH public key for ~/.ssh/authorized_keys
ssh-keygen -y -f private-key.pem
ssh-keygen -i -m PKCS8 -f public-key.pem      # same result from the public key

# PKCS#1 ("BEGIN RSA PRIVATE KEY") for older software
openssl rsa -in private-key.pem -traditional -out private-key-pkcs1.pem

# Inspect the key
openssl pkey -in private-key.pem -noout -text

Choosing Size and Algorithm

The algorithm setting decides what the browser lets the key do and what the JWK says: RSA-OAEP (with SHA-256) for encryption, RSASSA-PKCS1-v1_5 (with SHA-256, JWT RS256) for signatures. The PEM files carry no algorithm, so OpenSSL can use the same PEM key for either purpose.

Limits

• Key sizes 2048 and 4096 only; the public exponent is always 65537 (AQAB).
• The private key PEM is not encrypted with a passphrase. Store the file somewhere protected, or encrypt it with openssl pkcs8 -topk8 -in private-key.pem -out encrypted.pem.
• No OpenSSH private key format and no certificate or CSR output; use ssh-keygen and openssl req for those.
• Generation runs on the main thread; 4096-bit keys take noticeably longer than 2048-bit keys.

Check a Generated Export Pair

For a reproducible check, select 2048-bit, RSASSA-PKCS1-v1_5, and PEM. Generate a pair. The public output begins with -----BEGIN PUBLIC KEY-----; the private output begins with -----BEGIN PRIVATE KEY-----. Import the public output as SPKI and the private output as PKCS8, using SHA-256. The imported private key reports a 2048-bit modulus. Export those same imported keys as JWK: their n values match and both e values are AQAB. The private JWK contains d; the public JWK does not. Sign the UTF-8 message ZeroTool RSA export example with the private key. Verification with the public key succeeds for that message and fails after replacing it with ZeroTool RSA export changed.