The Well of Primes

Know this well and you feel in your body what "a longer key is safer" means. Multiplying two large primes takes an instant; dividing back from the product takes a thousand years—a well where falling is easy and climbing is hard.

The fediverse's old etiquette (draft-cavage's HTTP signatures) still draws water from this well. So fedify carries both RSA and Ed25519 keys—to speak with the new town and the old alike.

Highlights

• In the Mastodon-compatible world, an actor's public key is posted on the profile as publicKeyPem—the well's location is public, and the secret is only the two primes.
• A 2,048-bit key length is the current standard. Compared to the curve's 32 bytes, it takes much larger stones for the same strength.
• Once quantum computers become practical, this well (along with the Elliptic Curve Atelier) is slated to be filled in. Design of the successor well—post-quantum cryptography—has already begun.
• fedify's generateCryptoKeyPair defaults to RSASSA-PKCS1-v1_5, modulusLength 4096—laid with stones thicker than the standard 2,048.

A passage from the sutra

  if (algorithm == null || algorithm === "RSASSA-PKCS1-v1_5") {
    return crypto.subtle.generateKey(
      {
        name: "RSASSA-PKCS1-v1_5",
        modulusLength: 4096,
        publicExponent: new Uint8Array([0x01, 0x00, 0x01]),
        hash: "SHA-256",
      },
      true,
      ["sign", "verify"],
    );
  } else if (algorithm === "Ed25519") {
    return crypto.subtle.generateKey(
      "Ed25519",
      true,
      ["sign", "verify"],

Try it yourself