EdDSA (Ed25519) Encrypt & Decrypt

A: Both EdDSA and ECDSA are elliptic-curve signature algorithms, but they differ in important ways. ECDSA (used with NIST curves P-256, P-384) requires a random nonce (k) per signature — if the nonce is reused or weak, the private key can be completely recovered. This is exactly how the Sony PlayStation 3 private key was extracted in 2010. EdDSA uses a deterministic nonce derived from the private key and message hash, making nonce reuse mathematically impossible. EdDSA also uses safer Twisted Edwards curves (Curve25519 for Ed25519) designed to resist certain side-channel attacks. For new implementations, EdDSA is strongly preferred over ECDSA.

A: In ECDSA, the security of each signature depends on a fresh, cryptographically random nonce. If an attacker obtains two signatures that used the same nonce (nonce reuse), or if the nonce has insufficient entropy (weak randomness), the private key can be algebraically recovered using simple equations. This is not theoretical — it has broken real-world systems (PS3, Bitcoin wallets). Ed25519's deterministic signing derives the nonce from a hash of the private key seed and the message, so it is always unique and cannot be influenced by external randomness sources. There is no random number generator in the signing path — eliminating an entire class of vulnerabilities.

A: No. EdDSA (Ed25519) is a digital signature algorithm only — it cannot encrypt or decrypt data. Signing proves authenticity (who created the message) but provides no confidentiality (anyone can read the message). To encrypt data, use a symmetric cipher like ChaCha20-Poly1305 or AES-256-GCM. Note: X25519 (a related but distinct algorithm based on Curve25519) can be used for key exchange (Diffie-Hellman), but it is not the same as Ed25519 and the keys are not interchangeable.

A: Use OpenSSH's built-in key generation tool:
ssh-keygen -t ed25519 -C "your_email@example.com"
This generates ~/.ssh/id_ed25519 (private key) and ~/.ssh/id_ed25519.pub (public key). Ed25519 has been the recommended SSH key type since OpenSSH 6.5 (2014) and is now the default in most distributions. The keys generated by OpenSSH use a different encoding (PEM/OpenSSH format) than this tool's hex format, but they are based on the same cryptographic algorithm.

A: An Ed25519 signature consists of two 32-byte values: R (a compressed elliptic curve point, derived from the deterministic nonce) and S (a scalar value computed from the nonce, private key, and message hash). Together, R and S form the 64-byte signature. Verification reconstructs R using the public key, message, and S, and checks that it matches the R in the signature. The entire verification involves only fast field arithmetic on Curve25519 — no modular exponentiation like RSA.

A: No — not against a sufficiently large quantum computer. A quantum computer running Shor's algorithm could break Ed25519 by solving the discrete logarithm problem on elliptic curves. The same threat applies to RSA and ECDSA. However, a quantum computer capable of breaking 128-bit elliptic curve security would need thousands of logical qubits with very low error rates — well beyond current technology. For post-quantum signatures, NIST has standardized ML-DSA (formerly CRYSTALS-Dilithium) and SLH-DSA (SPHINCS+). Ed25519 remains the best practical choice for classical threat models and is widely deployed in production systems today.