xalgord Implementing Digital Signatures with Ed25519
Overview
Ed25519 is a high-performance digital signature algorithm using the Edwards curve Curve25519. It provides 128-bit security with 64-byte signatures and 32-byte keys, offering significant advantages over RSA and ECDSA including deterministic signatures (no random nonce needed), resistance to side-channel attacks, and fast verification. This skill covers implementing Ed25519 for document signing, code signing, and API authentication.
When to Use
- When deploying or configuring implementing digital signatures with ed25519 capabilities in your environment
- When establishing security controls aligned to compliance requirements
- When building or improving security architecture for this domain
- When conducting security assessments that require this implementation
Common Misconfigurations & Verification
- Verifying a hash instead of the message: Ed25519 hashes internally (SHA-512). Pass the full message to
verify(), not a pre-computed SHA-256 digest — pre-hashing yields a different (Ed25519ph) scheme and breaks interop or weakens binding. - Not validating the public key / accepting low-order points: reject malformed or low-order public keys; a small-subgroup point can make distinct signatures verify. Use a library that performs canonical point validation.
- Trusting an attacker-supplied public key: a signature verifies against whatever key you hand it, so verifying with the wrong/untrusted key proves nothing. Pin or authenticate the signer's public key out-of-band.
- Non-canonical / malleable signatures: enforce that
S < L(canonical encoding) so a signature cannot be tweaked into another valid one. - Key confusion (signing vs verifying key) or reusing one keypair across contexts: separate keys per purpose; store private keys encrypted at rest.
- The mandatory tests: (1) a valid signature verifies; (2) a signature over a tampered message is REJECTED; (3) a valid signature checked against the wrong public key is REJECTED; (4) a flipped byte in the 64-byte signature is REJECTED. Determinism: signing the same message twice yields identical bytes.
Prerequisites
- Familiarity with cryptography concepts and tools
- Access to a test or lab environment for safe execution
- Python 3.8+ with required dependencies installed
- Appropriate authorization for any testing activities
Objectives
- Generate Ed25519 key pairs for signing
- Sign messages and files with Ed25519
- Verify signatures against public keys
- Implement multi-signature verification
- Build a simple code signing system
- Compare Ed25519 performance with RSA and ECDSA
Key Concepts
Ed25519 vs RSA vs ECDSA
| Property | Ed25519 | RSA-3072 | ECDSA P-256 |
|---|---|---|---|
| Security | 128-bit | 128-bit | 128-bit |
| Public key size | 32 bytes | 384 bytes | 64 bytes |
| Signature size | 64 bytes | 384 bytes | 64 bytes |
| Key generation | ~50 us | ~100 ms | ~1 ms |
| Sign | ~70 us | ~5 ms | ~200 us |
| Verify | ~200 us | ~200 us | ~500 us |
| Deterministic | Yes | No (PSS) | No (unless RFC 6979) |
Key Properties
- Deterministic: Same message + key always produces same signature
- Collision-resistant: No separate hash function needed
- Side-channel resistant: Constant-time implementation
- Small keys: 32 bytes each (public and private)
Security Considerations
- Ed25519 does not support key recovery from signatures
- Verify the full message, not a hash (Ed25519 hashes internally)
- Public keys must be validated before use (check for low-order points)
- Private keys should be stored encrypted at rest
- Ed25519 is not yet approved for all NIST use cases (Ed448 is preferred for federal)
Validation Criteria
- Key pair generation produces valid Ed25519 keys
- Signature verification succeeds for valid message
- Signature verification fails for tampered message
- Signature verification fails for wrong public key
- Deterministic: same input produces same signature
- File signing and verification works correctly
- Performance meets or exceeds RSA-3072