crypto.go

package netpipe

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"
	"fmt"
	"io"
)

// encrypt encrypts plaintext using AES-256-GCM.
// The key can be any length - it is hashed to exactly 32 bytes with SHA-256.
// Returns: [12 bytes nonce][ciphertext + GCM auth tag]
func encrypt(plaintext []byte, key string) ([]byte, error) {
	// derive a fixed 32-byte key from whatever the developer passes
	keyHash := sha256.Sum256([]byte(key))

	block, err := aes.NewCipher(keyHash[:])
	if err != nil {
		return nil, fmt.Errorf("netpipe: aes cipher: %w", err)
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("netpipe: gcm: %w", err)
	}

	// random nonce - never reuse with the same key
	nonce := make([]byte, gcm.NonceSize()) // 12 bytes for GCM
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, fmt.Errorf("netpipe: random nonce: %w", err)
	}

	// Seal appends ciphertext + auth tag after the nonce
	ciphertext := gcm.Seal(nonce, nonce, plaintext, nil)
	return ciphertext, nil
}

// decrypt decrypts data produced by encrypt().
// Expects: [12 bytes nonce][ciphertext + GCM auth tag]
func decrypt(data []byte, key string) ([]byte, error) {
	keyHash := sha256.Sum256([]byte(key))

	block, err := aes.NewCipher(keyHash[:])
	if err != nil {
		return nil, fmt.Errorf("netpipe: aes cipher: %w", err)
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("netpipe: gcm: %w", err)
	}

	nonceSize := gcm.NonceSize()
	if len(data) < nonceSize {
		return nil, fmt.Errorf("netpipe: ciphertext too short")
	}

	nonce, ciphertext := data[:nonceSize], data[nonceSize:]
	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return nil, fmt.Errorf("netpipe: decrypt failed (wrong key?): %w", err)
	}

	return plaintext, nil
}