117 lines
3.7 KiB
Go
117 lines
3.7 KiB
Go
// Package secret encrypts sensitive settings values (passwords) at rest with
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// AES-256-GCM, keyed by a PBKDF2 key derived from a user-chosen passphrase.
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//
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// The passphrase — not the machine — is the key source, so an encrypted
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// database stays portable: copy the data folder to another PC, re-enter the
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// passphrase there, and the secrets decrypt. Encryption is opt-in: until the
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// user sets a passphrase, values are stored in clear (no behaviour change).
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//
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// Encrypted values are prefixed "enc:v1:" so reads can tell ciphertext from a
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// legacy plaintext value and migrate transparently.
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package secret
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import (
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"crypto/aes"
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"crypto/cipher"
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"crypto/pbkdf2"
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"crypto/rand"
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"crypto/sha256"
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"crypto/subtle"
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"encoding/base64"
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"errors"
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"strings"
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)
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// Prefix marks an encrypted value. Bump the version if the scheme changes.
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const Prefix = "enc:v1:"
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// verifierPlaintext is encrypted under the key and stored so a wrong passphrase
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// is detected on unlock (the decryption fails / mismatches) instead of silently
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// producing garbage secrets.
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const verifierPlaintext = "opslog-secret-verifier-v1"
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// pbkdf2Iter is the work factor. ~600k SHA-256 rounds ≈ sub-second derivation,
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// in line with current OWASP guidance for PBKDF2-HMAC-SHA256.
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const pbkdf2Iter = 600_000
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const keyLen = 32 // AES-256
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const saltLen = 16
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// NewSalt returns a fresh random salt to store alongside the verifier.
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func NewSalt() ([]byte, error) {
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s := make([]byte, saltLen)
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if _, err := rand.Read(s); err != nil {
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return nil, err
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}
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return s, nil
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}
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// DeriveKey turns a passphrase + salt into the AES key.
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func DeriveKey(passphrase string, salt []byte) ([]byte, error) {
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return pbkdf2.Key(sha256.New, passphrase, salt, pbkdf2Iter, keyLen)
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}
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// Cipher encrypts/decrypts individual values with the derived key.
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type Cipher struct{ gcm cipher.AEAD }
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// New builds a Cipher from a 32-byte key.
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func New(key []byte) (*Cipher, error) {
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block, err := aes.NewCipher(key)
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if err != nil {
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return nil, err
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}
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gcm, err := cipher.NewGCM(block)
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if err != nil {
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return nil, err
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}
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return &Cipher{gcm: gcm}, nil
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}
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// Encrypt returns "enc:v1:<base64(nonce||ciphertext)>".
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func (c *Cipher) Encrypt(plain string) string {
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nonce := make([]byte, c.gcm.NonceSize())
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if _, err := rand.Read(nonce); err != nil {
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return plain // extremely unlikely; never silently corrupt by returning bad data
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}
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ct := c.gcm.Seal(nonce, nonce, []byte(plain), nil)
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return Prefix + base64.StdEncoding.EncodeToString(ct)
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}
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// Decrypt reverses Encrypt. A value without the prefix is returned unchanged
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// (legacy plaintext). A malformed/forged value returns an error.
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func (c *Cipher) Decrypt(stored string) (string, error) {
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if !strings.HasPrefix(stored, Prefix) {
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return stored, nil
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}
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raw, err := base64.StdEncoding.DecodeString(stored[len(Prefix):])
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if err != nil {
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return "", err
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}
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ns := c.gcm.NonceSize()
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if len(raw) < ns {
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return "", errors.New("ciphertext too short")
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}
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pt, err := c.gcm.Open(nil, raw[:ns], raw[ns:], nil)
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if err != nil {
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return "", err
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}
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return string(pt), nil
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}
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// IsEncrypted reports whether a stored value is ciphertext.
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func IsEncrypted(v string) bool { return strings.HasPrefix(v, Prefix) }
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// MakeVerifier encrypts the known token so the passphrase can be validated later.
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func (c *Cipher) MakeVerifier() string { return c.Encrypt(verifierPlaintext) }
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// CheckVerifier returns true when c (derived from the entered passphrase)
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// decrypts the stored verifier back to the known token — i.e. the passphrase is
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// correct.
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func (c *Cipher) CheckVerifier(stored string) bool {
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p, err := c.Decrypt(stored)
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if err != nil {
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return false
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}
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return subtle.ConstantTimeCompare([]byte(p), []byte(verifierPlaintext)) == 1
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}
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