feat: decrypt Jasypt values in .env and Spring files

ENC(...) values are decrypted on reveal, copy or get with
jasypt-spring-boot's default algorithm (3.0+, PBEWITHHMACSHA512ANDAES_256).

- The password is, as in Spring, $JASYPT_ENCRYPTOR_PASSWORD, else the
  file's own jasypt.encryptor.password (JASYPT_ENCRYPTOR_PASSWORD in a
  .env), with ${VAR} and ${VAR:default} read from the environment.
- Errors say which password was used, or which one is missing.
- Secrets show a detail after their name: jasypt, or reference for a
  Spring ${VAR}; also in Go to and Favourites.
- A .env whose values are all Jasypt isn't marked as plain text in git.

internal/jasypt/jasypt.go

@@ -0,0 +1,67 @@
+// Package jasypt decrypts values encrypted with Jasypt the way
+// jasypt-spring-boot 3.0 and later does by default: ENC(...) around the
+// base64 of a random salt, a random IV and the ciphertext, with
+// PBEWITHHMACSHA512ANDAES_256 (PBKDF2-HMAC-SHA512, 1000 iterations,
+// AES-256-CBC).
+package jasypt
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/pbkdf2"
+	"crypto/sha512"
+	"encoding/base64"
+	"errors"
+	"strings"
+	"unicode/utf8"
+)
+
+const iterations = 1000
+
+// ErrDecrypt is a value that doesn't decrypt with the password: a wrong
+// password, or one encrypted with another algorithm.
+var ErrDecrypt = errors.New("doesn't decrypt: a wrong password, or not Jasypt's default algorithm (PBEWITHHMACSHA512ANDAES_256)")
+
+// Is reports whether v is a Jasypt value, ENC(...).
+func Is(v string) bool {
+	return len(v) > len("ENC()") && strings.HasPrefix(v, "ENC(") && strings.HasSuffix(v, ")")
+}
+
+// Decrypt decrypts an ENC(...) value.
+func Decrypt(v, password string) (string, error) {
+	b, err := base64.StdEncoding.DecodeString(strings.TrimSuffix(strings.TrimPrefix(v, "ENC("), ")"))
+	if err != nil {
+		return "", errors.New("not Jasypt: ENC(...) isn't base64")
+	}
+	const size = aes.BlockSize // the salt's and the IV's too
+	if len(b) < 3*size || len(b)%size != 0 {
+		return "", ErrDecrypt
+	}
+	salt, iv, data := b[:size], b[size:2*size], b[2*size:]
+	key, err := pbkdf2.Key(sha512.New, password, salt, iterations, 32)
+	if err != nil {
+		return "", err
+	}
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return "", err
+	}
+	plain := make([]byte, len(data))
+	cipher.NewCBCDecrypter(block, iv).CryptBlocks(plain, data)
+	// A wrong password decrypts to noise: its padding is almost never
+	// valid, and the rest rarely UTF-8.
+	n := int(plain[len(plain)-1])
+	if n == 0 || n > size {
+		return "", ErrDecrypt
+	}
+	for _, c := range plain[len(plain)-n:] {
+		if int(c) != n {
+			return "", ErrDecrypt
+		}
+	}
+	plain = plain[:len(plain)-n]
+	if !utf8.Valid(plain) {
+		return "", ErrDecrypt
+	}
+	return string(plain), nil
+}