package security

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"strings"
)

type CredentialCipher struct {
	current credentialKey
	keys    []credentialKey
}

type credentialKey struct {
	id   []byte
	aead cipher.AEAD
}

var credentialEnvelopeMagic = []byte("AGK1")

func NewCredentialCipher(encodedKey string) (*CredentialCipher, error) {
	return NewCredentialKeyring(encodedKey, nil)
}

func NewCredentialKeyring(encodedKey string, previous []string) (*CredentialCipher, error) {
	values := append([]string{encodedKey}, previous...)
	keys := make([]credentialKey, 0, len(values))
	seen := map[string]struct{}{}
	for _, value := range values {
		value = strings.TrimSpace(value)
		if value == "" {
			continue
		}
		key, err := base64.StdEncoding.DecodeString(value)
		if err != nil {
			return nil, fmt.Errorf("decode credential key: %w", err)
		}
		if len(key) != 32 {
			return nil, errors.New("credential key must be a base64-encoded 32-byte key")
		}
		block, err := aes.NewCipher(key)
		if err != nil {
			return nil, fmt.Errorf("create credential cipher: %w", err)
		}
		aead, err := cipher.NewGCM(block)
		if err != nil {
			return nil, fmt.Errorf("create credential AEAD: %w", err)
		}
		fingerprint := sha256.Sum256(key)
		id := base64.RawURLEncoding.EncodeToString(fingerprint[:6])
		if _, ok := seen[id]; ok {
			continue
		}
		seen[id] = struct{}{}
		keys = append(keys, credentialKey{id: []byte(id), aead: aead})
	}
	if len(keys) == 0 {
		return nil, errors.New("at least one credential key is required")
	}
	return &CredentialCipher{current: keys[0], keys: keys}, nil
}

func (c *CredentialCipher) Encrypt(plaintext string) ([]byte, error) {
	if plaintext == "" {
		return nil, errors.New("credential cannot be empty")
	}
	nonce := make([]byte, c.current.aead.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, fmt.Errorf("generate credential nonce: %w", err)
	}
	prefix := append(append([]byte{}, credentialEnvelopeMagic...), c.current.id...)
	sealed := c.current.aead.Seal(nil, nonce, []byte(plaintext), prefix)
	return append(append(prefix, nonce...), sealed...), nil
}

func (c *CredentialCipher) Decrypt(ciphertext []byte) (string, error) {
	if len(ciphertext) >= len(credentialEnvelopeMagic) && string(ciphertext[:len(credentialEnvelopeMagic)]) == string(credentialEnvelopeMagic) {
		idStart := len(credentialEnvelopeMagic)
		idEnd := idStart + 8
		if len(ciphertext) < idEnd {
			return "", errors.New("credential ciphertext is truncated")
		}
		prefix := ciphertext[:idEnd]
		for _, key := range c.keys {
			if string(key.id) != string(ciphertext[idStart:idEnd]) {
				continue
			}
			nonceEnd := idEnd + key.aead.NonceSize()
			if len(ciphertext) < nonceEnd {
				return "", errors.New("credential ciphertext is truncated")
			}
			plaintext, err := key.aead.Open(nil, ciphertext[idEnd:nonceEnd], ciphertext[nonceEnd:], prefix)
			if err != nil {
				return "", errors.New("decrypt credential: authentication failed")
			}
			return string(plaintext), nil
		}
		return "", errors.New("credential key is not present in the configured keyring")
	}
	for _, key := range c.keys {
		if len(ciphertext) < key.aead.NonceSize() {
			continue
		}
		nonce := ciphertext[:key.aead.NonceSize()]
		plaintext, err := key.aead.Open(nil, nonce, ciphertext[key.aead.NonceSize():], nil)
		if err == nil {
			return string(plaintext), nil
		}
	}
	return "", errors.New("decrypt credential: authentication failed")
}