97 lines
3.3 KiB
Go
97 lines
3.3 KiB
Go
package weixin_ilink
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import (
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"context"
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"crypto/aes"
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"encoding/base64"
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"errors"
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"fmt"
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"io"
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"net/http"
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"strings"
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)
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// ErrKeyTooShort is returned when the inbound derivation cannot yield an AES
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// key of the required block size.
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var ErrKeyTooShort = errors.New("weixin_ilink: derived key shorter than a block")
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// deriveInboundKey derives the AES-128 key for an inbound media payload. Per
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// design/08 §3.3 the key is extracted from the bot_token: base64-decode the
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// token and take the trailing 16 bytes. This is marked ⚠️ in the reference, so
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// DownloadMediaWithKey lets callers supply an explicit key when the platform
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// carries one per item instead.
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func deriveInboundKey(botToken string) ([]byte, error) {
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token := strings.TrimSpace(botToken)
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if token == "" {
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return nil, ErrInvalidConfig
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}
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b, err := base64.StdEncoding.DecodeString(token)
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if err != nil {
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return nil, fmt.Errorf("weixin_ilink: decode bot_token for key: %w", err)
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}
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if len(b) < aes.BlockSize {
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return nil, ErrKeyTooShort
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}
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return b[len(b)-aes.BlockSize:], nil
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}
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// DownloadMedia downloads an inbound CDN reference and returns the decrypted
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// plaintext using the key derived from the configured bot_token. It fails with
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// ErrMediaTooLarge when the ciphertext exceeds MaxMediaSize.
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func (a *Adapter) DownloadMedia(ctx context.Context, cdnURL string) ([]byte, error) {
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key, err := deriveInboundKey(a.cfg.BotToken)
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if err != nil {
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return nil, fmt.Errorf("weixin_ilink: derive inbound key: %w", err)
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}
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return a.downloadWithKey(ctx, cdnURL, key)
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}
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// DownloadMediaWithKey is DownloadMedia with an explicit per-item AES key,
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// used when the protocol delivers the key alongside the media reference.
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func (a *Adapter) DownloadMediaWithKey(ctx context.Context, cdnURL string, key []byte) ([]byte, error) {
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if len(key) != aes.BlockSize {
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return nil, fmt.Errorf("weixin_ilink: key must be %d bytes: %w", aes.BlockSize, ErrKeyTooShort)
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}
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return a.downloadWithKey(ctx, cdnURL, key)
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}
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// downloadWithKey fetches the CDN bytes and removes the AES-ECB layer.
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func (a *Adapter) downloadWithKey(ctx context.Context, cdnURL string, key []byte) ([]byte, error) {
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ciphertext, err := a.fetchBody(ctx, a.cdnURL(cdnURL))
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if err != nil {
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return nil, fmt.Errorf("weixin_ilink: fetch media: %w", err)
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}
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plaintext, err := aesECBDecrypt(key, ciphertext)
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if err != nil {
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return nil, fmt.Errorf("weixin_ilink: decrypt media: %w", err)
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}
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return plaintext, nil
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}
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// fetchBody GETs rawURL and returns the body, refusing anything over
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// MaxMediaSize so a single huge file cannot exhaust memory.
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func (a *Adapter) fetchBody(ctx context.Context, rawURL string) ([]byte, error) {
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req, err := http.NewRequestWithContext(ctx, http.MethodGet, rawURL, nil)
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if err != nil {
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return nil, fmt.Errorf("weixin_ilink: build media request: %w", err)
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}
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a.authorizationHeader(req)
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resp, err := a.httpClient.Do(req)
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if err != nil {
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return nil, fmt.Errorf("weixin_ilink: download media: %w", err)
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}
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defer resp.Body.Close()
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if resp.StatusCode < 200 || resp.StatusCode >= 300 {
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return nil, fmt.Errorf("weixin_ilink: download media: status %d", resp.StatusCode)
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}
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limit := a.cfg.MaxMediaSize + 1
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data, err := io.ReadAll(io.LimitReader(resp.Body, limit))
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if err != nil {
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return nil, fmt.Errorf("weixin_ilink: read media: %w", err)
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}
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if int64(len(data)) > a.cfg.MaxMediaSize {
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return nil, ErrMediaTooLarge
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}
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return data, nil
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}
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