A processing service of a provider network may protect media content from being tampered with when it is transmitted from the provider network/transcoder to untrusted networks (e.g., third-party networks/CDNs) and to a media player. The processing service (e.g., the transcoder) generates a public and a private key. The service uses the private key to digitally sign content portions (e.g., video frames) before distribution to untrusted CDNs. The provider network creates a manifest that includes the public key. To play the media content, the media player obtains a manifest that includes the public key (via a secure/trusted connection with the provider network). The media player may then obtain the media content from an untrusted edge server/CDN and validate it using the public key that was separately obtained from the manifest (to verify the content was not tampered with).