RFC 3161 defines a protocol for securely timestamping digital documents, and there are several free servers that provide this service. Here are some of them:
OpenTSA ( www.opentsa.org/ ): OpenTSA is an open-source timestamping server that supports both RFC 3161 and RFC 5816 (timestamping over HTTP). It is available under the MIT license.
SafeStamp ( www.safestamp.com/ ): SafeStamp is a free timestamping service that uses RFC 3161. It is provided by the SEC Consult Group and can be used with any document or data file.
Zeitserver ( zeitserver.org/ ): Zeitserver is a German-based timestamping service that supports RFC 3161. It is free for personal use, and charges a small fee for commercial use.
TimeStamp Authority ( sa.timestampauthority.com/ ): TimeStamp Authority is a US-based timestamping service that provides free RFC 3161 timestamps for individual users. It also offers commercial services such as timestamping for code signing.
It is important to note that these servers may have usage limitations or restrictions that should be carefully reviewed before use. Additionally, it is recommended to use trusted and reliable timestamping servers to ensure the integrity and authenticity of the timestamp.