  We present a concept to achieve non-repudiation for natural language
conversations over the Internet. The method rests on chained electronic
signatures applied to pieces of packet-based, digital, voice communication. It
establishes the integrity and authenticity of the bidirectional data stream and
its temporal sequence and thus the security context of a conversation. The
concept is close to the protocols for Voice over the Internet (VoIP), provides
a high level of inherent security, and extends naturally to multilateral
non-repudiation, e.g., for conferences. Signatures over conversations can
become true declarations of will in analogy to electronically signed, digital
documents. This enables binding verbal contracts, in principle between
unacquainted speakers, and in particular without witnesses. A reference
implementation of a secure VoIP archive is exhibited.
