Strong secrecy is a term used in formal proof-based cryptography for making propositions about the security of cryptographic protocols.
It is a stronger notion of security than syntactic (or weak) secrecy.
Strong secrecy is related with the concept of semantic security or indistinguishability used in the computational proof-based approach.
Bruno Blanchet provides the following definition for strong secrecy: For example, if a process encrypts a message m an attacker can differentiate between different messages, since their ciphertexts will be different.
The randomness incorporated into the encryption algorithm will yield different ciphertexts for the same value of m.