If I don't have access to your conversation and people know this, but I claim that I know you said something in such a conversation, people can ask how I know i

If I don't have access to your conversation and people know this, but I claim that I know you said something in such a conversation, people can ask how I know it and demand proof. If your conversations aren't encrypted and I have access to them, and people know this, then if I claim that you said a certain something in such a conversation, people are more likely to just believe me. Also, unencrypted data is data that can be altered by a third party.