Acquaintances have a chance to prepare for their secret-sharing, by sharing an encryption/decryption key over secure channels before they find themselves restricted to insecure channels.
Until a few decades ago, all cryptography relied on a-priori exchange of encryption keys, which were the very same keys that were used for decryption. Such ciphers are called symmetrical.
In modern time the most popular, most used, most universal cipher was DES, which was recently replaced by AES. DES stands for: data encryption system, and AES for: advanced encryption system. (Many replace 'system' with 'standard').
When computers became too fast for the 56 bits keys in DES, then for security concerns three DES ciphers where strung together, now requiring the cryptanalyst to decipher a 3*56 = 168 bits to crack it. This is large enough, yet DES was too slow in software, and the shadow of various 'conspiracy theories' mired its image.
Despite the fact that the DES algorithm was bombarded by academic cryptographic attacks, no successful compromise was ever published, yet NIST in 1997 announced a global effort to develop a replacement to DES called AES.