2. Security of encryption schemes (10+10 points). For each of the encryption schemes below, state whether the scheme is secure or not. Justify your answer in each case. (a) The message space is M = {0,1,...,6}. Algorithm Gen chooses a uniform key from the key space k = {0, 1, ...,7}. The encryption algorithm Encs (m) returns (sk + m) mod 7, and the decryption algorithm Deck(m) returns (c- sk) mod 7. (b) The message space is M = {1,3,5,..., 2019}. Algorithm Gen chooses a uniform key from the key space K = {2, 4, 6, ..., 2020}. The encryption algorithm Encak (m) returns (sk+m) mod 2020, and the decryption algorithm Decak (m) returns (c-sk) mod 2020. 2. Security of encryption schemes (10+10 points). For each of the encryption schemes below, state whether the scheme is secure or not. Justify your answer in each case. (a) The message space is M = {0,1,...,6}. Algorithm Gen chooses a uniform key from the key space k = {0, 1, ...,7}. The encryption algorithm Encs (m) returns (sk + m) mod 7, and the decryption algorithm Deck(m) returns (c- sk) mod 7. (b) The message space is M = {1,3,5,..., 2019}. Algorithm Gen chooses a uniform key from the key space K = {2, 4, 6, ..., 2020}. The encryption algorithm Encak (m) returns (sk+m) mod 2020, and the decryption algorithm Decak (m) returns (c-sk) mod 2020