To base the security of practical cryptographic schemes on weakened as-
sumptions (which are hence more likely to hold) and to improve their ef-
ficiency are general research goals in cryptography. In this thesis we con-
tinue this quest. We focus on the most traditional problems in cryptog-
raphy, namely that of assuring privacy and authenticity of data in the sym-
metric setting (where both the sender and the receiver share a secret key).