The modern age in cryptography started, when Claude Shannon entered the field.
He defined the word entropy in the information theory. This term desribes the amount of information hidden in a message.
In the year 1948 Shannon wrote the book "A Mathematical Theory of Communication" in which he defined this term. Shannon had in mind to use a communication channel as good as possible when he wrote this book. His idea was as follows. When you transmit only the information of a message over a channel and not the message itself (the data), you have the maximal throughput.
That's because nearly every message has redundance in it which makes the message larger but doesn't increase the information stored in this message.
In the following year (1949) he wrote his next famous article. "Communication Theory of Secrecy Systems". This book raised the cryptography to the status of a science. There he proved, that the only way to get a perfect encryption, is to use a key with the same length as the message. This cryptosystem is called One-Time-Pad (aka Vernam-Chiffre).
His work said, that an encrypted message should not lead the attacker to the plain message. That means, that the information stored in the message should be perfectly distributed (in average) in the encrypted message.
This idea was caught up by the cryptographers to build better algorithms for encrypting data. Modern cryptography was born.
Claude Elwood Shannon
Shannon was born in Petoskey, Michigan and was a distant relative of Thomas Edison. While growing up, he worked as a messenger for Western Union.
In 1932, Shannon began studying at the University of Michigan, where he eventually encountered a course that introduced him to the works of George Boole. He graduated from the university in 1936 with two bachelor's degrees, one in electrical engineering and one in mathematics, and he then moved to the Massachusetts Institute of Technology for graduate school, where he worked on Vannevar Bush's differential analyzer, an analog computer.
From 1956 to 1978 he was a professor at MIT. To commemorate his achievements, there were celebrations of his work in 2001, and there are currently three copies of a statue of Shannon: one at the University of Michigan, one at MIT in the Laboratory for Information and Decision Systems and one at Bell Labs.
Claude Shannon died on February 24, 2001