Up until the invention of public key cryptography in the 1970s, symmetric ciphers were the only encryption scheme available. A symmetric cipher refers to a cryptographic model where the communicating parties share a secret key to ensure the privacy of communications. Depending on the key, a symmetric encryption algorithm performs various permutations and substitutions to transform a given message into ciphertext. To recover the original message the decryption algorithm would perform the same operations in reverse, using the secret key. However, the key had to be sent through a secure channel, e.g. a trusted courier, registered mail, or best of all – agreed verbally in advance. There did not seem to be any other way for secure information exchange.

In 1974 Ralph Merkle, a computer science student at UC Berkeley attended a course in computer security. As part of the course he submitted a proposal addressing the issue of the distribution of a secret key [1]. The proposal was rejected several times, which led Merkle to drop the course and continue working on his idea independently [2]. Assuming that only an insecure channel is available for transmitting information, Merkle suggested that communications be based on a set of ‘puzzles’, solution to which would contain a unique puzzle ID and a unique puzzle key. If user

A puzzle could be a symmetric encryption of the puzzle key and ID, solvable by brute-force, i.e. by trying all possible keys. If one puzzle takes 2 minutes to solve, with

### References

[1] Merkle, R. C. *Secure communications over insecure channels*. Communications of the ACM 21.4 (1978): 294-299.

[2] Diffie, W. *The first ten years of public-key cryptography*. Proceedings of the IEEE 76.5 (1988): 560-577.

[3] Diffie, W. and Hellman, M. E. *New directions in cryptography*. Information Theory, IEEE Transactions on 22.6 (1976): 644-654.

[4] Rivest, R. L., Shamir, A. and Adleman, L. *A method for obtaining digital signatures and public-key cryptosystems*. Communications of the ACM 21.2 (1978): 120-126.

Author: Mante Zemaityte, mante.zemaityte@manchester.ac.uk