Links from Sorin Popescu's website http://www.math.sunysb.edu/~sorin/311-2006/

• An On-Line Encyclopedia of Integer Sequences.
• Fibonacci Numbers and Nature. Or Tony Phillips' "The most irrational number". Also "Who was Fibonacci?": a brief biography of Fibonacci.
• Primes: Lots of interesting facts about prime numbers.
• Mersenne Primes: interesting facts about Mersenne primes, perfect numbers, and related topics.
• Primes is P: about a recent polynomial time deterministic algorithm to test if an input number is prime or not.
• RSA: The RSA company's web page containing lots of interesting information about the RSA public key cryptosystem and cryptography in general, from both a technological and a socio-political viewpoint.
• The RSA factoring challenge.
• The Wikipedia entry for RSA cryptography.
• The Enigma machine.
• Handbook of Applied Cryptography, by Alfred J. Menezes, Paul C. van Oorschot and Scott A. Vanstone. It is a handbook for both novices and experts, introducing practical aspects of both conventional and public-key cryptography.
• The GCHQ Challenge (as of July 2005).
• Ron Rivest's Cryptography and Security links page.
• MIT Lecture Notes on Cryptography, by S. Goldwasser and M. Bellare.
• A brief history/introduction to error-correcting codes Digital Revolution - Error Correction Codes, Joseph Malkevitch, in What's New in Mathematics Feature Column of the AMS.
• Anatomy of Credit Card Numbers: a discussion of how to determine if a given credit card number might be valid or not.
• The Wikipedia entry for ISBN (that is International Standard Book Numbers). However, starting January 1, 2007, the book industry will begin using 13 digit ISBNs to identify all books in supply chain.
• A summary describing how information is encoded on Compact Discs. CDs use a modified form of the Reed-Solomon code called the Cross Interleave Reed-Solomon Coding (CIRC). Here is a short description of how Reed-Solomon codes are used for error-correction on a audio CD (Red Book Standard).
• FactorWorld is a web site dedicated to integer factorization results and algorithms. It also includes a list of recent factoring records.
• The list of the 221 currently known proofs of the Quadratic Reciprocity Law (from Legendre (1788) and Gauß (1801) to Szyjewski (2005))