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  Cryptography Definition Cryptography is the study of secure communications techniques that allow only the sender and intended recipient of a message to view its contents. The term is derived from the Greek word  kryptos,  which means hidden. It is closely associated to encryption, which is the act of scrambling ordinary text into what's known as ciphertext and then back again upon arrival. In addition, cryptography also covers the obfuscation of information in images using techniques such as microdots or merging. Ancient Egyptians were known to use these methods in complex hieroglyphics, and Roman Emperor Julius Caesar is credited with using one of the first modern ciphers. When transmitting electronic data, the most common use of cryptography is to encrypt and decrypt email and other plain-text messages. The simplest method uses the symmetric or "secret key" system. Here, data is encrypted using a secret key, and then both the encoded message and secret key are sent to...

  In   mathematics , the   greatest common divisor   ( GCD ) of two or more   integers , which are not all zero, is the largest positive integer that   divides   each of the integers. For two integers   x ,   y , the greatest common divisor of   x   and   y   is denoted   . For example, the GCD of 8 and 12 is 4, that is,   . [1] [2] In the name "greatest common divisor", the adjective "greatest" may be replaced by "highest", and the word "divisor" may be replaced by "factor", so that other names include  highest common factor  ( hcf ), etc. [3] [4] [5] [6]  Historically, other names for the same concept have included  greatest common measure . [7] Properties Every common divisor of  a  and  b  is a divisor of  gcd( a ,  b ) . gcd( a ,  b ) , where  a  and  b  are not both zero, may be defined alternatively and equivalently as the smallest ...