Google's recent announcement that they will begin to deprecate support for SHA-1 TLS/SSL digital certificates in Chrome is meeting resistance from certificate authorities (CAs). Google made their announcement on August 20 on their Security-dev mailing list, although they had been warning of this decision for months.
SHA-1 is a hash algorithm, a critical component of secure cryptography. A hash algorithm takes a block of data as input and outputs a value of a certain size (SHA-1 hashes are 160 bits long). This value is called a hash or digest. With a good hash algorithm, two different blocks of data will always produce a different hash, and even a small change in the input data will result in a significant change in the output. There should be no way to learn anything about the input data from the hash output.

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