• There are a number of popular message digest algorithms known as MD_{n} for various values of n.
• MD5 is the most popular and is fifth in a series of message digests designed by Ronald Rivest.
• The basic operation of MD5 is shown in fig.
• This algorithm operates on message 512 bits at a time.
• Messages not multiple of 512 bits are padded with:
1. A string consisting of 1 followed by zeroes, and
2. 64-bit integer that indicates the length of original message, to make the length of the composite message multiples of 512 bits.
• The message digest calculation begins with a digest value initialized to a constant.
• This value is combined with the first 512 bits of the message to produce a new value for the digest.
• The new value is than combined with the next 512 bits of the message using the same transformation.
• This process is repeated on each 512-bit block till the final value of digest is obtained from the last block of the message.
• The digest is 128-bit long for any message length.
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