Secure hash functions

• Used to reduce a message to a fixed size for signature (sometimes called a “message digest”)

  • also used in random number generators, password schemes (Unix passwd, S/key), and to design MACs

• Avoids some protocol problems, and lets you sign long messages with only one public key operation

• One-way: hard to find any x given only H(x)

• Collision-free: hard to find {x, y} where H(x)=H(y)

• Examples include MD5 (128 bits), SHA (160 bits)

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