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Digital Signature


Asymmetric Key - A file encrypted with a Private Key can only be decrypted with a matching Public Key. If you have access to both the encrypted and plain text version of the file together with the public key you can check your decryption produces a file identical to the original.

To avoid the overheads of encrypting the entire file, you can create a small but unique fingerprint of the file called a digest. The encryption of the digest with the private key is called a file signature

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The crytographic message digest algorithm MD5 reduces a file of any size to an 128-bit hash value. It is very unlikely any two files have the same hash value - the hash acts as a file fingerprint. MD5 has now been largely replaced by SHA-1. SHA-1 produces a 160-bit digest and SHA2 goes up to 256 bits.
A digital signature is created by encrypting the digest with the (secret) private key. A digital signature is sent with the original file as evidence that the file is valid.
A file can be authenticated by first re-creating the digest using the same hashing function on the original file. The original digest is recovered by decrypting the signature using the public key. If the two digests are identical then the file has not been changed since it was signed by the owner of the private key - it has been "authenticated". The public key is normally obtained from a digital certficate that can be freely published.

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