Doubt in Cryptography

Hi All

I'm going through Cryptography. I've some doubt in that. We use DigestMethod and SignatureMethod which use algorithms like SHA and RSA repectively. I'm not getting why we need two different algorithms for Digital Signature

Thanks
Navin

SHA is a digest (or hash) algorithm, while RSA is an encryption algorithm; they do very different things.

Thanks Ulf Dittmer

So as per my understanding, using DigestMethod we calculate the Hash value and using SignatureMethod we encrypt that value to keep the hash value confidential. Please correct me if I am incorrect.

Thanks,
Navin

Not quite. A hash value does not need encryption - it is already cryptologically secure and can be transmitted in clear.

SignatureMethod is an XML element that is used to carry information about the algorithms used for the signature; see http://www.w3.org/TR/xmldsig-core/#sec-SignatureMethod for more detail.

Thanks a lot Ulf Dittmer

Regards
Navin

Navin Pillu wrote:Thanks Ulf Dittmer

So as per my understanding, using DigestMethod we calculate the Hash value and using SignatureMethod we encrypt that value to keep the hash value confidential. Please correct me if I am incorrect.

Thanks,
Navin

Digital signatures are obtained using the techniques of public key cryptography, in which you have a public key that you give out to everybody and a private key that you must keep secret. To sign a message you must use your private key, and then resulting signature can be verified by anybody using your public key.

It turns out that the established algorithms for public key cryptography are very very expensive computationally, and the complexity is linear in the message length in both size and time. As an optimization in both, an alternative scheme was developed where the message to be signed is first digested and then the digest is signed rather than the original message. By contrast, digest algorithms such as SHA1 are much much faster to compute and their result is always the same relatively small size (160 bits in the case of SHA1) no matter how long the original message.