Secure Programming Cookbook for C and C++

7.13. Verifying Signed Data Using an RSA Public Key

Problem

You have some data, an RSA digital signature of that data, and the public key that you believe corresponds to the signature. You want to determine whether the signature is valid. A successful check would demonstrate both that the data was not modified from the time it was signed (message integrity) and that the entity with the corresponding public key signed the data (authentication).

Solution

Use the verification algorithm that corresponds to the chosen signing algorithm from Recipe 7.12. Generally, this should be included with your cryptographic library.

Discussion

Recipe 7.12 explains the basic components of digital signatures with RSA. When verifying, you will generally need to provide the following inputs:

• The signer’s public key.

• The signature to be verified.

• The message digest corresponding to the message you want to authenticate. If it’s a high-level API, you might be able to provide only the message.

• An indication of the message digest algorithm used in the signing operation. Again, this may be assumed in a high-level API.

The API should simply return indication of success or failure.

Some implementations of RSA signature verification are susceptible to timing attacks. Basically, if RSA private key operations do not happen in a fixed amount of time, such attacks are possible. A technique called blinding can thwart timing attacks. The amount of time it takes to decrypt is randomized somewhat by operating on a random ...