You can use public key to "encrypt" (or "decrypt" which is same in "textbook" RSA) the signature and get hashed message. If the hashed message equals hashed message, then you verified the message being correctly signed. You cannot use public key and message to recreate a signature that can pass the above verification though. P.S.

Dec 16, 2013 · What does an RSA signature look like? I found this in P1363 Public Key Cryptography. Its an older copy of the standard from 2000. It may (or may not) be current. On page 41, there is 8.2.6 IFSP-RSA2, which stands for Integer Factorization, Signing Primitive. RSA2 is the second approved method. In the algorithm, the signature is in the range:

Nov 15, 2015 · RSA is merely the only public-key cryptosystem that naively supports both public-key encryption and digital signatures. This usually confuses beginners since various sources/lecturers say. RSA decryption is the RSA signature. No, it is not! The confusion comes from the textbook RSA. the textbook RSA encryption;

Aug 10, 2012 · The RSA signature size is dependent on the key size, the RSA signature size is equal to the length of the modulus in bytes. This means that for a "n bit key", the resulting signature will be exactly n bits long. Although the computed signature value is not necessarily n bits, the result will be padded to match exactly n bits.

I would like to add a warning not directly related to your question: that image you showed is simplified, and is missing an important piece of the RSA signature process. After you perform the hash, it is essential that you pad the hash out before you give it to what they have labeled the 'encrypt hash with the signer's private key' step.

See, for example, Bernstein's RSA signatures and Rabin–Williams signatures:the state of the art. Bernstein provides a survey of the history of RSA-based signature schemes, and why things like hashing is required. From the paper, "Section 2, hashing (1979 Rabin): Messages are scrambled by a public hash function H. A signature of m is an eth ...

May 28, 2016 · But for example, if the hash function is a full-domain hash function (i.e., whose output range is $\{0, \dots, n-1\}$), the resulting signatures are secure. This is the well-known RSA-FDH signature scheme. $\endgroup$ –

You are right, the RSA signature size is dependent on the key size, the RSA signature size is equal to the length of the modulus in bytes. This means that for a "n bit key", the resulting signature will be exactly n bits long. Although the computed signature value is not necessarily n bits, the result will be padded to match exactly n bits.

Oct 22, 2022 · do we actually need padding in RSA Signatures?. Yes if we want to:. Sign the hash of a message computed with a standard cryptographic hash like SHA-256, and resist attack in a chosen-messages setup (where the attacker can obtain the signature of some messages of their choice, and succeeds by signing any other message of their choice; so-called EUF-CMA security).

Oct 23, 2015 · What is the difference between compute a signature with the following two methods? Compute a signature with Signature.getInstance("SHA256withRSA") Compute SHA256 with MessageDigest.getInstance("SHA-256") and compute the digest with Signature.getInstance("RSA"); to get the signature?