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I am trying to find information about the Signature Algorithm SHA512withRSA and have been unsuccessful so far.

In the current state, the signature is too long, so I would like to check the code for overhead and try to remove reduce the size.

Can anybody give me some pointers on how to achieve that.

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References

Related to your reference request: SHA512withRSA points to the RSA Signature Scheme with Appendix based on PKCS #1 v1.5 with SHA-512 hash function.

This means you’re looking for reference documentation describing RSA PKCS1 v1.5 (see: RFC2313) signatures with SHA512 (see: RFC6234) hash and X.509 encoding format.

Removing “overhead” from code

As for the part of your question on how to achieve the removal of overhead in your code: diving into that could quickly become off-topic for Crypto.SE and it would probably be more suitable for StackOverflow anyway. Nevertheless – since your question indicates you’re new to the reference documentations and RFCs describing SHA2, RSA, etc. – I would like to add my two cents to your code-modification idea…

Cryptographic implementations rarely contain or produce (what you call) “overhead”. Therefore, I would like to strongly discourage you from stripping or modifying any part of code that represents, interacts, or relates to cryptographical algorithms or schemes. Doing so has a high probability of introducing issues – which might range from simple software bugs, up to a dangerous situation where the removal of code is equal to the voiding (= removal) of cryptographic security. You surely don’t want the later to happen. Instead of fiddling with the code, you should simply rely a tested and well-vetted implementation. There are many of them out there, available in almost every programming language you could wish for.

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    $\begingroup$ Thank you for your time and writing me solid answer. I had this question on StackOverflow and was redirected here. The application which I am developing is a proof-of-concept only. I was expecting that SHA256withRSA will have approx 256 bits and SHA512 approx 512 bits. However the size of the signature is more than twice larger its size. What I will do now is try to simply do this manually - creating a SHA512 message digest and decrypt it with an RSA private key. I understand that the text-book RSA does this. I am aware of it might not being secure for this. $\endgroup$
    – KrNeki
    Sep 9 '14 at 21:41
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Since your problem is the signature size, I recommend you consider elliptic curve signatures instead of trying to roll your own RSASSA implementation.

With RSA a 512-bit signature requires you to use a 512-bit modulus, which has been considered insecure for more than a decade. It's equivalent to something like 50-80 bit security at best. These days the minimum recommendations start above 1024, with 2048 considered a secure choice.

In comparison, a 256-bit elliptic curve should give you about 128-bit security. The size of signatures varies, but e.g. Ed25519 has 512-bit signatures. You should see what 256-bit curves your crypto libraries support and whether the signature sizes are acceptable. If you need to save a few more bytes, you can go down to a 192-bit curve without it being obviously insecure, but otherwise I would stick with 256 bits for some security margin.

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  • $\begingroup$ My RSA key is 2048 bits, though my signature algorithm in java is SHA512withRSA. SHA is 512 bits, not my modulus. Or am I missing something? $\endgroup$
    – KrNeki
    Sep 11 '14 at 19:32
  • $\begingroup$ @KrNeki, With a 2048-bit RSA key the signatures are at least 2048 bits, regardless of which hash algorithm is used. My point was just that if you wanted 512-bit signatures, RSA would be insecure, while elliptic curves wouldn't. $\endgroup$
    – otus
    Sep 11 '14 at 20:57
  • $\begingroup$ You helped me a lot. I want shorter signatures with the same same security and elliptic curve has definitely a good advantage. CPU and memory are also not an issue with the system I am thinking of building, the only problem so far is that our Government only issues RSA certificates. Many thanks to you, I will write about this in my Thesis. $\endgroup$
    – KrNeki
    Sep 11 '14 at 21:12

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