AMCL JS create rsa_public_key from ASN1 formatted X.509 public key.

[CryptoMathician]

I want to create a RSA public key from a X.509 formatted RSA public key.

Is it possible to use the ASN.1/X.509 format directly to create a rsa_public_key? If that shouldn't be the case (it seems that it isn't the case), is it possible to set the modulus properly? I think that should be possible, but I am not sure how it will works, cause of the FF and BIG prototype/class which I am maybe not fully understand in detail yet (or I have missed something). Could you help me to understand how I can set the modulus in the correct way? Or better to say to create a proper n to set it correctly

If possible to create a proper n it would be nice to do something like:

var pub = new ctx.rsa_public_key(ctx.FF.FFLEN);
pub.n.copy(n);
pub.e.copy(65537);

Cause I want to create a public key with known e and n.

Thank you in advance!

Pascal

[mcarrickscott]

Yes, that should be possible. In rsa.js there would be a need to provide a new function

set_rsa_public_key = function(ctx,n,e)

similar to the existing

rsa_public_key = function(ctx)

Where the public key n is provided as a byte array extracted from an X.509 cert, and e=65537. Then use the ff.js function

FF.fromBytes = function(x, b)

to copy n into the public key structure.

(Note: we would regard X.509 cert processing as outside of the remit of this library)

[CryptoMathician]

Ok. Thank you!

Pascal

[CryptoMathician]

I am not sure, if that is wished, but maybe it could be useful to have getter and setter for the objects rsa_public_key and rsa_private_key. In the following is a suggestion how it could be done:

ctx.rsa_public_key.set(n,e);
ctx.rsa_public_key.set_e(e);
ctx.rsa_public_key.set_n(n);

ctx.rsa_public_key.get_instance(n,e);

Whereby the get_instance method creates a new rsa_public_key object and set the properties n and e. It is also possible to do respectively also getter and setter for the rsa_private_key and a get_instance method for setting the attributes manually. What do you think about the idea?

Maybe that would be better as creating a "new" rsa_public_key object and it is possible to use the existing structure. If you want I can create a Pull Request, depends on if you think that could be useful for the library.

[mcarrickscott]

Yes, OK, that sounds like a good plan. Make sure and test it on a real X.509 public key!

Mike

[CryptoMathician]

It seems that it is possible to store a public key in that structure. I used [1] as asn1 parser and stored the bytes directly into the rsa_public_key structure with the get_instance method.

Pull Request is on the way - without the asn1 parser, only the rsa key data strucutres.

Anyway I have a question regarding the data in the array which I get back from the FF data structure. Which representation have the content of the following array?

[ 190,  66, 142, 139,  46, 151, 187, 158, 173, 174, 147, 123, ... ]

is the the 190 (from index 0) in hex or in uint or something else? I am a bit confused about the representation and wanted to know which representation are used here.

Thank you in advance!

[1] https://git.coolaj86.com/coolaj86/asn1-parser.js

[mcarrickscott]

Those are just unsigned byte values, in decimal

[CryptoMathician]

Thank you.