[openssl-users] RSA Public Key error

[Erwann Abalea]

Bonjour,

Assuming the first 24 bytes you’re talking about are the very beginning of the SPKI structure (that is, the enclosing SEQUENCE, and the AlgorithmIdentifier), that means you’ve replaced up to the first byte of the BITSTRING containing the public key (this byte indicates the number of unused bits) for a 2048bits RSA key with 16 custom bytes.
That’s perfectly normal for OpenSSL to refuse to load that beast, and for asn1parse to return errors (the first bytes do not represent a correct DER encoding of anything).
Think of it as « I took a Jpeg file, replaced some bytes at the beginning by my own, and now I can’t open the file again ». Those bytes are there for a reason.

A quick solution would be to *add* your 16 bytes before the public key, and remove them when passing the rest of the bytes to OpenSSL.

Cordialement,
Erwann Abalea


De : openssl-users <openssl-users-bounces at openssl.org> au nom de prithiraj das <prithiraj.das at gmail.com>
Répondre à : "openssl-users at openssl.org" <openssl-users at openssl.org>
Date : mercredi 12 décembre 2018 à 08:08
À : "openssl-users at openssl.org" <openssl-users at openssl.org>
Objet : [openssl-users] RSA Public Key error

Hi,

I have a RSA public key(PKCS 1v1.5) that I have obtained from somewhere. That key has been obtained after removing the first 24 bytes from the originally generated RSA public key. Those 24 bytes are being replaced by some custom 16 byte information which is being used as some sort of identifier in some future task and those 16 bytes are playing no role in encryption. OpenSSL fails to read this key. asn1parse shows some parsing error and most importantly RSA encryption in OpenSSL using this key fails. The untampered version of the RSA public key generated from the same source and containing the original 24 bytes at the beginning of the key is successfully read by OpenSSL and the RSA encryption using that key is also successful in OpenSSL. But our requirement is to use the first key containing the custom 16 byte information.

My understanding is that the first 24 bytes of RSA public key following PKCS standards doesn't contain the modulus and exponent details required for RSA encryption.  But OpenSSL seems to require these 24 bytes for encryption. Can someone please confirm what kind of information is present in the first 24 bytes of RSA Public key and/or why does OpenSSL need it? If possible, please suggest a solution to work with that RSA public key containing custom 16 byte information at the beginning of the key.


Thanks and Regards,
Prithiraj
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