24 Oct
2023
24 Oct
'23
7:32 p.m.
Well, you can use mbedtls_rsa_export to obtain N, E and D, and then mbedtls_rsa_import with E and D swapped followed by mbedtls_rsa_complete.
But “encrypt with a private key” doesn't actually make sense. Mathematically, it works, because there's a way to make the core RSA operation work the same in both directions. But the resulting construction is unlikely to have any security properties. Calling mbedtls_rsa_pkcs1_encrypt with a private key doesn't actually “encrypt” since anyone can still obtain the plaintext.
Mbed TLS 3.x still provides the function mbedtls_rsa_public() and mbedtls_rsa_private() if you need some custom RSA-based encoding that is neither a PKCS#1 encryption mechanism nor a PKCS#1 signature mechanism.
Best regards,
--
Gilles Peskine
Mbed TLS developer
On 24/10/2023 15:59, Janos Follath via mbed-tls wrote:
>
> Hi Makejian,
>
> I am terribly sorry, but that is not supported anymore.
>
> To achieve that, you would need to swap the public and the private
> exponents in the RSA context, then remove the CRT coefficients. All
> these fields in the RSA context are private. That is, you would need
> to enable MBEDTLS_ALLOW_PRIVATE_ACCES and even if you manage to make
> it work like this, it is not guaranteed that it will keep working in
> later versions.
>
> Best regards,
>
> Janos
>
> (Mbed TLS developer)
>
> *From: *克坚马via mbed-tls mbed-tls@lists.trustedfirmware.org
> *Date: *Tuesday, 24 October 2023 at 13:22
> *To: *mbed-tls@lists.trustedfirmware.org
> mbed-tls@lists.trustedfirmware.org
> *Subject: *[mbed-tls] Re: How can I encrypt with a private key?
>
> hi Janos,
> Thanks for your reply!
>
> Due to application vendor restrictions, I need to use the public key
> to decrypt data encrypted with the private key from the server.
>
> I use 'mbedtls_rsa_check_privkey' to check whether it is a private key
> and decide to decrypt with private key or public key, which can
> temporarily solve this problem.
>
> But there is a problem that I cannot use the public key to encrypt
> when I have the private key.
>
> So do you have any good suggestions for supporting both encryption and
> decryption with private key and public key?
>
> Thanks again and looking forward to your reply!
>
> Best regards,
> makejian
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>