Decoder of ASN.1 format to read RSA public key

[lukin]

Hello,

I am using the STM32 Cryptolib. I need to get the modulus and exponent of RSA public key from a .pub or .pem files. After decoded from base64 I need to decode the DER format (in ASN.1 format). Has anyone any suggestion of which libray may I use? Cryptolib only accept modulus and exponent has two parameters. 

[STea]

Hello @lukin ,

can you explain more what is the type of conversion you will need to do and how you generated the keys in the first place ?

meanwhile i suggest you do key formatting with a python script to give as an input for the cryptolib accepted format this is explained in this thread .

BR 

[liaifat85]

Not sure but this thread may help you:

https://community.st.com/t5/stm32-mcus-security/stm32-cryptographic-library-rsa-pkcs-1-v1-5-encryption-unable-to/td-p/69453

[lukin]

Hi, I need to read PEM file, generated externally, which contain a RSA public key. I should extract the modulus and exponent in order to pass them to crypto library so to verify a signed file. I see openssl is a typical lib but size of the library is big. Any other way to retrieve this information?

Thank you 

[STea]

Hello ,

do you need to do the extraction in the MCU or it can be done externally ? because if you can do this on your computer it can be done using python and then it can be feed to you code in C formatted table in hex representation .

BR

[lukin]

Hello,

since pem or csr certificate would loaded externally on an SD card, it would be the best to decode the files. When using csr, I should also verify trust chain. 

[STea]

Hello @lukin ,

in this case i recommend you use opensll library to decode the RSA key stored into you SD card .

This can be done by :

• reading the key stored in the SD card using FatFS 
• decoding the buffer containing the RSA key with d2i_RSAPublicKey() function from openssl
• use the RSA_get0_key() function to separate it to public and private exponents and the modulus 

you will find in the attachment and example if this use-case hope this solves you problem.

BR 

[lukin · Accepted solution]

Hello everybody,

I solve by using wolfSSL library, which is available in STM32CubeIDE already. It allows to read pem public key and certificates. It allows to check certificate chain as well.

[lukin]

Hi, thank you. I then preferred to use wolfSSL. It should be lighter than openssl. Isn't it?

[STea]

Hello @lukin 

Indeed WolfSSL is lighter than OpenSSL  and it's more suited for your application if you are concerned about memory usage . this is explained in the following link . OpenSSL has the edge when it comes to the robustness of the solution 

because OpenSSL has a longer track record and a larger community of developers, which means it has been more extensively tested and audited.

The are basically the same in term of usage .

BR