Parse EC Public key

Question

I an working on ECIES and need to load peer public key. Load EC Public key

I an using ECDH and need to load peer public key. When I try to load public key from PEM file , seems no issue

Issue here:

EVP_PKEY * get_peer_key()  
{
     // base64 certificate data of alice_pub_key.pem
     char *buffer= "MFYwEAYHKoZIzj0CAQYFK4EEAAoDQgAEjWrT7F97QrSqGrlIgPK8dphNBicNO6gDLfOIMjhF2MiLuuzd7L7BP+bLCuNtKKe/2dOkgPqgXv4BFWqgp6PZXQ=="`
     // calculate buffer length
     int l = strlen(buffer)
     //create bio from buffer
     BIO *in = BIO_new_mem_buf(buffer,l)
     //gnerate ec key   
     EC_KEY *eckey = PEM_read_bio_EC_PUBKEY(in,NULL,NULL,NULL)` // ==> FAIL
     //need to convert to EVP format
     EVP_PKEY *peerKey = EVP_PKEY_new()
     //assign ec key evp
     if(EVP_PKEY_assign_EC_KEY(peerKey,eckey) != 1 )
         printf("\n error hapened");
     return peerKey;
}

Works fine:

EVP_PKEY * get_peer_key()
{
     //Load PEM format file
     char * infile = "alice_pub_key.pem";
     //create bio
     BIO *in = BIO_new(BIO_s_file());
     //read bio file
     BIO_read_filename(in , infile);
     //create eckey
     EC_KEY *eckey = PEM_read_bio_EC_PUBKEY(in,NULL,NULL,NULL); // ==> success
     // create peer key   
     EVP_PKEY *peerKey = EVP_PKEY_new();
     //assign public key
     if(EVP_PKEY_assign_EC_KEY(peerKey,eckey) != 1 )
         printf("\n error hapened");
     return peerKey;
}

Can some one suggest whats going wrong while reading base64 data of pem file

A PEM encoded key has a header and footer. The posted public EC key is in X.509/SPKI format and therefore has the header `-----BEGIN PUBLIC KEY-----` and footer `-----END PUBLIC KEY-----`, which are both missing from the code. Header and footer must each be on a _single_ line. In the body there is a line break after _every 64 characters_ (but the code runs also without this body formatting). — Topaco, Jun 04 '21 at 08:42
@Topaco I receive public key from network and the buffer is "MFYwEAYHKoZIzj0CAQYFK4EEAAoDQgAEjWrT7F97QrSqGrlIgPK8dphNBicNO6gDLfOIMjhF2MiLuuzd7L7BP+bLCuNtKKe/2dOkgPqgXv4BFWqgp6PZXQ==. What I know is that its ECC public key. Now how I can set this Public key for ECDH. — Baboo Patel, Jun 04 '21 at 08:53
I already described that, add header and footer and don't forget the line breaks. — Topaco, Jun 04 '21 at 08:58

score 1 · Answer 1 · answered Jun 04 '21 at 10:42

1

There are two ways of solving this:

Creating a PEM using a header and footer line and line breaks (at the 64th character;
Base 64 decoding the text and then handling it by parsing the resulting ASN.1 / DER binary;

I'd prefer the latter, as I abhor adding lines and such, it is error prone at best, and string manipulations should be avoided where possible.

Note that this assumes that the base 64 contains a SubjectPublicKeyInfo structure which I've shown you earlier. Otherwise you may have to find out how to parse a X9.62 structure or just a point.

answered Jun 04 '21 at 10:42

Maarten Bodewes

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Thanks d2i_PUBKEY() worked for me. Now I want to do ECIES through shared secret key. I generated my private , public key and also generated empirical key. How to I can encrypt as per ECIES. Please share me some link. – Baboo Patel Jun 08 '21 at 16:30
I think you misunderstand our relationship here. – Maarten Bodewes Jun 08 '21 at 17:30
I did not find much useful link for ECIES but your suggestion were really helpful in going step by step for implementing ECIES encryption, so I request in that way :). Thanks for useful suggestion till now .I will take care in future. – Baboo Patel Jun 09 '21 at 14:00

Parse EC Public key

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