encryption - Working with EVP and OpenSSL, coding in C -

i've seen many questions on openssl , evp, not many clear answers, figured i'd still post question here , hope better feedback.

the materials given me signed file "symmetrickey.bin", rsa key set "privatekey_a.pem", "publickey_a.pem", , other user's public key "publickey_b.pem".

what need is:

1. unsign symmetrickey.bin , store text file.
2. encrypt message.txt using symmetrickey.txt , algorithm aes example.
3. sign encrypted message privatekey_a.pem , write file cipher.bin.
4. after need unsign , verify signature on cipher.bin.
5. then decrypt message our symmetric key write file.

the issues i'm having understanding how implement openssl evp libraries. api page not clear values each function comes from. example, evp_openinit() getting ek or length of ek "ekl"? "prvi" private key? , how know type? these things i'm not given.

i've looked @ many implementations , don't answer questions or give crazy code little no explanation of what's going on or values coming from. i'm posting here last resort...

for sign/unsign key part need further information, how signature done? example, signature x byte length @ end of file , can removed?

for items 2-5 in list following code surely assist, based on examples openssl documentation more comments , adaptations needs. feel free ask if have questions not commented!

crpytor.c

#include <string.h> #include <stdio.h> #include <unistd.h> #include <openssl/evp.h>  #define appname "c"  #define chunk_size 512 int do_crypt(file *in, file *out, int do_encrypt) {     /* allow enough space in output buffer additional block */     unsigned char inbuf[chunk_size];     unsigned char outbuf[chunk_size + evp_max_block_length];     int inlen;     int outlen;     evp_cipher_ctx ctx;     /* bogus key , iv: we'd set these      * source.      */     unsigned char key[] = { 0x13, 0xa3, 0xb4, 0xc1, 0x24, 0x19, 0xf5, 0x23, 0x18, 0xef, 0xca, 0x12, 0x4c, 0x9f, 0x14, 0xfe };     unsigned char iv[] = { 0x92, 0x1c, 0x23, 0x3f, 0x5e, 0x10, 0x3d, 0x9a };     /* don't set key or iv because modify parameters */     evp_cipher_ctx_init(&ctx);     /* using blowfish encryption cbc algorithm, can use whichever supported in openssl if wish */     evp_cipherinit_ex(&ctx, evp_bf_cbc(), null, null, null, do_encrypt);     evp_cipher_ctx_set_key_length(&ctx, 16);     /* finished modifying parameters can set key , iv */     evp_cipherinit_ex(&ctx, null, null, key, iv, do_encrypt);     for(;;)     {         inlen = fread(inbuf, 1, chunk_size, in);         if(inlen <= 0) break;         if(!evp_cipherupdate(&ctx, outbuf, &outlen, inbuf, inlen))         {             /* error */             evp_cipher_ctx_cleanup(&ctx);             return -1;         }         fwrite(outbuf, 1, outlen, out);     }     if(!evp_cipherfinal_ex(&ctx, outbuf, &outlen))     {         /* error */         evp_cipher_ctx_cleanup(&ctx);         return -1;     }     fwrite(outbuf, 1, outlen, out);     evp_cipher_ctx_cleanup(&ctx);     rewind(in);     rewind(out);     return 0; }  /* standalone encryptor entry point */ int main(int argc, char** argv) {     file *encode_file;     file *decode_file;     int enc_or_dec;     if (argc < 4)     {         printf("usage: %s [plain file] [encrypted file] [0/1 deccrypt/encrypt]\n", argv[0]);         return -1;     }     encode_file = fopen(argv[1], "r");     decode_file = fopen(argv[2], "w+");     /* stupid decimal translation */     enc_or_dec = *argv[3]-48;      do_crypt(encode_file, decode_file, enc_or_dec);     return 0; } 

and makefile:

all:     gcc cryptor.c -o cryptor -g -lcrypto -i ../openssl-1.0.1f-host/include clean:     rm cryptor 

this code not use evp_openinit() because used decryption, while method (and needs) require both encryption or decryption. while can use evp_openinit() initialize decryption context, replaced single call suitable decryption 2 calls suitable both encryption , decryption.

from man page:

evp_openinit() initializes cipher context ctx decryption cipher type. decrypts encrypted symmetric key of length ekl bytes passed in ek parameter using private key priv. iv supplied in iv parameter. evp_openupdate() , evp_openfinal() have same properties evp_decryptupdate() , evp_decryptfinal() routines, documented on evp_encryptinit(3) manual page.

evp_openinit() key files

if signed file referring public key file in rsa/dsa or similar format, can use this stackoverflow question better method mine key extraction file automatically (and uses evp_openinit() require)