I'm writing a Go script that will decrypt some legacy data that is encrypted with EVP_aes_256_cbc and an RSA public key.
In C this would be something like:
key_size = EVP_OpenInit(&ctx, EVP_aes_256_cbc(), evp_key, eklen, iv, pkey);
//...
EVP_OpenUpdate(&ctx, destination, &len_out, buffer_in, buffer_size)
//...
EVP_OpenFinal(&ctx, destination+len_out, &len_out);
I have the evp_key and iv byte array equivalents in Go, but I must confess the order of how EVP works in OpenSSL eludes me (I'm fairly competent in C, but I can't get a grasp on the process by which this decryption happens from looking at the OpenSSL source.)
In Go, I can get this far:
pKey := //rsa.PrivateKey
eklen := 32
evpKey := "// hidden 32 byte array"
iv := "// hidden 16 byte array"
c, err := aes.NewCipher(iv)
cbc := cipher.NewCBCDecrypter(c, iv)
And here's where I get lost. I have an evpKey and the pKey, but I'm not sure how to decrypt the data from here. OpenSSL uses RSA_decrypt_old or something like that, but I'm unable to track down what that actually means.
Is there a Go equivalent or do I need to bust out the much-too-expensive cgo package and roll up my sleeves?
Update (Resolution):
For anyone looking to replicate the EVP behavior in Go or just wondering how EVP works exactly, the following is breakdown. If you know the C (or Java or whatever OpenSSL implementation) was encrypting with something like:
// pseudo-code: don't copypasta and expect amazing
EVP_PKEY_assign_RSA(pkey, public_key);
EVP_CIPHER_CTX_init(&ctx);
EVP_SealInit(&ctx, EVP_aes_256_cbc(), &evp_key, &evp_key_len, iv, &pkey, 1);
EVP_SealUpdate(&ctx, buffer_out, &encrypt_len, (unsigned char*)buffer_in, len);
EVP_SealFinal(&ctx, buffer_out+encrypt_len, &encrypt_len);
The "Seal" actually just encrypts the key with the RSA public key.
In Go to decrypt something like that:
evpKeyBytes := "// the rsa.PublicKey encoded evpKey"
evpKey, err := rsa.DecryptPKCS1v15(rand.Reader, PrivateKeyRSA, evpKeyBytes)
c, err := aes.NewCipher(evpKey)
cbc := cipher.NewCBCDecrypter(c, iv)
decryptedDataBytes := make([]bytes, 2048) // some message size
cbc.CryptBlocks(decryptedDataBytes, encryptedDataBytes)
data = string(decryptedDataBytes)
// data should have the expected decrypted result.
