Beispiel #1
0
func (ctx *decryptionCipherCtx) DecryptFinal() ([]byte, error) {
	outbuf := make([]byte, ctx.BlockSize())
	var outlen C.int
	if 1 != C.EVP_DecryptFinal_ex(ctx.ctx, (*C.uchar)(&outbuf[0]), &outlen) {
		// this may mean the tag failed to verify- all previous plaintext
		// returned must be considered faked and invalid
		return nil, errors.New("decryption failed")
	}
	return outbuf[:outlen], nil
}
Beispiel #2
0
// Open decrypts "in" using "iv" and "authData" and append the result to "dst"
func (g stupidGCM) Open(dst, iv, in, authData []byte) ([]byte, error) {
	if len(iv) != ivLen {
		log.Panicf("Only %d-byte IVs are supported", ivLen)
	}
	if len(in) <= tagLen {
		log.Panic("Input data too short")
	}
	buf := make([]byte, len(in)-tagLen)
	ciphertext := in[:len(in)-tagLen]
	tag := in[len(in)-tagLen:]

	// https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption#Authenticated_Encryption_using_GCM_mode

	// Create scratch space "context"
	ctx := C.EVP_CIPHER_CTX_new()
	if ctx == nil {
		log.Panic("EVP_CIPHER_CTX_new failed")
	}

	// Set cipher to AES-256
	if C.EVP_DecryptInit_ex(ctx, C.EVP_aes_256_gcm(), nil, nil, nil) != 1 {
		log.Panic("EVP_DecryptInit_ex I failed")
	}

	// Use 16-byte IV
	if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_SET_IVLEN, ivLen, nil) != 1 {
		log.Panic("EVP_CIPHER_CTX_ctrl EVP_CTRL_GCM_SET_IVLEN failed")
	}

	// Set key and IV
	if C.EVP_DecryptInit_ex(ctx, nil, nil, (*C.uchar)(&g.key[0]), (*C.uchar)(&iv[0])) != 1 {
		log.Panic("EVP_DecryptInit_ex II failed")
	}

	// Set expected GMAC tag
	if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_SET_TAG, tagLen, (unsafe.Pointer)(&tag[0])) != 1 {
		log.Panic("EVP_CIPHER_CTX_ctrl failed")
	}

	// Provide authentication data
	var resultLen C.int
	if C.EVP_DecryptUpdate(ctx, nil, &resultLen, (*C.uchar)(&authData[0]), C.int(len(authData))) != 1 {
		log.Panic("EVP_DecryptUpdate authData failed")
	}
	if int(resultLen) != len(authData) {
		log.Panicf("Unexpected length %d", resultLen)
	}

	// Decrypt "ciphertext" into "buf"
	if C.EVP_DecryptUpdate(ctx, (*C.uchar)(&buf[0]), &resultLen, (*C.uchar)(&ciphertext[0]), C.int(len(ciphertext))) != 1 {
		log.Panic("EVP_DecryptUpdate failed")
	}
	if int(resultLen) != len(ciphertext) {
		log.Panicf("Unexpected length %d", resultLen)
	}

	// Check GMAC
	dummy := make([]byte, 16)
	res := C.EVP_DecryptFinal_ex(ctx, (*C.uchar)(&dummy[0]), &resultLen)
	if resultLen != 0 {
		log.Panicf("Unexpected length %d", resultLen)
	}

	// Free scratch space
	C.EVP_CIPHER_CTX_free(ctx)

	if res != 1 {
		return nil, fmt.Errorf("stupidgcm: message authentication failed")
	}

	return append(dst, buf...), nil
}