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// Package stupidgcm is a thin wrapper for OpenSSL's GCM encryption and
// decryption functions. It only support 32-byte keys and 16-bit IVs.
package stupidgcm
// #include <openssl/evp.h>
// #cgo pkg-config: libcrypto
import "C"
import (
"fmt"
"log"
"unsafe"
)
const (
keyLen = 32
ivLen = 16
tagLen = 16
)
// stupidGCM implements the cipher.AEAD interface
type stupidGCM struct {
key []byte
}
func New(key []byte) stupidGCM {
if len(key) != keyLen {
log.Panicf("Only %d-byte keys are supported", keyLen)
}
return stupidGCM{key: key}
}
func (g stupidGCM) NonceSize() int {
return ivLen
}
func (g stupidGCM) Overhead() int {
return tagLen
}
// Seal - encrypt "in" using "iv" and "authData" and append the result to "dst"
func (g stupidGCM) Seal(dst, iv, in, authData []byte) []byte {
if len(iv) != ivLen {
log.Panicf("Only %d-byte IVs are supported", ivLen)
}
if len(in) == 0 {
log.Panic("Zero-length input data is not supported")
}
buf := make([]byte, 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_EncryptInit_ex(ctx, C.EVP_aes_256_gcm(), nil, nil, nil) != 1 {
log.Panic("EVP_EncryptInit_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_EncryptInit_ex(ctx, nil, nil, (*C.uchar)(&g.key[0]), (*C.uchar)(&iv[0])) != 1 {
log.Panic("EVP_EncryptInit_ex II failed")
}
// Provide authentication data
var resultLen C.int
if C.EVP_EncryptUpdate(ctx, nil, &resultLen, (*C.uchar)(&authData[0]), C.int(len(authData))) != 1 {
log.Panic("EVP_EncryptUpdate authData failed")
}
if int(resultLen) != len(authData) {
log.Panicf("Unexpected length %d", resultLen)
}
// Encrypt "in" into "buf"
if C.EVP_EncryptUpdate(ctx, (*C.uchar)(&buf[0]), &resultLen, (*C.uchar)(&in[0]), C.int(len(in))) != 1 {
log.Panic("EVP_EncryptUpdate failed")
}
if int(resultLen) != len(in) {
log.Panicf("Unexpected length %d", resultLen)
}
// Finalise encryption
// Because GCM is a stream encryption, this will not write out any data.
dummy := make([]byte, 16)
if C.EVP_EncryptFinal_ex(ctx, (*C.uchar)(&dummy[0]), &resultLen) != 1 {
log.Panic("EVP_EncryptFinal_ex failed")
}
if resultLen != 0 {
log.Panicf("Unexpected length %d", resultLen)
}
// Get GMAC tag and append it to the ciphertext in "buf"
if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_GET_TAG, tagLen, (unsafe.Pointer)(&buf[len(in)])) != 1 {
log.Panic("EVP_CIPHER_CTX_ctrl EVP_CTRL_GCM_GET_TAG failed")
}
// Free scratch space
C.EVP_CIPHER_CTX_free(ctx)
return append(dst, buf...)
}
// Open - decrypt "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")
}
// 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_EncryptUpdate 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)
}
// 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")
}
// 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
}
|
