250 lines
6.9 KiB
Go
250 lines
6.9 KiB
Go
// +build !without_openssl
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// Package stupidgcm is a thin wrapper for OpenSSL's GCM encryption and
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// decryption functions. It only support 32-byte keys and 16-bit IVs.
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package stupidgcm
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// #include <openssl/evp.h>
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// #cgo pkg-config: libcrypto
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import "C"
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import (
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"crypto/cipher"
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"fmt"
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"log"
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"unsafe"
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)
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const (
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// BuiltWithoutOpenssl indicates if openssl been disabled at compile-time
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BuiltWithoutOpenssl = false
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keyLen = 32
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ivLen = 16
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tagLen = 16
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)
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// StupidGCM implements the cipher.AEAD interface
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type StupidGCM struct {
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key []byte
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forceDecode bool
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}
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// Verify that we satisfy the cipher.AEAD interface
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var _ cipher.AEAD = &StupidGCM{}
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// New returns a new cipher.AEAD implementation..
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func New(keyIn []byte, forceDecode bool) cipher.AEAD {
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if len(keyIn) != keyLen {
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log.Panicf("Only %d-byte keys are supported", keyLen)
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}
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// Create a private copy of the key
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key := append([]byte{}, keyIn...)
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return &StupidGCM{key: key, forceDecode: forceDecode}
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}
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// NonceSize returns the required size of the nonce / IV.
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func (g *StupidGCM) NonceSize() int {
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return ivLen
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}
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// Overhead returns the number of bytes that are added for authentication.
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func (g *StupidGCM) Overhead() int {
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return tagLen
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}
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// Seal encrypts "in" using "iv" and "authData" and append the result to "dst"
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func (g *StupidGCM) Seal(dst, iv, in, authData []byte) []byte {
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if len(iv) != ivLen {
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log.Panicf("Only %d-byte IVs are supported", ivLen)
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}
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if len(in) == 0 {
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log.Panic("Zero-length input data is not supported")
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}
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if len(g.key) != keyLen {
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log.Panicf("Wrong key length: %d. Key has been wiped?", len(g.key))
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}
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// If the "dst" slice is large enough we can use it as our output buffer
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outLen := len(in) + tagLen
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var buf []byte
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inplace := false
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if cap(dst)-len(dst) >= outLen {
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inplace = true
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buf = dst[len(dst) : len(dst)+outLen]
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} else {
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buf = make([]byte, outLen)
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}
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// https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption#Authenticated_Encryption_using_GCM_mode
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// Create scratch space "context"
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ctx := C.EVP_CIPHER_CTX_new()
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if ctx == nil {
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log.Panic("EVP_CIPHER_CTX_new failed")
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}
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// Set cipher to AES-256
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if C.EVP_EncryptInit_ex(ctx, C.EVP_aes_256_gcm(), nil, nil, nil) != 1 {
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log.Panic("EVP_EncryptInit_ex I failed")
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}
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// Use 16-byte IV
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if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_SET_IVLEN, ivLen, nil) != 1 {
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log.Panic("EVP_CIPHER_CTX_ctrl EVP_CTRL_GCM_SET_IVLEN failed")
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}
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// Set key and IV
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if C.EVP_EncryptInit_ex(ctx, nil, nil, (*C.uchar)(&g.key[0]), (*C.uchar)(&iv[0])) != 1 {
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log.Panic("EVP_EncryptInit_ex II failed")
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}
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// Provide authentication data
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var resultLen C.int
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if C.EVP_EncryptUpdate(ctx, nil, &resultLen, (*C.uchar)(&authData[0]), C.int(len(authData))) != 1 {
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log.Panic("EVP_EncryptUpdate authData failed")
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}
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if int(resultLen) != len(authData) {
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log.Panicf("Unexpected length %d", resultLen)
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}
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// Encrypt "in" into "buf"
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if C.EVP_EncryptUpdate(ctx, (*C.uchar)(&buf[0]), &resultLen, (*C.uchar)(&in[0]), C.int(len(in))) != 1 {
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log.Panic("EVP_EncryptUpdate failed")
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}
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if int(resultLen) != len(in) {
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log.Panicf("Unexpected length %d", resultLen)
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}
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// Finalise encryption
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// Because GCM is a stream encryption, this will not write out any data.
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dummy := make([]byte, 16)
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if C.EVP_EncryptFinal_ex(ctx, (*C.uchar)(&dummy[0]), &resultLen) != 1 {
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log.Panic("EVP_EncryptFinal_ex failed")
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}
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if resultLen != 0 {
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log.Panicf("Unexpected length %d", resultLen)
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}
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// Get GMAC tag and append it to the ciphertext in "buf"
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if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_GET_TAG, tagLen, (unsafe.Pointer)(&buf[len(in)])) != 1 {
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log.Panic("EVP_CIPHER_CTX_ctrl EVP_CTRL_GCM_GET_TAG failed")
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}
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// Free scratch space
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C.EVP_CIPHER_CTX_free(ctx)
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if inplace {
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return dst[:len(dst)+outLen]
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}
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return append(dst, buf...)
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}
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// Open decrypts "in" using "iv" and "authData" and append the result to "dst"
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func (g *StupidGCM) Open(dst, iv, in, authData []byte) ([]byte, error) {
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if len(iv) != ivLen {
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log.Panicf("Only %d-byte IVs are supported", ivLen)
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}
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if len(g.key) != keyLen {
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log.Panicf("Wrong key length: %d. Key has been wiped?", len(g.key))
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}
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if len(in) <= tagLen {
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return nil, fmt.Errorf("stupidgcm: input data too short (%d bytes)", len(in))
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}
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// If the "dst" slice is large enough we can use it as our output buffer
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outLen := len(in) - tagLen
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var buf []byte
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inplace := false
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if cap(dst)-len(dst) >= outLen {
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inplace = true
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buf = dst[len(dst) : len(dst)+outLen]
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} else {
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buf = make([]byte, len(in)-tagLen)
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}
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ciphertext := in[:len(in)-tagLen]
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tag := in[len(in)-tagLen:]
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// https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption#Authenticated_Encryption_using_GCM_mode
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// Create scratch space "context"
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ctx := C.EVP_CIPHER_CTX_new()
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if ctx == nil {
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log.Panic("EVP_CIPHER_CTX_new failed")
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}
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// Set cipher to AES-256
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if C.EVP_DecryptInit_ex(ctx, C.EVP_aes_256_gcm(), nil, nil, nil) != 1 {
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log.Panic("EVP_DecryptInit_ex I failed")
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}
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// Use 16-byte IV
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if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_SET_IVLEN, ivLen, nil) != 1 {
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log.Panic("EVP_CIPHER_CTX_ctrl EVP_CTRL_GCM_SET_IVLEN failed")
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}
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// Set key and IV
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if C.EVP_DecryptInit_ex(ctx, nil, nil, (*C.uchar)(&g.key[0]), (*C.uchar)(&iv[0])) != 1 {
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log.Panic("EVP_DecryptInit_ex II failed")
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}
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// Set expected GMAC tag
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if C.EVP_CIPHER_CTX_ctrl(ctx, C.EVP_CTRL_GCM_SET_TAG, tagLen, (unsafe.Pointer)(&tag[0])) != 1 {
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log.Panic("EVP_CIPHER_CTX_ctrl failed")
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}
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// Provide authentication data
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var resultLen C.int
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if C.EVP_DecryptUpdate(ctx, nil, &resultLen, (*C.uchar)(&authData[0]), C.int(len(authData))) != 1 {
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log.Panic("EVP_DecryptUpdate authData failed")
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}
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if int(resultLen) != len(authData) {
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log.Panicf("Unexpected length %d", resultLen)
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}
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// Decrypt "ciphertext" into "buf"
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if C.EVP_DecryptUpdate(ctx, (*C.uchar)(&buf[0]), &resultLen, (*C.uchar)(&ciphertext[0]), C.int(len(ciphertext))) != 1 {
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log.Panic("EVP_DecryptUpdate failed")
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}
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if int(resultLen) != len(ciphertext) {
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log.Panicf("Unexpected length %d", resultLen)
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}
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// Check GMAC
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dummy := make([]byte, 16)
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res := C.EVP_DecryptFinal_ex(ctx, (*C.uchar)(&dummy[0]), &resultLen)
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if resultLen != 0 {
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log.Panicf("Unexpected length %d", resultLen)
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}
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// Free scratch space
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C.EVP_CIPHER_CTX_free(ctx)
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if res != 1 {
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// The error code must always be checked by the calling function, because the decrypted buffer
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// may contain corrupted data that we are returning in case the user forced reads
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if g.forceDecode == true {
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return append(dst, buf...), ErrAuth
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}
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return nil, ErrAuth
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}
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if inplace {
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return dst[:len(dst)+outLen], nil
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}
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return append(dst, buf...), nil
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}
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// Wipe tries to wipe the AES key from memory by overwriting it with zeros
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// and setting the reference to nil.
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//
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// This is not bulletproof due to possible GC copies, but
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// still raises to bar for extracting the key.
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func (g *StupidGCM) Wipe() {
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for i := range g.key {
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g.key[i] = 0
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}
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g.key = nil
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}
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