1 files changed, 184 insertions, 0 deletions
diff --git a/internal/stupidgcm/stupidgcm.go b/internal/stupidgcm/stupidgcm.go
new file mode 100644
index 0000000..6773a35
--- /dev/null
+++ b/internal/stupidgcm/stupidgcm.go
@@ -0,0 +1,184 @@
+// 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
+}