1 files changed, 98 insertions, 0 deletions
diff --git a/internal/stupidgcm/chacha.c b/internal/stupidgcm/chacha.c
new file mode 100644
index 0000000..c85cf78
--- /dev/null
+++ b/internal/stupidgcm/chacha.c
@@ -0,0 +1,98 @@
+#include <openssl/evp.h>
+#include <stdio.h>
+//#cgo pkg-config: libcrypto
+
+extern void panic1(void);
+
+static void panic(const char* const msg)
+{
+    fprintf(stderr, "panic in C code: %s\n", msg);
+    __builtin_trap();
+}
+
+// https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption#Authenticated_Encryption_using_GCM_mode
+int chacha20poly1305_seal(
+    const unsigned char* const plaintext,
+    const int plaintextLen,
+    const unsigned char* const authData,
+    const int authDataLen,
+    const unsigned char* const key,
+    const int keyLen,
+    const unsigned char* const iv,
+    const int ivLen,
+    unsigned char* const ciphertext,
+    const int ciphertextBufLen)
+{
+    // Create scratch space "context"
+    EVP_CIPHER_CTX* ctx = EVP_CIPHER_CTX_new();
+    if (!ctx) {
+        panic("EVP_CIPHER_CTX_new failed");
+    }
+
+    // Set cipher
+    if (EVP_EncryptInit_ex(ctx, EVP_chacha20_poly1305(), NULL, NULL, NULL) != 1) {
+        panic("EVP_EncryptInit_ex set cipher failed");
+    }
+
+    // Check keyLen by trying to set it (fails if keyLen != 32)
+    if (EVP_CIPHER_CTX_set_key_length(ctx, keyLen) != 1) {
+        panic("keyLen mismatch");
+    }
+
+    // Set IV length so we do not depend on the default
+    if (EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_IVLEN, ivLen, NULL) != 1) {
+        panic("EVP_CTRL_AEAD_SET_IVLEN failed");
+    }
+
+    // Set key and IV
+    if (EVP_EncryptInit_ex(ctx, NULL, NULL, key, iv) != 1) {
+        panic("EVP_EncryptInit_ex set key & iv failed");
+    }
+
+    // Provide authentication data
+    int outLen = 0;
+    if (EVP_EncryptUpdate(ctx, NULL, &outLen, authData, authDataLen) != 1) {
+        panic("EVP_EncryptUpdate authData failed");
+    }
+    if (outLen != authDataLen) {
+        panic("EVP_EncryptUpdate authData: unexpected length");
+    }
+
+    // Encrypt "plaintext" into "ciphertext"
+    if (plaintextLen > ciphertextBufLen) {
+        panic("plaintext overflows output buffer");
+    }
+    if (EVP_EncryptUpdate(ctx, ciphertext, &outLen, plaintext, plaintextLen) != 1) {
+        panic("EVP_EncryptUpdate ciphertext failed");
+    }
+    if (outLen != plaintextLen) {
+        panic("EVP_EncryptUpdate ciphertext: unexpected length");
+    }
+    int ciphertextLen = outLen;
+
+    // Finalise encryption
+    // Normally ciphertext bytes may be written at this stage, but this does not occur in GCM mode
+    if (EVP_EncryptFinal_ex(ctx, ciphertext + plaintextLen, &outLen) != 1) {
+        panic("EVP_EncryptFinal_ex failed");
+    }
+    if (outLen != 0) {
+        panic("EVP_EncryptFinal_ex: unexpected length");
+    }
+
+    // We only support 16-byte tags
+    const int tagLen = 16;
+
+    // Get MAC tag and append it to the ciphertext
+    if (ciphertextLen + tagLen > ciphertextBufLen) {
+        panic("tag overflows output buffer");
+    }
+    if (EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_GET_TAG, tagLen, ciphertext + plaintextLen) != 1) {
+        panic("EVP_CTRL_AEAD_GET_TAG failed");
+    }
+    ciphertextLen += tagLen;
+
+    // Free scratch space
+    EVP_CIPHER_CTX_free(ctx);
+
+    return ciphertextLen;
+}