Generate minimal certificate on the fly

README.md

@@ -1,5 +1,5 @@
 # ApkGolf
-This repository hosts the smallest Android APK in the world. The current size of the APK is *678 bytes*.
+This repository hosts the smallest Android APK in the world. The current size of the APK is *677 bytes*.
 
 To learn more about how this was achieved, please read the [blog post](https://fractalwrench.co.uk/posts/playing-apk-golf-how-low-can-an-android-app-go/).
 
@@ -34,8 +34,8 @@ Further byte-level optimisation of what remains of the manifest.
 
 Contributed by [Madis Pink](https://github.com/madisp) in this [Pull Request](https://github.com/fractalwrench/ApkGolf/pull/6)
 
-## Minimising APK Signing Certificate (678 bytes, 18% reduction)
-Manual removal of unnecessary fields present in the certificate used to sign the APK.
+## Minimising APK Signing Certificate (677 bytes, 18% reduction)
+Remove unnecessary fields present in the certificate used to sign the APK.
 
 Contributed by [klyubin](https://github.com/klyubin) in this [Pull Request](https://github.com/fractalwrench/ApkGolf/pull/15)
-
+Automated by [Adam Yi](https://github.com/adamyi)

build.sh

@@ -1,7 +1,5 @@
 #!/usr/bin/env bash
 
-set -e
-
 rm -rf build
 mkdir -p build/apk
 
@@ -18,6 +16,41 @@ recompress() {
 
 set -x
 
+echo "Compiling certificate generation program"
+
+success=0
+
+gcc -o build/generate_cert generate_cert.c -lssl -lcrypto -ldl
+
+if [ $? -eq 0 ]; then
+    echo "Generating cert"
+    cd build
+    ./generate_cert
+    if [ $? -eq 0 ]; then
+	cd ..
+        openssl x509 -in build/cert.pem -noout -text
+        if [ $? -eq 0 ]; then
+            openssl pkcs8 -topk8 -in build/key.pem -out build/key.pk8 -outform DER -nocrypt
+            if [ $? -eq 0 ]; then
+                echo "Success"
+                success=1
+            fi
+        fi
+    else
+	cd ..
+    fi
+fi
+
+if [ $success -eq 0 ]; then
+    echo "Failed to generate cert. Using pre-generated cert."
+    rm build/cert.pem
+    rm build/key.pk8
+    cp cert.pem build/
+    cp key.pk8 build/
+fi
+
+set -e
+
 echo "Creating base apk"
 cp app/AndroidManifest.xml build/apk/
 
@@ -27,7 +60,7 @@ zip -j -r build/app-unsigned.apk build/apk
 recompress build/app-unsigned.apk
 
 echo "Signing archive"
-$ANDROID_HOME/build-tools/26.0.2/apksigner sign --v1-signing-enabled false --key key.pk8 --cert key.x509.pem --in build/app-unsigned.apk --out build/signed-release.apk --min-sdk-version 24
+$ANDROID_HOME/build-tools/26.0.2/apksigner sign --v1-signing-enabled false --key build/key.pk8 --cert build/cert.pem --in build/app-unsigned.apk --out build/signed-release.apk --min-sdk-version 24
 
 set +x
 

cert.pem

@@ -0,0 +1,6 @@
+-----BEGIN CERTIFICATE-----
+MIGgMIGVAgEBMAMGAQAwCTEHMAUGAQATADAaFwswMDAxMDEwMDAwWhcLMDAwMTAx
+MDAwMFowCTEHMAUGAQATADBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IABOYMo2Ld
+mbwCoefiz7NRQ+du9obnpJNhRK11XPZHhk6jSRdFiOkqMhko2YI/XWlq6BkC3OPS
+r3QkiEeT3i+EXj8wAwYBAAMBAA==
+-----END CERTIFICATE-----

generate_cert.c

@@ -0,0 +1,91 @@
+// Generate minimal ECC cert
+// Required OpenSSL version < 1.0.1i
+// author: Adam Yi <[email protected]>
+
+#include <stdio.h>
+
+#include <openssl/pem.h>
+#include <openssl/x509.h>
+#include <openssl/asn1.h>
+
+EVP_PKEY * generate_key_ecc()
+{
+    BIO *outbio = NULL;
+
+    int eccgrp = OBJ_txt2nid("prime256v1");
+    EC_KEY *myecc = EC_KEY_new_by_curve_name(eccgrp);
+
+    //EC_KEY_set_asn1_flag(myecc, OPENSSL_EC_NAMED_CURVE);
+    EC_KEY_set_asn1_flag(myecc, 1);
+
+    if (! (EC_KEY_generate_key(myecc)))
+	BIO_printf(outbio, "Error generating the ECC key.");
+
+    EVP_PKEY *pkey = EVP_PKEY_new();
+    if (!EVP_PKEY_assign_EC_KEY(pkey, myecc))
+	BIO_printf(outbio, "Error assigning ECC key to EVP_PKEY structure.");
+
+    return pkey;
+}
+
+X509 * generate_x509(EVP_PKEY * pkey)
+{
+    X509 * x509 = X509_new();
+
+    ASN1_INTEGER_set(X509_get_serialNumber(x509), 1);
+
+    ASN1_TIME_set_string(X509_get_notBefore(x509), "0001010000Z");
+    ASN1_TIME_set_string(X509_get_notAfter(x509),  "0001010000Z");
+
+    X509_set_pubkey(x509, pkey);
+
+    // X509v1 cannot have empty issuer & subject DN
+    X509_NAME * name = X509_get_subject_name(x509);
+
+    X509_NAME_add_entry_by_NID(name, 0,  V_ASN1_APP_CHOOSE, "", 0, 0, 0);
+
+    X509_set_issuer_name(x509, name);
+
+    return x509;
+}
+
+void write_to_disk(EVP_PKEY * pkey, X509 * x509)
+{
+    FILE * pkey_file = fopen("key.pem", "wb");
+
+    PEM_write_PKCS8PrivateKey(pkey_file, pkey, NULL, NULL, 0, NULL, NULL);
+    fclose(pkey_file);
+
+    FILE * x509_file = fopen("cert.pem", "wb");
+
+    PEM_write_X509(x509_file, x509);
+    fclose(x509_file);
+
+    return;
+}
+
+int main(int argc, char ** argv)
+{
+    printf("Generating ECC key...\n");
+
+    EVP_PKEY * pkey = generate_key_ecc();
+    if(!pkey)
+	return 1;
+
+    printf("Generating X509 cert...\n");
+
+    X509 * x509 = generate_x509(pkey);
+    if(!x509)
+    {
+	EVP_PKEY_free(pkey);
+	return 1;
+    }
+
+    printf("Writting files....\n");
+
+    write_to_disk(pkey, x509);
+    EVP_PKEY_free(pkey);
+    X509_free(x509);
+
+    printf("Done!\n");
+}