Decrypt An RSA Message From Browser With Window.crypto.subtle APIs

I'm trying to decode an RSA 2048 bit message encoded with a public key using the corresponding private key. The environment is google chrome and I'm using the window.crypto.subtle

Solution 1:

There's only one flaw: The posted code currently uses OAEP with SHA256. The ciphertext can be decrypted with the posted key if OAEP with SHA1 is applied as padding.

In addition, the function fromBase64 can be used to Base64 decode the key into a TypedArray, so the function str2ab is actually not needed (but of course this is not an error, just redundant).

const fromBase64 = base64String => Uint8Array.from(atob(base64String), c => c.charCodeAt(0));
const getPkcs8Der = pkcs8Pem => {
    pkcs8Pem = pkcs8Pem.replace( /[\r\n]+/gm, "" );
    const pkcs8PemHeader = "-----BEGIN PRIVATE KEY-----";
    const pkcs8PemFooter = "-----END PRIVATE KEY-----";
    pkcs8Pem = pkcs8Pem.substring(pkcs8PemHeader.length, pkcs8Pem.length - pkcs8PemFooter.length);
    return fromBase64(pkcs8Pem);	
}		
	 
async function importPrivateKey(pkcs8Pem) {		
    return await window.crypto.subtle.importKey(
        "pkcs8",
        getPkcs8Der(pkcs8Pem),
        {
            name: "RSA-OAEP",
            hash: "SHA-1",          // Replace SHA-256 with SHA-1
        },
        true,
        ["decrypt"]
    );
}

async function decryptRSA(key, ciphertext) {
    let decrypted = await window.crypto.subtle.decrypt(
        {
            name: "RSA-OAEP"
        },
        key,
        ciphertext
    );
    const dec = new TextDecoder();
    return dec.decode(decrypted);
}

window.onload = init;

async function init() {

    const privateKey = 
        '-----BEGIN PRIVATE KEY-----\
        MIIEvwIBADANBgkqhkiG9w0BAQEFAASCBKkwggSlAgEAAoIBAQC3jmTi3O1k2YXs\
        AM6nNTTIzDq5YWkxYrYb6cpO9eYuzmphgRnVDR6a1YWRXMoCuCfuNXcDGywzudRn\
        bBMw0FHKLUqCttVHGpZYu0+0tRR10ubxiz/xnd/aCmRYHcmUNn8Qdh3KU59A9HK5\
        HhYFf1vhK8r3fkoO4CjoGo1ROzXyMybUSy+4mSNscUtt5LwrVn48vXvG5i5B4DRT\
        nM4cINmutEzA2s5cDt+dzU4Py71fKBRDRIGGn0vdVSoZKbWuhm5WewyRewCk7HFc\
        PALCi5/1A7VKDAHUC4FlXmuG2+wzdchEyxMj6oLR7+BkKFQaTmuMM/22cGBjVTVt\
        pSr3iDovAgMBAAECggEBAIuTQW+oovNu3IDq1DkdIjgV5AmW4tBkySlMi0OjhBbP\
        auEdtDDnOwBtoJU6Q3nx4psmGItKHEBw6+yAp88UeT0NV30x3delhfGO7Trx/s7h\
        Qi8lvcfSTqeUA11luSR0lAZGaryw/YX820eccw5XG9yK2ll7tIC/PxvPJOpB5fF2\
        XGxGrionTjHDzXJ1OWX0i0aZlNNufInJAHhlt7aT3GiQMKcQs+AUb/+bWxI3Hln8\
        KcL13EUlD4pJW8vtTK3gCnQNKKMoPB5Ugqe5BrU8ElkBz+zSKDnVwt5bgjrlucYz\
        rKJxWr6/qTRZkzmvkhaJeNzzepfwkFsQ/eHcxYrtuDECgYEA8OXkQ2SqYDZwizCd\
        SuVkx2zHm3zXYRSlRtnXYoUdJyTyuZ4k2GvXBrlwRsOJ14emVkHKyR5enmNjwrW5\
        dcD2tbBzavcqOYAMiHcKklcS/gWgPx0X5QFHU33vr8u51BQWCz75lxddWNKxVAN/\
        cUTugONtS4+EP4dSZhuxHt6RscsCgYEAwxA9QmZrI54hjMkIyqwmviRmIk42S5kk\
        OxbhxBbt5qVueLRB092JyGmCR2kYiqdHCYkGFDOE4kni6Bsszq5CSJvhiATFeZeX\
        ldFQeZqAiRY2rAd7xD1upMug/cK3ODA8k3k/e72CtyxtBTR01q29SnPx5p/57MrI\
        3ogddHlGvK0CgYEA3VqhELwjQh1D9OJK5lM683SlRd7FGdOauyvYmhKu4xU0ZBNI\
        0ATnpKoo3R04P+/JjGEQMRXS4794H6ZUMDuLdxAYPiW3ivZ6jbq04BtavEf3I4dc\
        OXWfULzbzbFpo9KBHvxS4974S3Hut8AvDqnEbnKML25EmwuBT4oKis8BGVkCgYEA\
        nusPDZbFeNou+TUbzYrdcZHUB+TyhTq58s4clxYbMgrbaslozAQ0aavT8Pvle6j2\
        zgTth+3FOFr72x+wrJ358I/W+Wrxu7NOU0eZqci/KXCIkDT0l5d5GhewDK3jeYqK\
        /5cLqnNmGHfARjpLak9X5V162erBwjIf3nTEkozvnW0CgYB6L1CX3DkTFH3OBcJe\
        SvV18RDUfNI8MpUKcpofmwwvgER3GrehSZHRVxVwNbnJOqbh/oiwmmNJieKrFsk9\
        EzCRBVWdZByHHYW2js+gCrAp+ghnl1QEAeCU7YTxCJ2fZIAmfB9X4u/7ARtVxnZY\
        mOWlm65KUYr5lf2Ws5plL4pCRA==\
        -----END PRIVATE KEY-----';

    const ciphertext =
        'F6/NwENdUZSl+vrgpWVkyWPQuYaTGDNZPIvj4KmIRHVx4qybxN24LPIgk0Rl84KHcLFadZWCjNpM\
        vg3l826OaKZAtwvIp9IxVrMbvtNOymY6A1koKvC9ema92SR4DC9hmTtMxhUB6r3XgACtRLFqMfg+\
        zYSHfFqQEGJg3yZ43hfzIq/gCfHPk5sZXASq5WY5b9yd4gRonn5D4OCD6xna/r5ovHfrpO/Fwe8N\
        eeY2gqTAdtzvtmOw/HLQhGANejpJYr1IriQbepM7jLjBkJX+uCn38O1MxpQb7s5RXTvGvoEoofWV\
        Cq8gNFhgnVFuurdZUiY0bn58UwaVFdwzEfDSUQ==';

    try {
        const key = await importPrivateKey(privateKey);
        const decrypted = await decryptRSA(key, fromBase64(ciphertext));
        console.log(decrypted);
    } catch(error) {
        console.log(error);
    }
}