Presigned URLs in S3

Image by ArtTower from Pixabay

S3 is the Amazon service to store files in the cloud. It is reliable, very reliable, the expected time to lost a single file from a group of 10 million of them is 10000 years. Even other services on Amazon uses internally S3 to store its files. On the bad side, as it is one of the first services that Amazon created, it can be a headache to fine grain permissions form all its capabilites and evolutions, making it difficult to be sure that a file is not accesible for those that should not be allowed.

In S3 you can define what they call a bucket, which is like a directory in a filesystem. The name of the bucket must be unique, not only in your account but in the global namespace from all AWS accounts in the world. That means you have to be creative when picking a bucket name.

A bucket can be private or publicly accessible. In the public side, one of the special uses is to serve static content from as a web server, even html pages from your custom domain. But what if you want to allow users to download files, for example an image, and you don’t want the user to be able to make it public sharing the link to the image?

I’ve played today with a very useful feature for that case. It allows to have a private bucket that can temporary allow the access to a single file to GET or even PUT/POST for a limited amount of time. You’ll need to use AWS SDK of your favourite supported programming language or AWS CLI from command line, to query AWS API for a temporary authorized url. Let’s see how with an example from scratch, installing and using AWS CLI in a Debian based environment.

Make sure you have access to an AWS account (you already have one if you have an account) and generate a pair of AWS Access Key and AWS Secret Access Key from web console.

$> sudo apt instal awscli
$> aws configure
AWS Access Key ID [None]:
AWS Secret Access Key [None]:
Default region name [None]: eu-west-1
Default output format [None]:

Create a local file called piticli with the content you prefer. Let’s create also a new S3 bucket using aws cli

# Create a convenience environment variable with a kind of random bucket name
$> BN="s3://thomarite-blog-test-$RANDOM"
# Let's actually create the bucket
$> aws s3 mb $BN 
make_bucket: thomarite-blog-test-1337
# Let's see it exists
$> aws s3 ls
2020-04-16 23:01:27 thomarite-blog-test-1337
# Now let's upload piticli into the new bucket
$> aws s3 cp piticli $BN
2020-04-17 23:01:45          26 piticli

Now let’s create a presigned url for piticli and store it in PRESIGNED_URL env var. As you can see, the temporary URL includes the bucket name, the file name and new AWS Access Key and signature, and a hint about the expiration date.

# Store the URL into a env var for future use
$> PRESIGNED_URL=$(aws s3 presign $BN/piticli)

That’s the public url and will be valid for 1h by default. You can set the expiration time in aws s3 presign command using the parameter --expires-in and set the seconds allowed until it expires.

Now you have a public url accessible by any browser. Let’s open it via curl:

$> curl -Ls $PRESIGNED_URL
piticli is now… sleeping

And finally to clean things up let’s remove all the files and the bucket in AWS

$> aws s3 rb --force $BN
delete: s3://thomarite-blog-test-1337/piticli
remove_bucket: thomarite-blog-test-1337


Today I’ve received a copy of Serious Cryptography and jumped straight ahead to Chapter 12 talking about Elliptic Curves. I’m more or less aware of how RSA works, but was intruigued for a few years now about how criptography was able to achieve same security capabilities with more efficiency while dealing with less information.

Basically that is a different kind of beast based on the hard it is to retrieve the exponent from a discrete logarithm, instead of rely on the factorization of a number composed from two large primes as it is with RSA. As the size of the numbers are much lower with the same hardness to attack, the keys involved are also way smaller.

Elliptic Curve comes after the properties of the equation that the field of work is based, an equation of the curve of the type y^2=x^3 + ax + b where a and b are pre-cooked parameters that must be carefully chosen to avoid security risks.

In fact, there are two main curves used nowadays. One are a family of curves that the NIST approved, the most famous known as P-256, being the most commonly used in the industry, but also had some critisism because those a and b params in the equation were defined by the NSA and the generation is not completely publicly known. Therefore it could have potentially unknown pre-cooked attacks, althought based on the book, the general consensus by the experts is that there is no problem.

The second one is the curve generated by Daniel J. Bernstein that have pretty much the security of the NIST one and arguabilly a bit more of performance speed. It is called Curve25519 because the (discrete) field it works on is based on the prime number 2^255-19, having a=486662 and b=x .