How Long Should I Make My API Key? An In-Depth Guide

When it comes to securing APIs, one of the most fundamental considerations is how to generate and manage the API keys used to authenticate requests. And one of the first questions developers often ask is, "How long should my API keys be?"

In this in-depth guide, we‘ll break down everything you need to know to make an informed decision about API key length for your application. We‘ll dive into the math behind calculating collision probabilities, discuss the various tradeoffs to consider, and share some best practices for securely generating and managing API keys. By the end of this post, you‘ll have a solid framework for picking the optimal key length to keep your API secure.

Understanding API Key Entropy

The first concept to understand when thinking about API key length is entropy. In the context of cryptography and security, entropy essentially means randomness. The more entropy in your API keys, the harder they are to guess or brute force.

Entropy is typically measured in bits. For each bit of entropy, the number of possible keys doubles. So a key with 10 bits of entropy has 2^10 (1,024) possible values, while a key with 50 bits of entropy has 2^50 (over 1 quadrillion) possible values.

The entropy of an API key depends on two factors:

1. Character set size: The number of possible characters that can appear in each position of the key. A larger character set provides more entropy per character.

2. Key length: The number of characters in the key. Longer keys provide more total entropy.

The formula for calculating the bits of entropy in an API key is:

entropy_bits = key_length * log2(character_set_size)

So, for example, a 32-character key using the full 62-character alphanumeric set (A-Z, a-z, 0-9) would have an entropy of:

32 * log2(62) ≈ 191 bits

In general, modern security best practices recommend a minimum of 128 bits of entropy for cryptographic keys. For API keys, which may face lower threat levels than other types of cryptographic keys, a lower entropy may be acceptable. However, it‘s generally a good idea to go beyond the minimum to provide a strong security margin.

Calculating Collision Probabilities

The next consideration in choosing an API key length is collision probability – the chance that two randomly generated keys will be the same. The math behind calculating collision probabilities was covered in depth in the previous post, so we won‘t rehash all the details here. However, it‘s worth refreshing on a few key points:

• The number of possible unique keys for a given key specification is calculated as: possible_keys = character_set_size ^ key_length

• The probability of a collision can be approximated using the formula: collision_probability ≈ 1 - e^(-(num_keys_needed^2)/(2*possible_keys))

• Each additional character of key length exponentially decreases the collision probability

To illustrate the impact of key length on collision probability, let‘s look at some examples:

Key Length	Character Set Size	Possible Keys	Estimated Keys Needed	Collision Probability
16	62	4.7e28	1 billion	1 in 23 million
24	62	1.3e43	1 billion	1 in 6.5e25
32	62	2.3e57	1 billion	1 in 1.1e41
40	62	6.7e71	1 billion	1 in 3.3e56

As you can see, even with a high estimate of 1 billion keys needed, a 32-character alphanumeric key provides an extremely low collision probability of 1 in 1.1e41.

Real-World API Key Length Data

So what API key lengths are used in practice by real-world APIs? Here are some examples from popular APIs and frameworks:

• Twitter API: 25-character alphanumeric keys
• Google API: 40-character alphanumeric keys
• AWS API: 20-character alphanumeric keys
• Ruby on Rails (default): 24-character alphanumeric keys
• Django REST Framework (default): 40-character alphanumeric keys

As you can see, most fall in the 20-40 character range, which aligns with the guidance of using at least 128 bits of entropy. However, the specific length choice often depends on the specific security needs and architectural constraints of the API.

Other Factors to Consider

While achieving a low collision probability and high entropy are the primary technical considerations in choosing an API key length, there are a few other factors to keep in mind:

• Storage and transmission overhead: Longer keys require more storage space in databases and take up more room in HTTP headers or request bodies. In most cases this overhead is negligible, but for APIs operating at massive scale, it may be a consideration.

• Usability: Longer keys may be harder for users to manage and work with. A 128-character API key could be annoying to deal with for command-line API clients. 32-40 characters is generally a good balance between security and usability.

• Compatibility: Some API frameworks, client libraries, or other tools may have built-in assumptions or limitations on key length. When in doubt, align with the conventions of your framework or ecosystem.

API Key Security Best Practices

Choosing a sufficiently long, high-entropy API key is just one piece of the API security puzzle. Here are some other best practices to follow to ensure your API keys are secure:

• Use a cryptographically secure random number generator: API keys should be generated using a vetted, cryptographically secure source of randomness. Don‘t use Math.random() or other weak pseudo-random functions.

• Encrypt keys in transit: Any transmission of API keys should use strong, up-to-date encryption like TLS 1.2+. Avoid sending keys over unencrypted channels like HTTP.

• Hash and salt keys at rest: Avoid storing plaintext API keys in databases. Instead, store a salted hash of the key using a secure hashing function like bcrypt, scrypt, or PBKDF2.

• Rotate keys regularly: Don‘t rely on the same API keys forever. Regularly cycling and replacing keys limits the blast radius if a key is ever compromised. Many APIs expire keys after 1-2 years by default.

• Monitor for anomalous activity: Use automated systems to monitor for suspicious API activity that could indicate a compromised key, such as a high rate of failed requests, abnormal usage patterns, or requests from unexpected IP ranges.

• Have a revocation plan: Ensure you have a way to quickly revoke API keys if needed in the case of a breach or inadvertent exposure. Avoid architectural patterns that make it difficult or impossible to invalidate existing keys.

The Importance of Secure API Key Management

In today‘s API-driven world, the security of your APIs is only as strong as your API key management practices. API keys act as the "keys to the kingdom" for accessing API functionality and data. A compromised key can allow an attacker to abuse API endpoints, make unauthorized requests, steal sensitive data, or even impersonate other users.

Given the high stakes of API security, it‘s critical that developers treat API keys with the appropriate level of sensitivity and care. Choosing a key length with a sufficiently low collision probability and high entropy is an important first step. But it must be combined with secure key generation, transmission, storage, and rotation.

For any developer building or managing an API, I highly recommend taking some time to audit your current API key management practices and ensure you are following all the best practices covered in this guide. The effort to implement proper API key hygiene is well worth the peace of mind of knowing your API and your users are protected.

Further Reading and Resources

For further exploration of the topics covered in this guide, I recommend the following resources:

• OWASP REST Security Cheat Sheet: In-depth guidance on REST API security best practices from the Open Web Application Security Project (OWASP)
• NIST SP 800-90A Rev. 1: The National Institute of Standards and Technology (NIST) standard for cryptographic random number generation
• Encryption at Rest: Google‘s best practices guide for securely encrypting data at rest
• The Twelve-Factor App: API Keys: Guidance on managing API keys in accordance with the Twelve-Factor App methodology

Wrapping Up

Congratulations, you now have a solid understanding of how to choose an appropriate API key length and manage your API keys securely!

To recap, the key takeaways are:

• Use a key length with at least 128 bits of entropy, which usually means 32-40 characters for alphanumeric keys
• Calculate your collision probability to ensure it is sufficiently low for your security needs
• Generate keys using a cryptographically secure method and avoid weak sources of randomness
• Encrypt keys in transit, hash and salt them at rest, and regularly rotate them
• Have a plan for monitoring key usage and revoking keys in the event of compromise

If you have any questions or feedback on this guide, feel free to reach out. And if you found it helpful, please share it with your fellow developers. Spreading API security best practices makes the web a safer place for everyone!