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Bytes	Lang	Time	Link
166	166 bytes	241117T212148Z	Anders K
290	Python's cryptography	241116T103212Z	user7610
392	openssl req	241113T233840Z	user7610
493	openssl req	241115T112306Z	Themooni

166 bytes

-----BEGIN CERTIFICATE-----
MFAwRgIBADADBgEAMAAwHhcNNTAwMTAxMDAwMDAwWhcNNDkxMjMxMjM1OTU5WjAAMBgwCwYJKoZIhvcNAQEBAwkAMAYCAQACAQAwAwYBAAMBAA==
-----END CERTIFICATE-----

This “certificate”, generated by hand with ascii2der, has an empty issuer, empty subject, 0-bit RSA public key, no extensions, and empty signature with algorithm null, but openssl x509 does load it:

Certificate:
    Data:
        Version: 1 (0x0)
        Serial Number: 0 (0x0)
        Signature Algorithm: 0.0
        Issuer: 
        Validity
            Not Before: Jan  1 00:00:00 1950 GMT
            Not After : Dec 31 23:59:59 2049 GMT
        Subject: 
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (0 bit)
                Modulus: 0
                Exponent: 0
    Signature Algorithm: 0.0
    Signature Value:

(I think we need much more clarification about what “valid” means. After such clarification is provided, I’m happy to delete or revise this.)

Python's `cryptography`, 290 bytes

I could not beat my previous result using openssl req, so I switched to Python's cryptography module, which is somehow able to produce even shorter certs. I again used elliptic curve-based certs because the curves are smaller than RSA. openssl is still happy to load such certificate.

from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import ed25519
from cryptography.x509 import CertificateBuilder
from cryptography import x509

import datetime

def generate_minimal_certificate() -> bytes:
    private_key = ed25519.Ed25519PrivateKey.generate()

    subject = x509.Name([
        # x509.NameAttribute(x509.NameOID.COMMON_NAME, 'localhost')
    ])

    cert = (
        CertificateBuilder()
        .subject_name(subject)
        .issuer_name(subject)
        .public_key(private_key.public_key())
        .serial_number(1)
        .not_valid_before(datetime.datetime.now(datetime.UTC))
        .not_valid_after(datetime.datetime.now(datetime.UTC) + datetime.timedelta(days=365))
        .sign(private_key, None)
    )

    pem_cert = cert.public_bytes(encoding=serialization.Encoding.PEM)
    return pem_cert

if __name__ == '__main__':
    cert = generate_minimal_certificate()
    print(cert.decode())
    print(len(cert))

`openssl req`, 737 558 526 497 392 bytes

% openssl ecparam -name secp112r1 > ec.params                                          
% openssl req -nodes -x509 -newkey ec:ec.params -days 1 -set_serial 1 -out ca-cert.pem -subj "/"

With the above I am getting certs that are either 392 or 396 bytes long. So I picked one of the shorter ones.

changelog

-newkey rsa:512, use smallest possible private key size
-subj "/", the / seems to be the shortest possible accepted argument for -subj
-set_serial 1, the default is a large random number that takes up bytes
-newkey ec:ec.params, picked an elliptic curve from the list openssl ecparam -list_curves that is giving me the smallest key, after trying each once
-days 1, does not seem to have impact, but smaller numbers are usually better, copied from Themoonisacheese's answer

`openssl req`, 518 493 bytes

-25 bytes: --set-serial 1 by user7610

generated using:

openssl req -nodes -x509 -key key490.pem -days 1 -subj "/" --set-serial 1> cert.pem

with key490.pem a 490-bit rsa key generated using:

import rsa
import sys
pubkey,privkey = rsa.newkeys(int(sys.argv[1]))
print(privkey.save_pkcs1('PEM').decode('UTF-8'))

While 512 bits is the smallest openssl will allow you to generate, an RSA key can be as small as 16 bits (!). However, as the key gets smaller, you can encrypt less and less data with it, and openssl is not happy about that. I performed a manual binary search between 16 and 512 to find that 490 bits is the smallest possible key using this technique.

166 bytes

Python's cryptography, 290 bytes

openssl req, 737 558 526 497 392 bytes

changelog

openssl req, 518 493 bytes

Python's `cryptography`, 290 bytes

`openssl req`, 737 558 526 497 392 bytes

`openssl req`, 518 493 bytes